CBSE Notes

By going through these CBSE Class 12 Biology Notes Chapter 9 Strategies for Enhancement in Food Production, students can recall all the concepts quickly.

Strategies for Enhancement in Food Production Notes Class 12 Biology Chapter 9

→ The agricultural practice of breeding and raising livestock is termed animal husbandry. It mainly deals with the care and breeding of commercially important livestock like cows, buffaloes, pigs, cattle, horses, sheep, camels, goats, and poultry farming, and fisheries. Bees, silk-worm, prawns, crabs, birds are also used for their products like honey, milk, and meat, etc.

→ The dairy food industry deals with the management of animals for milk production and its products for the consumption of human beings. Milk yield depends on the quality of breeds with high yielding potential and disease-resistant features.

Those processes and systems are practiced which would increase the yield and improve the quality of milk. The cattle should be well housed, given adequate water, and maintained disease-free. Feeding is done with special emphasis on the quality and quantity of fodder in a scientific manner. Cleanliness and hygiene are of paramount importance while milking, storage, and transport of milk and milk products.

→ Poultry includes all those birds which can be raised under domestication for economic purpose. Poultry farming is highly advantageous. It yields quick returns, requires less space, and easy to manage. Birds like chickens, ducks, geese, turkey, etc. are raised for their eggs and meat.

→ Poultry farm management includes the selection of disease-free and suitable breeds, proper and safe farm conditions, proper feeding and water, hygienic and healthy care.

→ To increase the yield and get desirable qualities in animals, breeding of animals is done. A group of animals that are similar in most characters such as general appearance, features, size, configuration, etc. is said to belong to a breed.

→ When breeding is done between the animals of the same breed, it is called inbreeding. When breeding is done between animals of different breeds it is called cross-breeding or out-breeding.

→ Inbreeding is done by carrying out mating between closely related individuals within the same breed for 4 to 6 generations. Superior males and females are identified and mated in pairs. The progeny is evaluated and superior males and females are selected for further mating. This is repeated for some generations, it leads to the development of pure lines. It increases homozygosity. In cattle, a superior female is one that produces more milk and a superior male is one that gives rise to superior progeny.

→ Inbreeding depression occurs because of continued, close inbreeding, which reduces fertility and productivity. This can be solved by outbreeding.

→ The breeding of unrelated animals is called outbreeding. It may be between some breeds with no ancestral link or between different breeds called cross-breeding. It may happen in different species, called interspecific hybridization.

→ Out-crossing is done within the same breed. It helps to overcome inbreeding depression and the best breeding method to increase productivity.

→ Crossbreeding allows the desirable qualities of two different breeds to combine together. The hybrids are either used commercially or allowed to inbreed so as to select a new stable breed that is superior to the existing breeds.

→ Interspecific hybridization allows the mating of male and female, of two different species. The progeny will have the combined features of both the parents, e.g. mule.

→ Controlled breeding is done with artificial insemination. The semen is collected from males and injected into the reproductive tract of females. This helps in desirable mating and ensures good quality progeny. The semen obtained from a single bull can inseminate a few thousand cows at far-off places.

→ Superovulation and embryo transplantation are new techniques employed for cattle improvement. For this, the cow is administered hormones to induce follicle maturation and superovulation. Due to which several eggs are produced per cycle. These are artificially inseminated, the fertilized eggs are surgically removed at an 8 – 32 celled stage and transferred to the surrogate mother.

→ Apiculture is the maintenance of hives of honey bees for honey production. A. dorasata, Apis, florea, A. indica species are reared for honey. It is an age-old cottage industry. Beehives can be kept in courtyards, or bee pastures of some wild shrubs, fruit orchards, and cultivated crops.

It is not labor intensive but it requires some specialized knowledge and training. Before starting apiculture practice, knowledge of the nature and habits of honey bees, selection of suitable location for keeping the beehives, catching and hiving of a group of bees, management of hives during different seasons, handling and collection of honey and beeswax, is very important.

→ Along with honey, honey bees also produce beeswax, which is used for cosmetics, polished, and various other industries. Honey is aromatic, viscid, and sweet material. Its constituents are :
(a) Levulose – 41.80%,
(b) Glucose and fructose (grape sugar) – 35.40%,
(c) Water – 15.43%,
(d) Sucrose – 5.39%,
(e) Bee wax and pollen grains – 1.68%.

Pure honey is dissolved in water, makes a thread through the depth of glass while impure honey gets dissolved in water.

→ Pisciculture is the rearing and breeding of fish. It includes catching, processing, or selling fishes, shellfish, or other aquatic animals such as prawns, crab, oysters, etc. The flesh of fish has 60 – 80% water, 13 – 20% protein, and some amount of fat. Fish is a very good source of protein and forms an excellent food as it has very little fat and more protein, has a good quantity of vitamin A and D, a rich source of iodine, and easily digested than other meals.

→ In addition to providing food, the fishing industry yields a number of by-products that are of commercial importance.
Some are:
(a) liver oil, which is extracted from fish liver. It’s a good source of vitamin A, C, D, and E,

(b) fish meal is prepared from the waste of fish oil or canning industry or whole fish. It is used as a major food for domestic animals. It is rich in proteins, calcium, and phosphorus,

(c) fish proteins are 80-90% proteins with no fats. These are used in icecreams, pharmaceuticals, paints, varnishes, textile, paper, and cosmetics. Fish protein concentrate (FPC) is also used as a diet supplement,

(d) fish flour is used in biscuits, bread, cakes sweets, and soup. It is easily digested by the infants,

(e) fish fertilizers are wastes of fish meal preparation. These are used as manures for tea, coffee, and tobacco plantation,

(f) fish rees are rich in thymine, lecithin, cholesterol, tyrosine, xanthine, hypoxanthine, and vitamin B, C, D, and E. These are valuable foods.

→ Agriculture is the science or the practice of farming or cultivation, reaping, and management of farm products. Horticulture is the branch of agriculture that deals with the art of growing vegetables, fruits, and ornamental plants.

→ Plant breeding is the purposeful manipulation of plant species in order to create desired plant types that are better suited for

→ cultivation, give better yields, and disease resistant. The main objective of plant breeding is to reproduce new superior crop varieties with respect to crop yield and quality, increased tolerance to environmental stress such as salinity, extreme temperature, and drought, resistance to pathogens like viruses, fungi, and bacteria, and increased tolerance to insect pests.

→ Plants which are both self and cross-pollinated can produce homozygous and heterozygous population and are most suitable of breeding experiments.

→ First of all collection and preservation of all different wild varieties, species, and relatives of cultivated species is done to properly use the natural available genes. The entire collection of plants or seeds with all the diverse alleles for all genes in a given crop is called germplasm collection.

→ The germplasm is evaluated to identify the plants with a desirable combination of characters. Selection is the oldest breeding method and is the basis of crop improvement.

It may be of two types:
(a) Natural selection and
(b) Artificial selection.

Natural selection is a continuous process that is operating in nature. Here the fittest survives and the remaining ones vanish away.

→ In artificial selection, various types of plants are selected from bulk by plant breeders. This is purposefully done to have a better crop from a mixed population, in a. short duration of time as compared to the natural selection process. This may be achieved by mass selection, pure line selection, and clonal selection.

→ Mass selection is based on phenotypes. The best plants from the population are selected at the time of harvesting, with desired traits. The seeds from all such plants are collected and mixed to form a bulk. This process may be repeated for some generations to get the desired improvement.

→ Pure line selection is also called single plant selection. In it, several single plants with desired traits are selected from a population. This is the best method to improve the variety in self-pollinated plants and the crop is of uniform appearance.

→ Clonal selection is used to ensure purity to race, e.g. mangoes, apples, etc., or due to lack of seed formation, e.g. banana. It is the best method for vegetatively propagated plants

→ The plants are selected by using a suitable selection method. These are then multiplied and used further for hybridization.

→ Hybridization is the crossing of two plants differing from each other genotypically in one or more traits.

The purposes of doing hybridization are:
(a) to exploit and utilize the hybrid vigor and
(b) to increase and generate the genetic variations through recombination.

→ In hybridization, greater success is obtained with healthy and vigorous parents. Selfing is done to reduce heterozygosity. The selected inbreeds are used for hybridization.

→ Hybridization involves emasculation and bagging (discussed in Chapter 2) techniques, which are removal of stamens and bagging male and female flowers to prevent foreign pollen contamination. After these crossing is done by collected pollen grains from the male parent and dusting them on stigma. The same desired characters for hybridization are high protein content from one parent and disease resistance from another parent. The hybrid plant is a genetic combination of the two parents.

→ Hybridization is followed by a selection of hybrids that have desired characters. This step needs expertise and careful scientific evaluation. These plants are superior to both the parents. These are further self-pollinated for several generations to attain homozygosity and to ensure that the desired characters will not segregate in the progeny.

→ After successful experimentation, the crop is grown in the research field under ideal fertilizer application, irrigation, and crop management practices. This is followed by testing in farmer’s fields in different locations for three growing seasons. These are evaluated for their yield and quality, disease resistance qualities, etc.

→ Wheat, rice, and maize are the chief cereals of the world. These belong to the Gramineae family. Wheat flour is used for making chapatis, bread, cakes, biscuits, etc. Its straw is used as fodder and for stuffing. Sonalika and Kalyan Sona are two high-yielding and disease-resistant varieties of wheat.

→ Rice is a staple food for half of the world. It is a semi-aquatic crop. It’s used in various preparations like idli, dosa, kheer, etc. It’s straw issued for making hats. Rice flavor is used in the cosmetic industry. IR-8 and Taichung Native-1, Jaya, and Ratna are few varieties of rice.

→ Maize or corn is rich in carbohydrates, fats, and proteins. It lacks binding gluten so can’t be used for bread making. It can be boiled, roasted, or popped up.

→ Smaller grained cereals are called millets, e.g. ragi, Jowar, and Bajra.

→ Sugarcane is a hybrid obtained from crossing between Saccharum Barberi and Saccharum officinarum. It is the chief source of sugar. Baggage (leftover) is used in the fuel and paper industry. The molasses is used in alcohol anti vinegar preparation.

→ Resistance of the host plant is the ability to prevent the pathogen from causing disease. Some plants are disease resistant while some are susceptible, it is determined by the genetic constitution of the host plant. These pathogens Rhay be fungal, bacterial, or viral, e.g. brown rust of wheat (fungi), red rot of sugarcane, late blight of potato (fungi), black rot of crucifers (bacteria), tobacco mosaic, turnip mosaic (virus). Breeding helps to develop disease-resistant varieties and reduce the fungicides and bactericides treatment.

→ Breeding for disease resistance is achieved by the usual steps like screening for disease-resistant plants through germplasm, hybridization of selected parents, selection and evaluation of the hybrids, followed by testing and finally release of new varieties.

→ Due to a limited number of disease resistance genes in a species, conventional breeding has a limited scope. Other breeding methods such as induced mutations, selection among somaclonal variants, and genetic engineering have promising results in producing desirable characters. Initially, mutations are induced in plants, these are screened for disease resistance genes. Such plants are either directly multiplied or used inbreeding.

→ The mutation is a sudden, stable, and heritable change that alters the genotype of an organism. Mutations are induced artificially in plants which change base sequence in genes, creating genetic variation, this results in new traits in the progeny.

→ The application of induced mutations for crop improvement is called mutation breeding. Mutations can be induced chemically or by radiations, e.g. UV rays, X-rays, cosmic rays, gamma rays, ⁶⁰cobalt, and ¹³⁷caesium, nitrous acid, EMS, mustard gas, colchicine, etc.

→ Plant breeding experiments are used for developing pest and insect-resistant varieties. These are dependent on morphologica1, biochemical, and physiological characters of the host plant. For example, hairy leaves are associated with resistance to insect pests, solid stems of wheat are resistant to stem sawfly, sugar content in maize leads to resistance for maize stem borers. Breeding methods are the same as discussed earlier.

→ Plant breeding is also practiced for improved food quality, it is done for improving protein content and quality, oil content, and quality, for increasing vitamin content, for micronutrient and mineral content. For example, Atlas 66 wheat has high protein content, maize hybrids with more amino acids, lysine and tryptophan and rice with more iron content have been developed.

→ Single-cell proteins (SCPs) are an alternative source of proteins for animal and human nutrition.

→ Microbes like spirulina which can be grown on waste materials can serve as rice food for proteins, minerals, fats, carbohydrates, and vitamins and reduces environmental pollution too. Microbes are grown increasingly on large scale to serve as a good source of proteins.

→ Plant tissue culture is a technique of growing cells, tissues, or organs in sterilized nutrient media under controlled aseptic conditions. The plant part which is used for culture preparation is called explant. Plant cells are totipotent i.e: they can form a complete plant under suitable conditions. In specific culture medium and sterile conditions, many plant cells and tissues divide and grow to form an unorganized mass of cells called callus.

→ The nutrient medium must provide a source for carbon and inorganic salts, vitamins, amino acids, and growth regulators like cytokinins and auxins, etc.

→ Micropropagation or cloning is a novel technique devised to produce vast quantities of strong and healthy plantlets by rapid vegetative multiplication under controlled conditions. Each of these plants is genetically identical to the parent plant and is called some clones.

→ Tissue culture is useful for

1. Micropropagation,
2. Production of disease-free plants,
3. Androgenic haploids and their use in breeding,
4. Embryo rescue for successful hybridization,
5. Induction and selection of mutants,
6. Somaclonal variations,
7. Protoplast technology.

→ Culture of explant on an agar medium containing 2, 4-D results in the formation of callus. Suspension cultures are obtained by culturing pieces of callus in a liquid medium which is constantly agitated so that the larger masses are dissociated into smaller clumps and single cells.

→ The callus and suspension cultures are commonly used for achieving cell biomass production. Single cells can be isolated either from suspension culture or directly from explants by mechanical or enzymatic methods. They are cultured in liquid or semi-solid media by using filter paper raft-nurse tissue technique or by Bergmann’s plating technique. Plantlets can be regenerated from callus or suspension cultures either by shoot regeneration or by somatic embryo regeneration technique.

→ The regenerated plantlets can be transferred and established in the field. The term ‘micropropagation’ or ‘cloning’ is applied to a novel technique devised to produce vast quantities of strong and healthy plantlets by rapid vegetative multiplication under controlled conditions.

Haploid plants can be produced by another culture or by culturing microspores. These haploids can be converted to homozygous diploids by doubling their chromosomes using certain chemicals.

→ Young embryos can be cultured on artificial culture media containing specific nutrients. This technique is helpful in raising the plants from interspecific crosses as well as for ‘embryo rescue’. The multiple shootlet production techniques are useful in getting disease-free healthy plants.

→ Genetic vacations occur at all levels of the tissue culture process. They are termed somaclonal variations. These variations are useful for the improvement of crops if they are heritable and have agronomic traits.

→ Naked protoplasts of plant cells can be isolated by digesting their cell walls using certain hydrolytic enzymes. The isolated protoplasts can be cultured in a liquid or semi-solid medium. Naked protoplasts of two different plants can be allowed to fuse to form hybrids. This is called protoplast fusion and somatic hybridization.

The plant tissue culture technique is also used for the production of artificial seeds in those plants which either do not bear seeds or produce a small number of seeds.

→ Artificial seeds are used for direct field delivery of select elite genotypes, hand-pollinated hybrids, genetically engineered plants, sterile and fertile genotypes, etc.

→ Cultivation of plant cells in culture media promises great potential in large-scale production of secondary metabolites which are otherwise produced in minute amounts and their extraction from the plants is difficult and expensive.

→ Allotetraploid: AllotetraploidS are produced by the multiplication of chromosomç sets that are initially derived from two different species.

→ Apiculture: Rearing and management of beehives for obtaining honey and wax.

→ Aquaculture: Production of useful plants and animals.

→ Concentrates: They include cotton seeds, oil cakes, cereal grains, etc.

→ ElectrapOration: Genetic transformation by applying high electric potential for a few microseconds to change the porosity of protoplast to take up DNA.

→ EmasculatiOn: Removal of stamens from bisexual flowers before they burst and shed their pollen.

→ Hybrid vigor: The increased vigor often exhibited by hybrid progeny.

→ Livestock: The animals like cattle, sheep goat, camel, horse, pig, and poultry form livestock.

→ Pisciculture: The rearing of fishes for obtaining meat and oil.

→ Poultry: Rearing of fowls, ducks, turkeys for their eggs and meat.

→ Roughage: It includes straw of cereals such as wheat, rice, Jowar, and oat, etc.

→ Teratoma The partially organized tumors are called a teratoma.

By going through these CBSE Class 12 Biology Notes Chapter 8 Human Health and Disease, students can recall all the concepts quickly.

Human Health and Disease Notes Class 12 Biology Chapter 8

→ Health is a state of complete physical, mental and social well-being. It is not just the absence of diseases. Health may be defined as a state of the body when all the organs and systems are functioning properly and a perfect balance is maintained between the environment and the body.

→ The disease may be identified as any condition which impairs health or interferes with the normal functioning of the body due to one or another reason. It is any functional change from the normal state which causes discomfort or disability.

→ Any factor or substance which causes disease by its excess or deficiency or absence is called a disease agent.

→ The disease agents may be biological (e.g. virus, bacteria, fungi, protozoans, helminths, and anthropods) they are called pathogens; or Nutrient agents, e.g. food components viz. proteins, fats, carbohydrates, minerals, vitamins, and water; or chemical agents, may be exogenous or endogenous ones. Exogenous agents enter the body from outside e.g.-pollutants such as fumes, dust, metals, gases, etc., or allergens such as pollen and spores.

Endogenous chemical agents are formed inside the body, e.g. hormones, enzymes, uric acid, etc.; or Physical agents such as humidity, pressure, radiation, heat, cold, electricity, and sound; or mechanical agents such as chronic friction or other mechanical forces which cause a sprain, fracture or dislocation, etc; err genetic agents which cause genetic disorders or underdeveloped organs.

→ Diseases are broadly classified into two types as

1. Congenital disease and
2. Acquired diseases.

→ Congenital disease is anatomical or physiological abnormalities that are present at the time of birth. These may be single gene mutation, e.g. phenylketonuria, albinism, sickle-cell anemia, color blindness, hemophilia, etc., or chromosomal aberrations e.g. Down’s syndrome, Klinefelter’s syndrome, or Turner’s syndrome. These are further transmitted to the children.

→ Harelip and cleft palate are some environment-related congenital diseases that are not transferred to children.

→ Acquired diseases: These develop after birth. They may be communicable or non-communicable diseases.
Classification:

→ Acquired diseases are further of two types – Communicable and non-communicable. Communicable diseases are infectious which are caused by pathogens. Non-communicable diseases are non-infectious ones that remain confined to the diseased person only.
These are of four types:

1. Organic diseases occur due to the malfunctioning of important organs such as heart diseases, epilepsy.
2. Deficiency diseases are produced by a deficiency of minerals, nutrients, vitamins, and hormones such as beriberi, diabetes, goiter, etc.
3. Allergies are caused when the body becomes hypersensitive to some foreign substance e.g. Hay fever,
4. Cancer is caused by the uncontrolled growth of certain tissues in the body.

→ Communicable diseases are classified into seven types:

1. Viral diseases e.g. smallpox, chickenpox, polio, common cold, etc.
2. Rickettsial diseases are caused by obligate intracellular parasitic organisms known as rickettsias, e.g. typhus fever, Q fever, Rocky Mountain spotted fever, etc.
3. Bacterial diseases e.g. diphtheria, tetanus, food poisoning, etc.
4. Spirochaetal diseases are caused by long, spiral, corkscrew-shaped bacteria called spirochaetes e.g. syphilis,
5. Protozoan diseases are caused by protists, e.g. amoebic dysentery, malaria, kala-azar, sleeping sickness, etc.
6. Fungal diseases are caused by non-green heterotrophic organisms, fungi e.g. ringworm and athlete’s foot,
7. Helminth diseases are caused by flatworms and ringworms e.g. liver rot, taeniasis, etc.

→ Typhoid is caused by gram-negative bacteria, Salmonella those commonly found in the human intestine. It is a communicable disease, spread through the intestinal discharge of typhoid carriers. Its incubation period is 1 – 3 weeks.

Its symptoms include headache, typical typhoid fever which increases in the afternoon and enhances every day, in a second week its high and decreases during the third and fourth week, lesions of the intestinal mucosa, hemorrhage, ulcer of the intestine, red rashes on chest and upper abdomen. It is confirmed by S.typhi bacterium detection in stools or blood and also by the Widal test.

As typhoid spreads through contaminated food and water, precautions should be taken for screening of water and food sources, proper sanitation of human feces.
Its therapy includes

1. Thyphoral is an oral vaccine,
2. TAB – this vaccine provides immunity for 3 years,
3. Chloramphenicol, ampicillin, and chloromycetin are effective drugs against typhoid bacterium.

→ Pneumonia is a lung infection caused by bacteria like Streptococcus pneumonia and Haemophilus influenza or mycoplasma or viruses. This is an airborne disease spread through cysts present in the patient’s sputum. Its incubation period is 1 – 3 days. Pneumonia is characterized by lymph collection and mucus in the bronchioles and alveoli, thus decreasing the respiratory efficiency of the lungs. It includes fever, chills, cough, and headache. In severe cases, the fingernails and lips may turn grey to bluish.

The bacteria present in the body secretion of the patient are released out as aerosols or droplets which are inhaled by healthy person and cause infection. It also spread by glasses and utensils sharing with the patient so the best precaution is the isolation of the infected person. It is treated, with penicillin, flucloxacillin, etc. antibiotics.

→ The common cold is caused by a large number of viruses that belong to a group ‘rhinoviruses. It is the most infectious human disease. These viruses infect the nose and respiratory passage but not the lungs of the patient. Its symptoms are nasal congestion, discharge, sore throat, cough, headache, tiredness, etc. It lasts for 3 to 7 days. The infection spreads through droplets from sneezes or cough, through contaminated objects like books, cups, doorknobs, computer keyboard pens, mice, etc.

Salmonella typhosa

Diplococcus pneumoniae

→ Amoebic dysentery or Enteritis or Amoebiasis is caused, by an intestinal endoparasitic protozoan, Entamoeba histolytic which inhabits the large intestine. It may also spread to the lower part of the small intestine, liver, lungs, and brain. E. histolytic occurs in two forms (diagonal) minute form, its an absorptive stage and undergoes encystment and magma form, its large-sized, destroys epithelial cells and holozoic stage. Its main sources of infection are drinking water and food, contaminated by the fecal matter of infected person. Houseflies act as carriers and transfer the parasite from feces to food products.

Stool detection of the patient shows the presence of E. histolytic. The symptoms include constipation, abdominal pain, and cramps, stools with excess mucous, and blot clots. The protozoan secretes leading to ulcers.

Its precautions include

1. proper sanitary conditions,
2. coverage of tables to prevent contamination by vectors,
3. proper washing of fruits and vegetables before eating. Its treatment is done by antibiotics like Terramycin, erythromycin, aureomycin, and metronidazole.

→ Malaria is a vector-borne disease which is caused by the protozoan parasite Plasmodium. It is caused by toxic pigment hemozoin from RBC’s Hb. This is produced when RBC are destroyed by developing stages of the Plasmodium. Plasmodium is digenetic (two hosts) and triphasic. The primary host is the female Anopheles mosquito and the secondary host is human beings.

Three phases include asexual schizogony (in liver and RBCs of man), sexual cosmogony (starts in RBCs and completes in mosquito’s stomach), asexual sporogony (stomach of mosquito). Malaria is of several types which are caused by different species of Plasmodium. Malaria caused by Plasmodium falciparum is most dangerous, it is called malignant malaria and it can be fatal.

Plasmodium enters the human body through the Anopheles mosquito. The parasite multiplies within the RBCs and ruptures the cells. Symptoms include headache, chill, shivering, nausea, muscular pain, recurring fever every 3rd or 4th day, increase in pulse and respiration rate, sweating, and anemia.

Its symptoms are divided into three stages:

1. cold state,
2. hot stage and
3. sweating stage.

The mosquitoes acquire the parasite from infected persons and transmit it to healthy individuals.

Its precautions include

1. wire gauzing of doors and windows etc.,
2. using mosquito nets,
3. use of mosquito- repellents.
4. spraying insecticides like D.D.T. and B.H.C.,
5. do not allow any stagnant water. Its treatment is done by quinine, chloroquine, palustrine, Daraprim, etc., anti-malarial drugs.

→ Filariasis is caused by the filarial worm, Wuchereria ban crafti and Wuchereria malayi. The worm is digenetic, its primary host is man, and the secondary or vector host is mosquitoes Aedes and Culex. It causes slowly chronic inflammation of organs for many years.

They are found in lymph vessels and lymph nodes and due to infection the lymphatic vessels of lower limbs become enlarged due to blockage and chronic inflammation. The disease is called elephantiasis or filariasis. Genital organs are also affected. This causes enormous swelling of legs, scrotal sacs, breasts, etc.

The pathogens are transmitted through the bite of female mosquito vectors. Its precautions are the same as for malaria. Its treatment includes Hetrazan and diethylcarbamazine.

→ Ascariasis is caused by intestinal endoparasite of man, Ascaris lumbricoid known as roundworm. It is a monogenetic helminth that spreads through water, vegetables, fruits that are contaminated with the eggs of the parasite. Its symptoms include abdominal pain, indigestion, weakness, anemia, nausea, vomiting, diarrhea, internal bleeding, muscular pain, fever, etc.

Heavy infection may cause pneumonia, hepatitis, blockage of the intestine, convulsions, etc. Flies and cockroaches serve as vectors for spreading the diseases. It is diagnosed by doing stool test, Dermol test or Scratch test, its precautions are same as for Amoebic dysentery. Its treatment is done by Hexyl-resorcinol crystals, a mixture of oil of chenopodium and tetrachloroethylene.

→ Ringworms are a common infectious disease caused by fungi Microsporum, Trichophyton, Candida, and Epidermophyton. Their symptoms include dry and scaly lesions on skin, nails, and scalp. These are accompanied by intense itching.

Moisture and warmth help in fungal growth. It’s most likely to grow in skin folds like the groin and between the toes. These generally spread from soil or sharing towels, clothes, or comb with an infected person. These are treated with antifungal ointments.

→ Vaccination is a common method of preventing the infection of microorganisms like bacteria and viruses. Infectious diseases like polio, diphtheria, pneumonia, and tetanus are controlled to great extent by vaccination. Smallpox is completely eradicated by vaccines and immunization programs. Various antibiotics are also very effective against infectious diseases.

→ Disease resistance or immunity is the ability of an organism to resist disease. Immunology is the branch of science which deals with immunity. The immune system forms the third line of defense. The immune system can differentiate between self (body cells) and non-self (foreign microbes).

→ Any foreign substance entering the body stimulates an immune response it is called an antigen. The protective chemicals (proteins) produced by the body in response to antigens are called antibodies.

→ Immunity is of two main types:

1. Innate immunity and
2. Acquired immunity.

1. Innate immunity or inborn or natural immunity is non-specific and present at the time of birth. It is inherited from the mother through the placenta. It provides four different types of barriers to the foreign agents as (a) physical barriers: Skin is the foremost barrier that prevents the entry of microorganisms, similarly, the mucus coating of the epithelial layer lining the respiratory, gastrointestinal, and urogenital tracts also defend the entry of microorganisms,

2. Physiological barriers such as gastric acids, saliva in the mouth, tears, all prevent microbial growth.

3. Cellular barriers, some leukocytes (WBC) like polymorphonuclear leukocytes (PMNL-neutrophils) and monocytes and some lymphocytes (e.g. natural killer) from blood and macrophages from tissue can phagocytose and destroy the invading microbes,

4. Cytokine barriers, virus-infected cells produce anti-viral proteinous molecules called interferons, which protect the unattacked cells from viral attack.

→ Acquired or adaptive immunity is not present from birth but is acquired or developed by the organisms in response to a disease. It is pathogen-specific and is characterized by memory. When the body encounters any antigen (pathogen) the lymphocytes produce antibodies which are termed as a primary response, it’s of less intensity.

Lymphocytes also provide immunity against further attack, a subsequent encounter with the same pathogen produces a highly intensified secondary or anamnestic response. The acquired immunity may be temporary (influenza) or permanent (measles, mumps, polio, smallpox, etc.) for life long.

→ Acquired immunity is of two types:

1. Active or natural immunity is developed by antibodies from the individual’s body,
2. Passive or artificial immunity: In it, readymade antibodies are obtained from human or animal serum, who had recovered from the same infection and injected into the patient. It is known as inoculation
   e.g. tetanus treatment, snake bite, etc.

→ The primary and secondary immune responses are carried out with lymphocytes. Lymphocytes are a kind of agranulocytes of leucocytes (WBC) of blood. Lymphocytes are of two types: B-lymphocytes and T-lymphocytes. Both types of lymphocytes are formed from the stem cells called haemocytoblasts in the bone marrow and undergo further processing.

Showing formation of lymphocytes

→ B-lymphocytes produce antibodies when stimulated by an antigen. After stimulation one B-lymphocyte synthesize RNA, divide rapidly and differentiate into the RER-rich histologically distinctive plasma cells. A group of plasma cells is called a clone. They produce antibodies at a rate of about 2000 molecules per second. These antibodies circulate in the lymph to fight antigens. They form the humoral immune system or Antibody-Mediated Immune System (AMIS).

→ An antibody molecule is made up of four peptide chains, two small peptide chains called light chains, and two long heavy chains. An antibody may be designated as H₂L. There are five classes of antibodies (immunoglobulins) namely IgM, IgG, IgA, IgD, and IgE. IgG is not abundant in man whereas IgM and IgG cause the lysis of foreign cells.

→ The second type of acquired immune response is Cell-Mediated Immune System (CMIS). It is mediated by T-lymphocytes. T-cells do not secrete antibodies but help B-cells to produce them. T-lymphocytes attack directly the pathogenic microorganisms entering the body or grafts such as transplanted kidney or skin grafts. It also protects the body from its own cells which have become cancerous.

→ Vaccination or immunization is based on the property of memory of the immune system. A preparation of antigenic proteins of pathogen or weakened or inactive pathogen is given. The body produces antibodies against these antigens and neutralizes the pathogenic agent during actual infection. The vaccines also produce memory B and T cells which recognize the pathogen or subsequent exposure and counteract with massive production of antibodies. Vaccines are also produced by recombinant DN A technology where bacteria or yeast produce antigenic polypeptides of pathogens, e.g. hepatitis B vaccine.

→ Allergies are an exaggerated immune response to certain antigens called allergens e.g. dust mites, pollens, animal dung, fur, venom, etc. Allergy symptoms are inflammation of mucous membranes, sneezing, watery eyes, running nose, irritation of upper respiratory tract, rashes, itching, etc. In allergic people, the body develops a primary immune response on encounter with the allergen. B-cells produce antibody IgE. On the next exposure, the body produces a secondary immune response called allergy.

→ The allergens combine with antibody-bound mast cells and basophils. This complexing ruptures the mast cells and basophils, releasing histamine from vesicles called granules. The process of the release of histamine is termed degranulation. The histamine acts as an allergy mediator. It increases the permeability of capillaries constricts smooth muscles in many parts such as those around the bronchioles, causing breathing problems; and stimulates mucous glands. It also widens the small arteries but not the veins.

Thus, the affected area (where histamine has been released receives more blood than it returns. As a result of this, fluid accumulates in the tissue which swells up. This causes allergy.

Showing allergic reaction and release of histamines by rupturing of mast cells

→ The immune system sometimes rejects certain tissues of the body as non-self. Due to genetic and other unknown reasons the body attacks self-cells. This is called autoimmunity. The body tissues act as autoantigens and autoantibodies, e.g. Hashimoto’s thyroiditis, systemic lupus erythematosus, rheumatoid arthritis.

→ The immune system plays a role in allergic reactions, autoimmune diseases, and organ transplantation. The immune system consists of lymphoid organs, tissues, cells, and soluble molecules like antibodies. Lymphoid organs help in the origin or maturation and proliferation of lymphocytes.

The primary lymphoid organs are bone marrow and thymus, here immature lymphocytes differentiate into antigen-specific lymphocytes. After maturation lymphocytes migrate to secondary lymphoid organs which are the spleen, lymph nodes, tonsils, appendix, and Peyer’s patches of the small intestine. Here the lymphocytes interact with antigens and then proliferate to become effector cells.

→ Bone marrow provides a suitable environment for the development and maturation of B-lymphocytes. Bone marrow is the main lymphatic organ where all blood cells, as well as lymphocytes, are produced.

→ The thymus is a lymphatic organ, located near the heart beneath the chest bone. It is prominent in children but keeps on reducing with age, by puberty it becomes very small in size. Development and maturation of T-lymphocytes take place in the thymus. It educates the fetal lymphocytes to distinguish between self and non-self.

→ The spleen is the largest component of the lymphatic system. It is bean-shaped, vascular, dark red in color, located in the abdomen below the diaphragm. It consists of red pulp and white pulp. It helps in the destruction of worn-out RBCs, as a reservoir of RBCs, production of antibodies, disposal of foreign elements.

→ Lymph nodes are present at intervals in the course of lymphatic vessels. They contain lymphocytes, plasma cells, and macrophages. They trap the microorganisms and antigens and stimulating the immune response.

→ Lymphatic tissue present in the lining of respiratory, digestive, and urogenital tracts is known as mucosal-associated lymphoid tissue (MALT). Lymphatic tissue present in the gut is called get-associated lymphoid tissue (GALT).

AIDS is an acquired immunodeficiency disease. A detailed account of the retrovirus and the disease has been discussed in detail in previous chapters. Its control and prevention were also discussed.

The figure shows the aids virus.

Diagram of AIDS virus

Cancer is a malignant growth or tissue enlargement due to unlimited and uncontrolled mitotic division of some cells known as cancerous cells.

Showing development stages of cancer due to smoking in lung epithelium

Cancerous cells grow rapidly and form a mass of cells called tumors. Tumors are of two types: Benign or non-malignant tumors these remain confined to one place and cause little damage, cancerous or malignant tumors – these are a mass of proliferating cells known as neoplastic cells. They grew rapidly, damage the nearby normal tissue. Cancerous cells divide actively and compete for nutrients with the normal tissue. These cells reach another place through blood and form new tumors. This is called metastasis.

→ Cancer is neither contagious nor hereditary. Factors that cause cancer are called carcinogenic agents. These may be ionizing radiations, e.g. x-rays and gamma rays or non-ionizing rays like U.V rays, chemical carcinogens like beetle and tobacco, aniline dyes, heavy smoking, viral oncogenes, and cellular oncogenes. Cellular oncogenes (c-onc) or protooncogenes are present in normal cells which get activated under certain conditions and become oncogenes.

→ Detection of cancer is done by biopsy and histopathological studies of tissues, blood, and bone marrow for increased cell count (leukemia). X-rays, CT scans, and MRIs are used to detect cancer of internal organs. Mammography is done to detect breast cancer. Pap test is done for cancer detection in the cervix and other genital parts. Monoclonal antibodies against the cancer-specific antigen are coupled to the suitable radioisotope, these antibodies are used further for cancer detection.

→ Cancer is treated with surgery, radiation therapy and immunotherapy, and chemotherapy. Biological response modifiers such as γ – interferons are used to activate the immune response to destroy the tumor.

→ Drugs are normally used for the treatment of diseases. An addictive drug modifies the biological, psychological, or social behavior of an individual by stimulating, depressing, or distorting mind and body functions. This habit of taking drugs beyond voluntary control is called an Addition. Commonly abused drugs include opioids, cannabinoids, and coca alkaloids.

TABLE: SHOWING MAJOR GROUPS OF PSYCHOTROPIC DRUGS WITH EXAMPLES AND EFFECTS

Opioids bind to opioid receptors in the central nervous system and gastrointestinal tract. These suppress brain activity and relieve pain. These are called pain killers e.g. opium and its derivatives.

(a) Somniferum (Opium poppy) plant (b) Chemical structure of Morphine

Heroin or smack is diacetylmorphine. It is a white, odorless, bitter crystalline compound. It is formed by the acetylation of morphine, which is extracted from poppy latex. It is a depressant and slows down body functions.

→ The products of the Hemp plant Cannabis Sativa are Bhang, Ganja, Charas, Hashish, and Marijuana. These are less harmful but may lead to opiates addiction. Their active compound is delta-9- tetrahydrocannabinol or THC.

(a) Leaf of Cannabis Sativa
(b) Skeletal structure of cannabinoid molecule

Cocaine or coke is an alkaloid from Erythroxylum coca. It has a potent stimulating effect on the central nervous system, produces a sense of euphoria and increased energy.

→ Datura spp. and Atropabeliadona also have hallucinogenic properties. Cannabinoids are misused by some sportspersons.

→ Other abused drugs are barbiturates, amphetamines, benzodiazepines, and lysergic acid diethyl amides (LSD).

→ The adolescence period is very vulnerable to the mental and psychological development of an individual. Curiosity, adventure and excitement, and experimentation motivate youngsters towards drug and alcohol misuse.

TABLE: SHOWING EFFECTS OF ALCOHOL AND DRUGS

→ Smoking paves the way to hard drugs. Tobacco is derived from Nicotiana tabacum and N, Rustica leaves. Its main constituent is the toxin alkaloid Nicotine. It stimulates the release of adrenaline and nor-adrenaline which cause high blood pressure and increased heartbeat. It is associated with lung cancer, bronchitis, emphysema, coronary heart disease, urinary bladder cancer, throat cancer, gastric ulcer, and oral cavity cancer.

→ Alcohol is obtained from the fermentation of cereals or grains by yeast Saccharomyces cerevisiae and further distillation. It has depressant effects on the central nervous system.

→ The symptoms of addiction include undue excitement, irritable and violent nature, exhausted and drowsy appearance, loss of interest in studies, poor concentration and memory, reduced appetite, vigor, and weight.

→ Parents should keep a watch on their children and seek medical and professional help if needed.

→ Allografting: A graft taken from another person. e.g. skin.

→ Antibody: The proteinous molecules produced to counter the specific antigen.

→ Antigen: A foreign agent which stimulates the immune system to produce antibodies.

→ ARC: AIDS-related complex.

→ Autoimmunity: An immune disorder when the immune system of a person rejects its own body cells.

→ Addiction: Dependency on certain drugs.

→ Anesthetic: Chemical which causes the loss of sensation, e.g. opiates.

→ Benign tumor: Cäncer type which remains localized in the organ affected.

→ Communicable disease: The disease which can be transmitted from an infected person to a healthy person.

→ Cancer: Disease characterized by uncontrolled growth and division leading to tumor formation.

→ Carcinogens: Cancer-causing agents e.g. radiations.

→ Carcinomas: Cancers located in the epithelial tissues. e.g. breast cancer.

→ Congenital diseases: Inborn diseases caused by gene or chromosomal mutations, e.g. hemophilia.

→ Cirrhosis: A liver disease caused by the storage of fats.

→ Co-poisoning: CO of tobacco smoke binds lib of RBCs and decreases their Oxygen carrying capacity.

→ Depressant: Chemical which lowers the activity of CNS, e.g. alcohol.

→ Disease: A condition impairing health.

→ Epidemiology: Mode of transmission of disease.

→ Epidemic: A disease that attacks a large number of people at the same time and in the same region e.g. cholera.

→ Endemic: A disease restricted to a certain region, e.g. sleeping sickness.

→ Gastritis: Inflammation of gastric mucosa.

→ hallucinogens: Chemicals which change thoughts, feelings, and perceptions of a person e.g. LSD.

→ Health: A state of complete physical, mental and social norms.

→ Hypertension: High blood pressure.

→ Immunodeficiency: An immune disorder characterized by a deficiency of B-cells and T-cells in the body.

→ Immunoglobulin: Antibodies (γ-globulins) produced against the antigen.

→ Immunity: Resistance of body towards diseases.

→ Immunosuppression: Immune disorder in which the immune system rejects transplanted organs.

→ Infection: Entry and multiplication of living pathogenic microorganisms in some part of the host’s body.

→ Infestation: Presence of a large number of parasitic organisms on the surface of the host’s body or clothing.

→ Incubation period: Period between infection and appearance of the first symptom of a disease.

→ Leukemia: Blood cancer, increased WBC count of the blood.

→ Metastasis: Spread of cancer.

→ Lymphocytes: Type of WBCs which produce antibodies.

→ Lysozyme: A bacterial protein.

→ Neoplasm: A tumor formed by uncontrolled growth of the tissue in cancer.

→ Neuritis: Inflammation of nervous tissue of the brain.

→ Oncogenes: Cancer-causing genes.

→ Prophylaxis: Prevention of occurrence of a disease.

→ Psychotropic drug: mood-altering drug.

→ Pyrexia: Increase in temperature.

→ Pathogen: An organism causing disease.

→ Pyrogen: A fever-causing substance.

→ Serology: Study f antigen-antibody interactions.

→ Sedative: A chemical that decreases functional efficiency and has a calming effect, e.g. alcohol.

→ Therapy: Mode of treatment of a disease.

→ Tranquilizer: A drug having a calming effect without inducing sleep e.g. valium.

→ Vaccine: An inoculation with dead or weakened or virulent germs, which stimulates the formation of antibodies of the host’s body.

→ Vector: Organism which spreads the disease.

By going through these CBSE Class 12 Biology Notes Chapter 7 Evolution, students can recall all the concepts quickly.

Evolution Notes Class 12 Biology Chapter 7

→ The study of the history of life forms on earth is called Evolutionary Biology.

→ The origin of life on earth can be understood only against the background of the origin of the universe especially earth.

→ The universe is vast. The earth is almost a speck. The universe is about 20 billion years old. The universe comprises huge clusters of galaxies.

→ The Big Bang theory explains to us the origin of the universe. It states that a singular huge explosion laid the foundation of the universe.

→ Most scientists believe that the formation of biomolecules preceded the appearance of the first cellular forms of life.

→ Conventional religious literature tells us that the theory of creation has connotations.

• One, that all living organisms that we see today, were created as such.
• Two, the diversity was always the same since creation and will be the same in the future also.
• Three, that the earth is about 4000 years old.

→ Charles Darwin concluded that existing living forms share similarities to varying degrees not only among themselves but also with life forms that existed millions of years ago.

→ According to Darwin, fitness refers ultimately and only to reproduce fitness. Thus, those who are a better fit in an environment, leave more progeny than others. There therefore will survive more and hence are selected by nature. He called it natural selection and implied it as a mechanism of evolution.

→ Comparative anatomy and morphology show similarities and differences between organisms of today and those that existed millions of years ago. Such similarities can be interpreted to understand common ancestors were shared or not.

→ The diversity of life forms on earth has been changing over millions of years.

→ It is believed that variations in a population either due to meiotic recombination during gametogenesis or germ live mutations result in variable fitness in learning fertile progeny.

→ Branching descent and natural selection are the two key concepts o.f the Darwinian Theory of Evolution.

→ Hardy-Weinberg Principle states that allele frequencies in a population are stable and constant from generation to generation. The gene pool remains a constant.

→ Homology is accounted for by the idea of branching descent; study of comparative anatomy, fossils comparative, biochemistry provide evidence for evolution.

→ The story of the evolution of modern man among the stories of the evolution of individual species is most interesting. It appears to parallel the evolution of the human brain and language.

→ Evolutionary Biology: Study of history of life forms on earth.

→ Big Bang Theory: Theory that explains the origin of the universe.

→ Fossils: Remains of hard parts of life forms found in rocks.

→ Divergent evolution: In animals, the same structure developed in different directions due to adaptations to different needs.

→ Homology: It indicates common ancestry.

→ Adaptive Radiation: The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography.

By going through these CBSE Class 12 Biology Notes Chapter 6 Molecular Basis of Inheritance, students can recall all the concepts quickly.

Molecular Basis of Inheritance Notes Class 12 Biology Chapter 6

→ Gene is a macromolecule attached to an undifferentiated protein thread (chromonema) that can pass from one cell to another and can be transmitted from one generation to another without being changed at all.

→ Gene is a unit of heredity and responsible for inheritance.

→ Gene is a part of DNA and is made up of nucleotides. It can grow, reproduce and mutate. Gene is the unit of recombination. Genetic information is conveyed from DNA to mRNA.

→ Genes mainly act by producing proteins or enzymes which are required at various levels of metabolism. Proteins are made up of polypeptides which are formed from amino acids. Genes determine the physical and physiological characteristics of living beings.

→ DNA and RNA are two types of nucleic acids, which are polymers of nucleotides. DNA is the genetic material in most organisms. RNA is found only in some viruses as genetic material, it acts as a messenger to transfer the genetic information from DNA to proteins. It also functions as an adapter, structural and catalytic molecule.

→ DNA (deoxyribonucleic acid) is the polymer of deoxyribonucleotides. The haploid content of human DNA is 3.3 × 10⁹ bp. DNA is chemically and structurally more stable than RNA.

→ Nucleic acids were first isolated by Friedrich Miescher (1869) from pus cells. These were named nuclein. Due to their acidic nature, they were named nucleic acid, and Altmann (1899) named them nucleic acids. Fisher (1880s) discovered the presence of purine and pyrimidine bases. Levene (1910) found deoxyribose nucleic acid contains phosphoric acid and deoxyribose sugar. He characterized four types of nucleotides.

Chargaff (1950) found that purines are equal to pyrimidines in DNA, and adenine = thymine, and guanine = cytosine. W.T. Astbury discovered through X-ray diffraction that DNA is a polynucleotide with nucleotides arranged perpendicular to the long axis of the molecule and are 0.34nm away from one another.

Wilkins and Franklin (1953) through X-ray photographs found that DNA was a helix with 2.0nm width. One turn of the helix was 3.4 nm with 10 layers of bases stacked in it. Through these photographs, Watson and Crick (1 953) built a three-dimensional model of DNA. They were awarded Nobel Prize in 1962. They purposed that DNA consists of a double helix with two chains having sugar-phosphate as the backbone and nitrogen bases on the inner sides.

The nitrogen bases of two chains form complementary pairs with purines of one and pyrimidine of the other held together by hydrogen bonds. The two chains are antiparallel with 5′ → 3′ orientation and 3′ → 5′ orientation. The two chains are twisted helically as a rope ladder with rigid steps twisted into a spiral. Each turn contains 10 nucleotides. In Vitro, DNA synthesis was carried out by Komberg (l 959). DNA has got an implicit mechanism for replication and copying.

→ A nucleotide is made up of three components: a nitrogenous base, a pentose sugar (ribose in RNA, deoxyribose in DNA), and a phosphate group. Nitrogenous bases are of two types: purines and pyrimidines. Purines are Adenine and Granite and pyrimidines are Cytosine, Uracil, and Thymine. Thymine is present in DNA. Thymine is replaced by Uracil in RNA.

→ A nucleoside is formed by a nitrogenous base linked to pentose sugar by an N-glycosidic bond, e.g. deoxyadenosine or adenosine, deoxycytidine, or cytidine.

Structure of DNA.

A. Chemical Structure and bonding of different constituents of DNA in the two chains.
B. sequence of nucleotides in a part of the double helix of DNA. C. coiling in double helix or duplex of DNA.

→ Two nucleotides are bound to each other through a phosphodiester bond. The phosphate group provides acidity to the nucleic acids. In Bacteria nucleotide, the DNA is covalently closed at its two ends. It’s called circular DNA. Its also present in mitochondria, plastids, and some viruses. Eukaryotic cells have free linear DNA. Both types of DNA are coiled and supercoiled to get into small space.

→ DNA replication is autocatalytic, it occurs during the S-phase of the cell cycle. Watson and Crick proposed a semiconservative model of replication for DNA. During which one strand of the daughter strand is derived from the parental duplex and the other strand is formed a new. Taylor et al (1957) worked with Broad Bean root tips, using

The experiment of Mcselson and Stahi (i958) to prove semi-conservative replication of DNA

Semi-conservative replication of DNA showing Zipper duplication

radiolabelled thymine and showed DNA replication is semi-conservative. In 1958 Nesselson and Stahl proved the semi-conservative mode of replication. They worked with E.Coli, using a heavy isotope of nitrogen, N¹⁵. E.Coli was grown with N¹⁵ for several generations till the bacteria became completely labeled with N¹⁵. They were then shifted to medium with normal N¹⁴ nitrogen.

The sample was taken after every generation and tested for the presence of N¹⁵ and N¹⁴ using density gradient centrifugation with CsCl. The first generation was found to be a hybrid between N1¹⁵ and N¹⁴. The second-generation contained two types of DNA, 50% light (N¹⁴) and 50% intermediate. This is only possible if two strands separate during replication and act as a template for the synthesis of new complementary strands. Thus proving a semiconservative mode of replication.

→ The process of replication starts at a particular spot known as the origin of replication. The deoxyribonucleotides are first phosphorylated and activated. Energy and phosphorylase enzymes are required for activation. Enzymes’ topoisomerases are specialized to break and reseal the DNA strands. Enzyme helicases unwind the DNA helix and separate the two strands. DNA binding proteins bind on the separated strands. The whole of DNA does not open up for replication, the point of separation proceeds from one end to another. The replication fork appears Y- Y-shaped during the process of replication.


Replication of DNA continuous over one strand and discontinuous over other strands

DNA polymerase enzyme requires an RNA primer to initiate the process/ RAN primer is synthesized at the 5′ end of new DNA strand by enzyme primase. The two separated strands act as templates. The new nucleotides are added according to the law of base pairing i.e. A pairs with T and G pairs with C.

Energy is utilized in forming hydrogen bonds between the free nucleotides and nitrogen bases of templates. Elongation of the DNA chain requires enzyme DNA polymerase III in the presence of Mg²⁺ and ATP. The adjacent nucleotides attach to one another by phosphodiester bonds. As replication proceeds, the strands unwind and at the end, the RNA primer is removed and the gap is filled with the help of DNA polymerase 1. This is also called Zipper duplication.

DNA polymerase can act only in the 5′ → 3′ direction so replication is continuous on the strand (3′ → 5′) known as the leading strand. The other strand called lagging strand 1 (5′ → 3′) replicates itself in small stretches called Okazaki fragments. These fragments are joined by the DNA ligase enzyme. Any mismatch or mutation if occurs during replication can be corrected by proofreading and DNA repair mechanisms

→ Frederick Griffith (1928) by doing transforming experiments on mice with Streptococcus pneumonia found out the transfer of genetic material from a heat-killed infectious stain to the normal non-infectious stain.

→ Earlier the genetic material was thought to be protein. In 1933 – 44 Oswald Avery, Colin Macleod, and Maclyn McCarty discovered that the transforming biochemical in Griffith’s experiments was DNA.

→ Alfred Hershey and Martha Chase (1952) proved that DNA is the hereditary material. They worked with bacteriophage T, which infects E.Coli.

→ Francis Crick proposed the central dogma in molecular biology which explains the flow of genetic information
i.e. DNA → RNA → Protein.

→ The formation of RNA over a DNA template is called transcription. Transcription is meant for taking the coded information from DNA to the site where it is required for protein synthesis. Only one DNA strand transcribes RNA, it is called a sense strand. Transcription requires enzyme RNA polymerase. Prokaryotes have only one RNA polymerase. Eukaryotes have three RNA polymerases.

RNA polymerase 1 synthesizes rRNA, 28S, 18S, and 5.8S, RNA polymerase II synthesizes hnRNA, mRNA, and snRNAs, and RNA polymerase III synthesizes tRNA, 5SRNA, and siRNAs. A transcription unit consists of a promoter, the structural gene, and a terminator.

Schematic Structure of a Transcription Unit

→ In eukaryotes, the primary transcript contains exons and introns so it is non-functional. Exons are coding sequences whereas introns are intervening sequences. These introns are removed by splicing. It is followed by capping, methyl guanosine triphosphate addition to 5′ end and of hnRNA and tailing around 200 – 300 adenyl residues are added at 3′ end. The fully processed hnRNA is called mRNA and is transported out of the nucleus.

→ The relationship between the sequence of amino acids in a polypeptide and nucleotides sequence of DNA or mRNA is called genetic code.

→ George Gamow proposed that the genetic code is a triplet in nature. Marshall and Nirenberg’s method for cell-free protein synthesis helped to decipher the code. The chemical method developed by Har Gobind Khorana helped in synthesizing RNA molecules with defined base combinations. Severo Ochoa’s enzyme polynucleotide phosphorylase helped to polymerize RNA with defined sequences. All these methods helped in deciphering the genetic code.

→ The translation is the process of polymerization of amino acids to form a polypeptide. mRNA sequence decides the sequence of amino acids. Amino acids are joined through peptide bonds. Ribosomes are considered as the protein-synthesizing cellular factory. It requires amino acids, mRNA, tRNA, aminoacyl tRNA synthetase, and various enzymes along with ribosomes to complete the process of protein synthesis.

The process of translation occurs in the cytoplasm. The process takes place in several steps like activation of amino acids, initiation, elongation, and termination. Polyribosomes or polysomes help to produce a number of copies of the same polypeptide. In it, different ribosomes are held together by a strand of messenger RNA. These ribosomes usually form rosette or helical groups during active protein synthesis and are known as polyribosomes.

Ribosomes as a protein factory. A relationship between the various components. B. synthesis of the polypeptide on ribosome connected with the endoplasmic reticulum.

Cistron is a segment of DNA consisting of a stretch of base sequences that codes for one polypeptide chain, one transfer RNA (tRNA), ribosomal RNA tRNA) a molecule or performs any other specific function in connection with transcription, including controlling the functioning of other cistrons (operon model of gene action).

Regulation of gene expression may occur at various steps like

1. transcriptional level,
2. processing level (splicing),
3. transport of mRNA from the nucleus to the cytoplasm.
4. translational level.

An operon is a part of genetic material (or DNA) which acts a: a single regulated unit having one or more structural genes, an operator gene, a promoter gene, a regulator gene, a repressor, and an inducer or corepressor (from outside). Operons are of two types, inducible and repressive.

Lac operon is an example of an inducible operon system. In E.Col; the enzyme beta-galactosidase catalyzes the hydrolysis of lactose into glucose and galactose, which is used as a source of energy. If the bacteria do not have lactose in the medium they won’t need beta-galactosidase enzyme. Thus the production of an enzyme (protein) is regulated by the presence of lactose (inducer).

→ Anticodon: A triplet of bases present on tRN A complementing with mRNA codon is called the anticodon.

→ A-site: It’s the site where the second and next amino-acyl-tRNA enters the ribosome.

→ Codon: Triplet of bases on mRNA, which codes for one amino acid.

→ Central dogma: Unidirectional flow of information from DNA to RNA to protein.

→ Cistron: A segment of DNA that determines a single polypeptide chain.

→ DNA polymerase: The enzyme playing a pivotal role in adding the building blocks to the primer in a sequence as guided by the DNA template. It can polymerize nucleotides only in the 5’ → ‘3’ direction.

→ Frameshift mutation: The mutations which are caused by shifting the entire reading frame by addition or deletion on the segment of DNA.

→ Genetic code: The genetic presentation of codon through which the information in RNA is decoded in a polypeptide chain.

→ Glycosylation: The addition of sugar residues by modification of certain proteins, which are released in the lumen and are trapped in Golgi vesicles.

→ Hydrogen bond: The bond between nitrogenous bases of DNA binding two nucleotide chains. It is a weak bond.

→ Inducer: An effector molecule responsible for the induction of enzyme synthesis at recognition sites, to prevent self-cleavage by a modified enzyme that recognizes the sites and methylates specific nucleotides at each site.

→ Jumping genes: These are genes that shuffle from one location to another.

→ Phosphodiester bond: The bond between two adjacent nucleotides of two adjacent sugar moieties at 3’ and 5’ positions with phosphoric acid.

→ Phenocopy: When a normal gene under a different set of environmental conditions copies down the phenotypic characters of a mutant.

→ Restriction enzyme: An endonuclease that recognizes specific nucleotide sequences in DNA and makes a double-strand cleavage of DNA molecule.

→ Regulatory gene: Any gene which regulates or modifies the activity of other genes.

→ Repression: The phenomenon in which the synthesis of a set of enzymes leading to a product is shut down if the product is present in plentiful amounts.

→ Rho-factor: The factor which is required for termination of RNA synthesis at some sites.

→ Structural gene: A gene that codes for a polypeptide.

→ Silent mutation: This kind of mutation does not cause any change in the protein.

→ Thalassemia: Haemoglobin-based genetic disorder which involves frameshift mutation in β-chain of hemoglobin.

→ Transformation: The process in which the cell takes up the segment of the naked DNA from its surroundings and incorporates it in its hereditary material and ultimately expresses the character specified by incoming DNA.

→ Wobble position: The position on the codon where mutation occurs at 3rd base of triplet which still permits the normal interaction with anticodon.

By going through these CBSE Class 12 Biology Notes Chapter 5 Principles of Inheritance and Variation, students can recall all the concepts quickly.

Principles of Inheritance and Variation Notes Class 12 Biology Chapter 5

→ Genetics is a branch of biology which deals with principles of inheritance arid is practices. Progeny resembling the parents in morphological and physiological features has attracted the attention of many biologists.

→ During the mid-nineteenth century, Gregor Mendel conducted hybridization experiments on garden peas and proposed the laws of inheritance in living organisms.

→ Mendel proposed the principles of inheritance, which today are referred to as Mendel’s Law of inheritance. He proposed that the factors (genes) regulating the characters are found in pairs known as alleles. The expression of the characters in the offspring follows a definite pattern in first-generation (F₁), second-generation (F₂), and so on.

→ The dominant characters are expressed when the factors are in heterozygous conditions (Law of Dominance). The recessive characters are only expressed in homozygous conditions. The characters never blend in heterozygous conditions. A recessive character that was not expressed in heterozygous condition may be expressed again when becomes homozygous. Due to this reason, characters segregate while the formation of gametes (Law of Segregation).

→ Among all characters, some show incomplete dominance while some show co-dominance.

→ Mendel found that the factors independently assort and combine in all permutation and combination (Law of Independent Assortment.)

→ Different combinations of gametes are theoretically represented in a square tabular form known as ‘Punnett Square’.

→ The factors on chromosomes regulating the characters are called the genotype. The physical expression of characters is called the phenotype.

→ Later the Mendel’s Law was extended in the form of the ‘Chromosomal Theory of Inheritance’. However, later on, Mendel’s law of independent assortment does not hold true for the genes that were located on the same chromosomes. These genes were called ‘linked genes’.

→ Different genes were linked to sexes also. These are called sex-linked genes. Both males and females have a set of chromosomes. One set was common while the other set was different. The chromosomes that were different in the two sexes were named as sex chromosomes and the remaining set was named autosomes. In human beings, a normal male has 22 pairs of autosomes and a pair of sex chromosomes as XY. A female has 22 pairs of autosomes and a pair of sex chromosomes XX.

→ An analysis of traits in several generations of a family is called pedigree analysis. In the pedigree analysis, the inheritance of a particular trait is represented in the family tree over generations.

→ Genetic disorders may be grouped into two categories: Mendelian disorders and Chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in a single gene. The pattern of inheritance of Mendelian disorders can be traced in a family by the pedigree analysis. Some examples of Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle cell anemia, Colour blindness, Phenylketonuria, Thalassemia, etc.

Chromosomal disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes. Some common examples of chromosomal disorders are Down’s syndrome, Turner’s syndrome, Klinefelter’s syndrome, etc.

→ Hybridization: Mating between two (or more) individuals differing ¡n genotype.

→ Mutation: It is a sudden and heritable change in a character of an organism.

→ Heterozygous: Hybrids that contain alleles that express contrasting traits.

→ Punnett square: A graphical representation to calculate the probability of all possible genotypes öf offspring in a genetic cross.

→ Law of dominance: It explains the expression of only one of the parental characters in a monohybrid cross in the F₁ and the expression of both in the F₂.

→ Law of segregation: It ¡s based on the fact that the alleles do not show a blending and that both the characters are recovered as such in the F₂ generation though one of there is not seen at the F₁ stage.

→ Incomplete dominance: When the F₁ has a phenotype that does. not resemble either of the two parents and is in between the two it is incomplete dominance.

→ Aneuploidy: Failure of segregation of chromatin during cell division results in the gain or loss of a chromosome(s) is called aneupLoidy.

→ Pedigree traits: An analysis of traits in several generations of a family.

→ Mendelian disorders: Disorders mainly determined by alteration or mutation in a single gene.

→ Chromosomal disorders: Disorders caused due to the absence or excess or abnormal arrangement of one or more chromosomes.