Class 12 Biology — Important Board Questions with Answers

The two reproductive whorls of a flower are the androecium and the gynoecium. The androecium is the male reproductive whorl made up of stamens; each stamen has an anther that produces pollen grains (which contain the male gametes). The gynoecium (pistil) is the female reproductive whorl made up of one or more carpels; each carpel has an ovary containing ovules (which contain the female gamete, the egg), a style and a stigma that receives the pollen.

Microsporogenesis is the process of formation of microspores (pollen grains) from a microspore mother cell (pollen mother cell) through meiosis; each diploid mother cell gives four haploid microspores arranged in a tetrad. A mature pollen grain has a two-layered wall: an outer hard exine made of sporopollenin (very resistant) with germ pores, and an inner thin intine of cellulose and pectin. Inside, a mature pollen grain is two-celled, having a larger vegetative cell (with food and a tube nucleus) and a smaller generative cell that later forms two male gametes.

A typical mature embryo sac (female gametophyte) is 7-celled and 8-nucleate. At the micropylar end there is the egg apparatus consisting of one egg cell and two synergids (with special thickenings called filiform apparatus). At the chalazal end there are three antipodal cells. In the centre there is a large central cell containing two polar nuclei. Thus, although there are 8 nuclei, there are only 7 cells because the central cell has two nuclei.

Pollination is the transfer of pollen grains from the anther to the stigma of a flower. Self-pollination (autogamy) is the transfer of pollen from the anther to the stigma of the same flower or another flower of the same plant; it maintains parental characters. Cross-pollination (allogamy) is the transfer of pollen to the stigma of a flower on a different plant of the same species; it brings about variation. Cross-pollination is carried out by agents such as wind, water, insects and birds.

Double fertilization is a characteristic feature of flowering plants in which two fusion events take place inside the embryo sac using the two male gametes from one pollen tube. One male gamete fuses with the egg cell to form a diploid zygote; this is called syngamy (true fertilization). The other male gamete fuses with the two polar nuclei of the central cell to form a triploid (3n) primary endosperm nucleus; this is called triple fusion. Since two fusions (syngamy and triple fusion) occur, the process is called double fertilization.

After fertilisation: (1) the zygote develops into the embryo; (2) the primary endosperm nucleus develops into the endosperm, which nourishes the developing embryo; (3) the ovule develops into the seed and its integuments harden into the seed coat; and (4) the ovary develops into the fruit and its wall becomes the pericarp. Other parts of the flower such as the petals, stamens, style and stigma usually wither and fall off.

Apomixis is the formation of seeds without fertilisation; it is a form of asexual reproduction that mimics sexual reproduction (for example, in some species of grasses and Citrus). The seeds so formed are genetically identical to the parent. Polyembryony is the occurrence of more than one embryo in a single seed; it is commonly seen in Citrus and mango, where the extra embryos may develop from the cells of the nucellus.

Many flowering plants have devices to encourage cross-pollination (outbreeding) because continued self-pollination leads to inbreeding depression, and cross-pollination introduces genetic variation, which is important for adaptation and evolution. Two such outbreeding devices are: (1) unisexuality - the male and female flowers are separate (or on separate plants), so self-pollination is not possible; and (2) different timing of maturation of the anther and the stigma (dichogamy - protandry or protogyny), so that they are not receptive at the same time. Another device is self-incompatibility.

The human male reproductive system consists of: a pair of testes (the primary sex organs, which produce sperms and the male hormone testosterone), housed in the scrotum; a system of ducts, namely the epididymis, vas deferens, and urethra, that carry and store the sperms; and the accessory glands - the seminal vesicles, the prostate gland and the bulbourethral (Cowper's) glands - whose secretions form the seminal fluid. The penis is the copulatory organ.

The human female reproductive system consists of: a pair of ovaries (the primary sex organs, which produce ova/eggs and the hormones oestrogen and progesterone); a pair of oviducts (fallopian tubes), which carry the egg and are the usual site of fertilisation; the uterus (womb), where the embryo develops; the cervix, which connects the uterus to the vagina; and the vagina, which is the copulatory canal and birth canal.

Spermatogenesis is the process of formation of sperms (spermatozoa) from the germ cells in the testes, which begins at puberty. The diploid spermatogonia multiply by mitosis and some enlarge to form primary spermatocytes (2n). Each primary spermatocyte undergoes the first meiotic division to form two haploid secondary spermatocytes, which then undergo the second meiotic division to form four haploid spermatids. The spermatids are transformed into mature sperms by a process called spermiogenesis. It is stimulated by the hormones FSH and testosterone.

Oogenesis is the process of formation of the mature female gamete (ovum) from the germ cells in the ovary; it begins during the embryonic stage. An oogonium forms a primary oocyte, which undergoes the first meiotic division to form a secondary oocyte and a small first polar body; the second meiotic division (completed only after fertilisation) gives one large ovum and a second polar body. Thus, one primary oocyte forms only one functional ovum, whereas in spermatogenesis one primary spermatocyte forms four functional sperms.

A human sperm is a microscopic, motile cell with three main parts: (1) a head, which contains the haploid nucleus and is capped by the acrosome (containing enzymes that help the sperm penetrate the egg); (2) a middle piece, which contains many mitochondria that provide the energy for movement; and (3) a tail (flagellum), which lashes to propel the sperm. A plasma membrane covers the whole body of the sperm.

The menstrual cycle is the cyclic change in the reproductive system of a human female, of about 28 days. Its phases are: (1) Menstrual phase (days 1-5) - the uterine lining breaks down and is shed as menstrual flow. (2) Follicular (proliferative) phase (days 6-13) - under FSH the ovarian follicle matures and the uterine lining is rebuilt, and oestrogen rises. (3) Ovulation (about day 14) - a surge of LH causes the release of the ovum from the ovary. (4) Luteal (secretory) phase (days 15-28) - the corpus luteum secretes progesterone, which maintains the uterine lining; if fertilisation does not occur, it degenerates and the next cycle begins.

Fertilization is the fusion of a sperm with the ovum to form a diploid zygote; it normally takes place in the ampullary region of the fallopian tube. The zygote then undergoes repeated cell divisions (cleavage) as it moves down the tube to form a hollow ball of cells called the blastocyst. Implantation is the attachment and embedding of the blastocyst into the thick, prepared wall (endometrium) of the uterus, where the embryo then develops further.

The placenta is a special disc-shaped structure that connects the developing foetus (embryo) to the wall of the mother's uterus; it is formed from the tissues of both the foetus and the mother and is connected to the foetus by the umbilical cord. Functions: (1) it allows the exchange of materials - it supplies oxygen and nutrients from the mother to the foetus and removes carbon dioxide and wastes from the foetus; and (2) it acts as an endocrine gland, secreting hormones such as hCG, oestrogen and progesterone to maintain the pregnancy.

Oestrogen is secreted mainly by the growing ovarian follicle; it stimulates the repair and thickening of the uterine lining after menstruation, develops secondary sexual characters, and (at high levels) triggers the LH surge that causes ovulation. Progesterone is secreted mainly by the corpus luteum after ovulation; it maintains and further thickens the uterine lining (endometrium) to prepare it for implantation and, along with other hormones, maintains pregnancy if fertilisation occurs.

Reproductive health means a total well-being in all aspects of reproduction - physical, emotional, behavioural and social. A reproductively healthy society is one whose people have physically and functionally normal reproductive organs and healthy attitudes towards reproduction. Two measures to achieve it are: (1) creating awareness through education about reproduction, safe practices and available facilities (for example the RCH - Reproductive and Child Health - programmes); and (2) providing medical facilities and care for pregnancy, contraception, birth control and treatment of sexually transmitted infections.

Contraceptive methods are used to prevent unwanted pregnancy. The main types are: (1) natural methods, such as the rhythm method (periodic abstinence) and withdrawal; (2) barrier methods, such as condoms and diaphragms, which prevent the sperm and ovum from meeting; (3) intrauterine devices (IUDs), such as the copper-T; (4) hormonal methods, such as oral contraceptive pills; and (5) surgical methods (sterilisation), such as vasectomy in males and tubectomy in females. An ideal contraceptive should be safe, effective and reversible.

Sexually transmitted diseases (STDs), also called venereal diseases, are infections that are transmitted mainly through sexual contact. Examples include gonorrhoea, syphilis, genital herpes, hepatitis B and AIDS (caused by HIV). They can be prevented by: avoiding sexual contact with unknown or multiple partners, using condoms, avoiding sharing needles, and getting early diagnosis and complete treatment. Awareness and safe practices are the best prevention.

Assisted reproductive technologies (ART) are special medical techniques used to help infertile couples have a child. In IVF (in vitro fertilisation), the ovum and sperms are collected and fertilisation is carried out outside the body in the laboratory (in a culture dish); the resulting early embryo is then transferred into the uterus (embryo transfer). A baby born by such a technique is commonly (though misleadingly) called a test-tube baby. Other ART methods include GIFT, ZIFT and ICSI.

Law of dominance: characters are controlled by discrete units called factors (genes) which occur in pairs; in a dissimilar pair (heterozygous) of factors, one factor is dominant and expresses itself while the other is recessive and remains masked. Law of segregation: during the formation of gametes, the two factors (alleles) of a pair separate (segregate) so that each gamete receives only one factor of the pair; the factors do not blend and are passed on unchanged.

Mendel's law of independent assortment states that when two pairs of contrasting characters (traits) are considered together, the alleles of one pair of characters segregate (assort) independently of the alleles of the other pair during the formation of gametes. In other words, the inheritance of one character is not affected by the inheritance of another. This law is based on the results of a dihybrid cross, which gives a phenotypic ratio of 9 : 3 : 3 : 1 in the F2 generation.

In a monohybrid cross, a pure tall plant (TT) is crossed with a pure dwarf plant (tt). All the F1 plants are Tt and are tall, because T (tall) is dominant over t (dwarf). When the F1 plants (Tt) are self-pollinated, the gametes T and t combine in all possible ways to give the F2 generation: TT, Tt, Tt and tt. This gives a phenotypic ratio of 3 tall : 1 dwarf and a genotypic ratio of 1 TT : 2 Tt : 1 tt.

In a dihybrid cross, two pairs of contrasting characters are studied together (for example, seed shape and seed colour in peas: round yellow RRYY crossed with wrinkled green rryy). All F1 plants are round and yellow (RrYy). When the F1 are self-pollinated, the F2 generation shows four phenotypes in the ratio 9 (round yellow) : 3 (round green) : 3 (wrinkled yellow) : 1 (wrinkled green). This 9 : 3 : 3 : 1 ratio illustrates Mendel's law of independent assortment.

F2 dihybrid ratio = 9 : 3 : 3 : 1

A test cross is a cross in which an individual showing a dominant phenotype (whose genotype is unknown) is crossed with a homozygous recessive individual. It is used to determine whether the individual with the dominant phenotype is homozygous or heterozygous. If all the offspring show the dominant character, the individual is homozygous; if the offspring show a 1 : 1 ratio of dominant to recessive phenotypes, the individual is heterozygous.

In incomplete dominance, neither allele is completely dominant, so the heterozygote shows an intermediate (blended) phenotype; for example, in the four o'clock plant (Mirabilis jalapa) a cross between red and white flowered plants gives pink flowers in the F1. In codominance, both alleles express themselves fully and independently in the heterozygote (there is no blending); for example, in the human ABO blood group, the AB blood group shows both the A and the B antigens.

Linkage is the tendency of two or more genes located on the same chromosome to be inherited together (as a group) because they do not assort independently; such genes are said to be linked. Recombination is the formation of new combinations of genes (different from the parental combinations) in the offspring, mainly as a result of crossing over between homologous chromosomes during meiosis. The frequency of recombination between two genes is used to estimate the distance between them on a chromosome.

In humans, sex is determined by the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY); this is the XX-XY type of sex determination. All the eggs produced by the female carry one X chromosome, while the sperms are of two types: half carry an X chromosome and half carry a Y chromosome. If an X-bearing sperm fertilises the egg, the child is a female (XX); if a Y-bearing sperm fertilises the egg, the child is a male (XY). Thus the father (male) determines the sex of the child.

Sex-linked disorders are genetic disorders caused by genes located on the sex chromosomes (usually the X chromosome), so their inheritance is linked to the sex of the individual. Haemophilia is an X-linked recessive disorder in which the blood does not clot properly (a clotting factor is missing), so there is prolonged bleeding even from minor injuries. Colour blindness (red-green) is also an X-linked recessive disorder in which a person cannot distinguish between red and green colours. Because these genes are on the X chromosome, such disorders are more common in males (who have only one X chromosome).

Down syndrome is caused by the presence of an additional copy of chromosome number 21 (trisomy 21, i.e. 47 chromosomes); affected persons show mental retardation and characteristic facial features. Turner syndrome is caused by the absence of one X chromosome in females (45, X0), giving sterile females with underdeveloped features. Klinefelter syndrome is caused by an additional X chromosome in males (47, XXY), giving males with some feminine features and usually sterile.

Pleiotropy is the phenomenon in which a single gene affects (controls) more than one character or trait; for example, the gene for sickle-cell anaemia (or the gene causing phenylketonuria) affects several traits at once. Polygenic inheritance is the phenomenon in which a single character or trait is controlled by two or more genes, each adding to the effect; for example, human skin colour and human height are controlled by many genes and show a range of variation.

According to the Watson and Crick model (1953), DNA is a double helix made of two polynucleotide chains coiled around a common axis. The two strands are antiparallel (run in opposite directions) and have a sugar-phosphate backbone on the outside, with the nitrogenous bases pointing inward. The two strands are held together by hydrogen bonds between complementary bases (A pairs with T by two hydrogen bonds, and G pairs with C by three hydrogen bonds). The helix has about 10 base pairs per turn, with a pitch of about 3.4 nm.

In DNA, the base pairing rule (complementary base pairing) states that adenine (A) always pairs with thymine (T) through two hydrogen bonds, and guanine (G) always pairs with cytosine (C) through three hydrogen bonds. As a consequence, Chargaff's rule states that in a DNA molecule the amount of adenine equals the amount of thymine (A = T) and the amount of guanine equals the amount of cytosine (G = C); therefore the total purines equal the total pyrimidines.

A = T; G = C

Semiconservative replication is the method of DNA replication in which each of the two strands of the parent DNA acts as a template, and the newly formed DNA molecule contains one old (parental) strand and one newly synthesised strand. During replication, the enzyme helicase unwinds and separates the two strands; then the enzyme DNA polymerase adds new nucleotides to each template strand following complementary base pairing, forming two identical daughter DNA molecules. This mode was experimentally proved by Meselson and Stahl.

The central dogma of molecular biology, proposed by Francis Crick, states that the genetic information generally flows in one direction: from DNA to RNA to protein. DNA is first copied into a messenger RNA (mRNA) by the process of transcription, and the information in the mRNA is then used to synthesise a protein by the process of translation. (In some viruses called retroviruses, the flow can be reversed from RNA to DNA by reverse transcription.)

DNA -> RNA -> protein

Transcription is the process of copying the genetic information from one strand of DNA into a molecule of messenger RNA (mRNA). Only one strand of the DNA (the template strand) is copied. The enzyme RNA polymerase binds to the promoter region and moves along the template, adding ribonucleotides according to complementary base pairing (A of DNA pairs with U of RNA, and so on), forming the RNA. The three steps are initiation, elongation and termination. In eukaryotes the RNA is then processed (splicing) before it leaves the nucleus.

The genetic code is the set of rules by which the sequence of bases in mRNA is translated into the sequence of amino acids in a protein. Its features are: (1) it is a triplet code - each codon of three bases codes for one amino acid (there are 64 codons); (2) it is degenerate - one amino acid may be coded by more than one codon; (3) it is universal - the same codons code for the same amino acids in almost all organisms; (4) it is non-overlapping and read in a continuous manner; and (5) AUG is the start codon, and there are three stop (termination) codons.

Translation is the process of synthesis of a protein (polypeptide) from the information present in the mRNA. It takes place on the ribosomes. The mRNA attaches to a ribosome, and transfer RNA (tRNA) molecules bring specific amino acids according to the codons on the mRNA (each tRNA has an anticodon complementary to a codon). The amino acids are joined one by one by peptide bonds as the ribosome moves along the mRNA (initiation, elongation and termination), forming a polypeptide chain. Translation stops when a stop codon is reached.

The three main types of RNA are: (1) messenger RNA (mRNA), which carries the genetic message (codons) from the DNA in the nucleus to the ribosomes for protein synthesis; (2) transfer RNA (tRNA), also called the adaptor molecule, which brings specific amino acids to the ribosome during translation according to the codons; and (3) ribosomal RNA (rRNA), which is a structural and functional (catalytic) component of the ribosomes where proteins are synthesised.

The lac operon is a segment of DNA in the bacterium E. coli that controls the metabolism of lactose; it consists of a promoter, an operator and three structural genes (z, y and a). It is an inducible operon. In the absence of lactose, a repressor protein binds to the operator and blocks transcription, so the genes are switched off. When lactose is present, it acts as an inducer: it binds to the repressor and inactivates it, so the repressor leaves the operator and transcription of the genes takes place, allowing the bacterium to use lactose.

DNA fingerprinting is a technique used to identify an individual on the basis of the unique patterns in certain highly variable regions of their DNA (called repetitive DNA or VNTRs), which differ from person to person (except in identical twins). Applications: (1) in forensic science, to identify criminals or victims from biological samples such as blood, hair or semen; and (2) in settling disputes of parentage (paternity or maternity testing). It is also used to study genetic diversity and evolution.

Darwin's theory of natural selection (survival of the fittest) states that: (1) organisms produce far more offspring than can survive, so there is a struggle for existence for limited resources; (2) individuals within a species show variations, some of which are useful (favourable); (3) individuals with favourable variations are better adapted, so they survive, reproduce more and pass on these variations to their offspring (natural selection); and (4) over many generations these favourable variations accumulate, leading to the origin of new species.

(1) Fossil evidence (palaeontological): fossils are the preserved remains of organisms of the past found in rocks; the study of fossils in different layers shows a gradual change in organisms over time and provides direct evidence of evolution. (2) Morphological and anatomical evidence: the presence of homologous organs (similar in structure and origin but different in function, such as the forelimbs of a whale, bat and human) shows common ancestry (divergent evolution), while vestigial organs (reduced, functionless organs like the appendix in humans) also indicate evolution.

Homologous organs are organs that have the same basic structure and origin (developmental plan) but may perform different functions; they indicate common ancestry and divergent evolution. Example: the forelimbs of a human, a whale and a bat. Analogous organs are organs that have different basic structure and origin but perform the same function; they indicate convergent evolution. Example: the wings of a butterfly (insect) and the wings of a bird, which both serve for flight but are structurally different.

The Hardy-Weinberg principle states that in a large, randomly mating population, the allele frequencies and genotype frequencies remain constant from generation to generation, provided there are no disturbing factors; such a population is said to be in genetic equilibrium. If p is the frequency of one allele and q the frequency of the other (with p + q = 1), then the genotype frequencies are given by the equation p^2 + 2pq + q^2 = 1, where p^2 and q^2 are the frequencies of the two homozygotes and 2pq is the frequency of the heterozygote.

p^2 + 2pq + q^2 = 1

The factors that change allele frequencies and disturb the Hardy-Weinberg genetic equilibrium (and thus cause evolution) are: (1) gene mutation (a source of new alleles); (2) gene migration or gene flow (movement of alleles into or out of a population); (3) genetic drift (random change in allele frequencies, especially in small populations); (4) natural selection (differential reproduction of favourable variations); and (5) non-random (assortative) mating. When these operate, the population evolves.

Adaptive radiation is the process of evolution of different species from a single common ancestor in a given geographical area, where each new species becomes adapted to a different habitat or way of life. The classic example is Darwin's finches of the Galapagos Islands: from an original seed-eating ancestor, a variety of finches evolved with different types of beaks suited to different diets (seeds, insects, etc.). Another example is the Australian marsupials, which radiated into many forms from a common ancestor.

The Oparin-Haldane theory (theory of chemical evolution) proposes that life originated from non-living organic molecules that were formed spontaneously in the primitive earth's conditions; the first form of life arose from these molecules in the ancient ocean (the primordial soup). To test this, Miller and Urey performed an experiment in which they created conditions similar to the primitive atmosphere (a mixture of methane, ammonia, hydrogen and water vapour) in a closed flask and passed electric sparks through it. After a week, they found that simple organic molecules, including amino acids, had formed, supporting the theory of chemical evolution.

Typhoid is caused by the bacterium Salmonella typhi, which infects the intestine and is spread through contaminated food and water. Malaria is caused by the protozoan parasite Plasmodium (such as Plasmodium vivax and Plasmodium falciparum), which is transmitted by the bite of an infected female Anopheles mosquito. Pneumonia is caused mainly by the bacteria Streptococcus pneumoniae (and also Haemophilus influenzae), which affect the lungs (alveoli).

Immunity is the ability of the body to resist and fight against disease-causing organisms (pathogens) and their harmful products. Innate (natural or non-specific) immunity is present from birth and provides a general, non-specific defence through barriers such as the skin, mucous membranes, phagocytic cells and inflammation. Acquired (adaptive or specific) immunity develops during a person's lifetime after exposure to a specific pathogen; it is specific to that pathogen, involves antibodies and memory cells, and includes active immunity (produced by one's own body after infection or vaccination) and passive immunity (ready-made antibodies received from outside).

Antibodies (immunoglobulins) are Y-shaped protein molecules produced by the B-lymphocytes (plasma cells) in response to an antigen; they bind specifically to the antigen and help destroy it. Each antibody molecule is made of four polypeptide chains: two identical longer heavy (H) chains and two identical shorter light (L) chains, so it is represented as H2L2. Each antibody has a variable region (which binds the specific antigen) and a constant region. The main classes are IgG, IgA, IgM, IgE and IgD.

antibody structure = H2L2

AIDS (Acquired Immuno Deficiency Syndrome) is caused by the Human Immunodeficiency Virus (HIV), a retrovirus that attacks and destroys the helper T-lymphocytes, weakening the body's immune system. It is transmitted by: (1) unprotected sexual contact with an infected person; (2) transfusion of infected blood or sharing infected needles/syringes; and (3) from an infected mother to her child during pregnancy, birth or breast-feeding. It can be prevented by using disposable needles, screening blood before transfusion, using condoms and creating awareness.

Cancer is a disease in which there is an uncontrolled division of cells that lose the property of contact inhibition, forming a mass of cells called a tumour; malignant tumours can spread to other parts of the body (metastasis). Agents that cause cancer are called carcinogens; examples are: (1) physical agents such as ionising radiations (X-rays, gamma rays) and ultraviolet rays; (2) chemical agents such as those in tobacco smoke; and (3) certain viruses (oncogenic viruses). Cancer arises when proto-oncogenes are activated into oncogenes.

Vaccination (immunisation) is the process of introducing a vaccine - a preparation of weakened, killed or a part of a pathogen (antigenic proteins) - into the body to develop immunity against a specific disease. It works on the principle of the memory of the immune system: the vaccine acts as an antigen and stimulates the body to produce antibodies and memory cells against that pathogen. Then, if the actual pathogen ever enters the body, the memory cells recognise it and quickly produce a large amount of antibodies to destroy it, preventing the disease.

An allergy is an exaggerated (hypersensitive) response of the immune system to certain substances called allergens (such as pollen, dust or certain foods) that are otherwise harmless; it releases chemicals like histamine, causing symptoms such as sneezing, watery eyes and rashes (for example, asthma or hay fever). An autoimmune disease is a condition in which the body's immune system mistakenly attacks its own cells and tissues (the body fails to distinguish self from non-self); an example is rheumatoid arthritis.

Drug and alcohol abuse is the use of drugs or alcohol for a purpose other than medicine, in amounts or ways that harm the body and mind, often leading to addiction and dependence. Harmful effects include: (1) physical and mental harm - damage to the nervous system, liver (cirrhosis) and other organs, and impaired judgement and coordination; and (2) social and psychological problems - poor academic or work performance, aggression, depression, and strained relationships with family and society; severe cases can be fatal.

Curd is produced from milk by lactic acid bacteria (LAB), mainly Lactobacillus, which convert the sugar lactose into lactic acid and also improve its nutritional quality (vitamin B12). Bread is made using the fungus baker's yeast (Saccharomyces cerevisiae), whose fermentation produces carbon dioxide that makes the dough rise (puffed up). Cheese is produced by the action of specific bacteria and fungi; for example, the characteristic flavour and large holes of Swiss cheese are due to a bacterium (Propionibacterium), and Roquefort cheese is ripened using a fungus (Penicillium roqueforti).

(1) Ethanol (alcohol): the yeast Saccharomyces cerevisiae ferments sugars to produce ethanol, used in alcoholic beverages and as a fuel. (2) Antibiotics: the fungus Penicillium notatum (or chrysogenum) produces the antibiotic penicillin, and Streptomyces bacteria produce antibiotics such as streptomycin. (3) Organic acids and enzymes: for example, Aspergillus niger produces citric acid, and Acetobacter aceti produces acetic acid (vinegar). These are made in large fermenters (bioreactors).

Antibiotics are chemical substances produced by certain microorganisms (such as bacteria and fungi) that can kill or inhibit the growth of other disease-causing microorganisms; they are used to treat bacterial infections. Penicillin, the first antibiotic, was discovered by Alexander Fleming (in 1928) from the fungus (mould) Penicillium notatum, which he noticed inhibiting the growth of bacteria. It was later developed as a drug by Ernst Chain and Howard Florey.

Sewage (the waste water from towns and cities) is treated in sewage treatment plants (STPs) mainly with the help of heterotrophic microbes. Primary treatment is a physical process in which floating and suspended solids are removed by sequential filtration and sedimentation, forming primary sludge; the liquid part is the primary effluent. Secondary (biological) treatment is where aerobic microbes are grown as flocs and consume the organic matter in the effluent, greatly reducing its BOD (biochemical oxygen demand); the microbial mass then settles as sludge, part of which is digested by anaerobic bacteria to produce biogas.

Biogas is a mixture of gases (mainly methane, along with carbon dioxide and hydrogen sulphide) produced by the anaerobic breakdown of organic waste such as cattle dung (gobar) and plant material; it is used as a fuel for cooking and lighting. It is produced by a group of anaerobic bacteria called methanogens (for example, Methanobacterium), which are also present in the rumen of cattle and in anaerobic sludge. The biogas plant used to make it is commonly called a gobar gas plant.

Biofertilizers are living organisms (microbes) that enrich the nutrient quality of the soil, mainly by fixing atmospheric nitrogen or making phosphorus available; examples include the bacterium Rhizobium (in root nodules of legumes), cyanobacteria (blue-green algae) and mycorrhiza (a fungus-root association). Biocontrol agents are organisms used to control plant diseases and pests biologically, instead of chemical pesticides; examples include the bacterium Bacillus thuringiensis (used against insect larvae), the ladybird beetle (which eats aphids) and the fungus Trichoderma.

Biotechnology is the use of living organisms, cells or their components (such as enzymes) to make products or processes useful to human beings (for example, medicines, vaccines and improved crops). Recombinant DNA (rDNA) technology, also called genetic engineering, is the technique of combining (joining) DNA from two different sources to form a new recombinant DNA molecule, which is then introduced into a host organism where it expresses the desired character. It is the core technique of modern biotechnology.

The three main tools of recombinant DNA technology are: (1) restriction enzymes (molecular scissors), which cut the DNA at specific recognition sites, and DNA ligase, which joins DNA fragments; (2) cloning vectors (such as plasmids and bacteriophages), which are vehicles used to carry the desired (foreign) DNA into the host cell and help it multiply; and (3) a competent host organism (such as the bacterium E. coli), into which the recombinant DNA is introduced so that it can replicate and express the gene.

Restriction enzymes (restriction endonucleases) are enzymes obtained from bacteria that cut DNA at specific recognition sequences, which are usually palindromic (read the same on both strands). Each restriction enzyme recognises a particular base sequence and cuts the DNA within or near it (for example, EcoRI). They are called molecular scissors because they cut the DNA strands at precise positions, often producing sticky ends that help in joining DNA fragments from different sources during genetic engineering.

A cloning vector is a DNA molecule (commonly a plasmid or a bacteriophage) that is used as a carrier to transfer a desired (foreign) DNA fragment into a host cell and to replicate it there. The essential features of a good cloning vector are: (1) an origin of replication (ori), which enables the vector to replicate inside the host; (2) a selectable marker (such as a gene for antibiotic resistance), which helps to identify and select the host cells that have taken up the vector; and (3) unique restriction (recognition) sites where the foreign DNA can be inserted.

The main steps are: (1) isolation of the desired DNA (gene) and of the vector DNA; (2) cutting both the desired DNA and the vector with the same restriction enzyme to get compatible (sticky) ends; (3) joining (ligation) of the desired DNA into the vector using DNA ligase to form the recombinant DNA; (4) introduction (transformation) of the recombinant DNA into a suitable host cell; and (5) selection and multiplication of the transformed host cells so that the gene is expressed and the desired product is obtained (using a bioreactor and downstream processing).

PCR (polymerase chain reaction) is a technique used to make multiple copies (amplify) of a specific segment of DNA in a test tube (in vitro). It uses primers and a heat-stable DNA polymerase (Taq polymerase from Thermus aquaticus). Its three steps, repeated in cycles, are: (1) denaturation - the DNA is heated to separate the two strands; (2) annealing - the primers bind to the complementary sequences on the single strands at a lower temperature; and (3) extension - Taq polymerase extends the primers to synthesise new strands. Repeating the cycles increases the DNA exponentially.

A bioreactor is a large vessel (often thousands of litres) in which the raw materials are biologically converted into specific products by microbes, plant or animal cells or enzymes, under optimum controlled conditions of temperature, pH, oxygen and so on. The most commonly used type is the stirred-tank bioreactor. Downstream processing refers to all the processes carried out after the product has been formed in the bioreactor, such as separation and purification of the product, and its formulation with preservatives and quality control, before it is marketed as a finished product.

Genetically modified organisms (GMOs) are plants, animals or microbes whose genes have been altered (modified) by genetic engineering to introduce a desired character. Advantages of GM (genetically modified) crops: (1) they can be made more tolerant to abiotic stresses such as cold, drought, salt and heat; (2) they can be made resistant to pests, reducing the need for chemical pesticides (for example Bt cotton); (3) they can have improved nutritional value (biofortification, e.g. golden rice rich in vitamin A) and a longer shelf life; and (4) they can increase crop yield.

Bt cotton is a genetically modified variety of cotton that carries a gene (a cry gene) taken from the soil bacterium Bacillus thuringiensis (Bt). This gene codes for a protein (the Bt toxin) that is toxic to certain insect pests such as the cotton bollworm. The toxin is produced as an inactive protoxin (crystal), which, when eaten by the insect, is activated in the alkaline gut of the insect, binds to the gut cells and kills the insect. Thus Bt cotton is protected from the pest without using chemical insecticides.

Human insulin is produced by recombinant DNA technology (the product is called Humulin). Insulin consists of two short polypeptide chains, A and B, joined by disulphide bonds. In the method developed by the company Eli Lilly, the DNA sequences coding for the A and B chains were prepared and introduced separately into the bacterium E. coli; the bacteria produced the two chains separately. These chains were then extracted and joined together by disulphide bonds to form functional human insulin. This insulin is identical to human insulin and does not cause the allergies that animal insulin sometimes did.

Gene therapy is a collection of methods that allow the correction of a gene defect that has been diagnosed in a person; a normal (healthy) functional gene is introduced into the individual (or embryo) to replace or compensate for the non-functional defective gene, so that the normal function is restored. The first clinical gene therapy was given in 1990 to a girl suffering from adenosine deaminase (ADA) deficiency, a disorder of the immune system, in which functional ADA genes (in her lymphocytes) were introduced into her body.

Transgenic animals are animals whose DNA has been altered (manipulated) to carry and express an extra (foreign) gene from another organism. Uses: (1) they are used to study how a gene contributes to the development of a disease and to study normal physiology (for example, transgenic mice used to study diseases); (2) they are used to produce useful biological products, such as human proteins in their milk (for example, a transgenic cow producing human alpha-lactalbumin); and (3) they are used for testing the safety of vaccines and the toxicity of chemicals.

Biopiracy is the unauthorised use and exploitation of the bio-resources (plants, animals and microbes) and the traditional knowledge of a country or community by other organisations or nations, usually without proper permission or fair payment (for example, patenting the medicinal use of a plant known traditionally in India). The GEAC (Genetic Engineering Appraisal Committee) is an Indian government body that makes decisions regarding the validity and safety of genetically modified (GM) research and the safety of introducing GM organisms for public use.

Population density is the number of individuals of a population present per unit area or volume at a given time. Natality (birth rate) is the number of births (new individuals added to the population) per unit time. Mortality (death rate) is the number of deaths (individuals lost from the population) per unit time. A population grows when natality and immigration exceed mortality and emigration.

Exponential (geometric) growth occurs when resources (food, space) are unlimited; the population grows without any check and its growth curve is J-shaped. Logistic growth occurs when resources are limited; the population grows slowly at first, then rapidly, and finally slows down and levels off at the carrying capacity (the maximum number the environment can support), giving an S-shaped (sigmoid) curve. In nature, resources are usually limited, so logistic growth is more realistic.

The main population interactions between two species are: (1) Mutualism - both species benefit (for example, lichens, and pollination of flowers by insects). (2) Competition - both species are harmed as they compete for the same resources. (3) Predation - one species (predator) kills and eats the other (prey), benefiting the predator. (4) Parasitism - one species (parasite) benefits at the expense of the other (host), which is harmed. (5) Commensalism - one species benefits while the other is neither harmed nor benefited (for example, an orchid growing on a tree).

An adaptation is any attribute (morphological, physiological or behavioural) of an organism that enables it to survive and reproduce in its habitat. For example, the desert kangaroo rat is adapted to conserve water: it can meet almost all its water requirement from the internal oxidation of the food it eats (fat) and it produces very concentrated urine, so it loses very little water. Similarly, desert plants have thick cuticles and sunken stomata to reduce water loss (transpiration).

The carrying capacity (denoted by K) of an environment is the maximum population size of a species that the resources of that particular environment can support and sustain indefinitely, without being degraded. When a population grows logistically, it increases until it reaches the carrying capacity, and then it levels off, because the limited resources cannot support any further increase. If the population exceeds K, it declines back due to shortage of resources.

An age pyramid is a diagram that shows the distribution of the individuals of a population in different age groups (pre-reproductive, reproductive and post-reproductive) at a given time. Its shape indicates the growth status of the population: a broad-based (triangular) pyramid indicates a rapidly growing (expanding) population with many young individuals; a bell-shaped pyramid indicates a stable population; and an urn-shaped (narrow-based) pyramid indicates a declining population.

Mutualism is a close, positive interaction between two different species in which both the species are benefited. Examples: (1) lichens, which are a mutualistic association between a fungus and an alga (the fungus provides shelter and absorbs water and minerals, while the alga provides food by photosynthesis); and (2) mycorrhiza, an association between a fungus and the roots of higher plants; and (3) the pollination of flowers by insects, where the plant is pollinated and the insect gets nectar as food.

An ecosystem is a functional unit of nature in which the living organisms (the biotic community) interact among themselves and with their physical (non-living) environment, exchanging energy and matter. Its two basic components are: (1) the abiotic (non-living) components, which include physical factors such as light, temperature, water, air, soil and inorganic nutrients; and (2) the biotic (living) components, which include producers (green plants), consumers (herbivores and carnivores) and decomposers (microbes).

A food chain is a linear sequence of organisms through which food energy passes from one trophic (feeding) level to the next, starting from the producers, for example: grass -> grasshopper -> frog -> snake -> hawk. A food web is a network formed by the interconnection of many food chains in an ecosystem, because most organisms feed on more than one type of food and are eaten by more than one type of predator. A food web gives greater stability to an ecosystem than a single food chain.

The flow of energy in an ecosystem is unidirectional: it starts from the sun, is trapped by producers (green plants) during photosynthesis, and then passes from the producers to herbivores (primary consumers) and then to carnivores (higher consumers) through the food chain. At each transfer, most of the energy is lost as heat in respiration and other activities. According to Lindeman's ten per cent law, only about 10 per cent of the energy present at one trophic level is transferred to and stored at the next higher trophic level; the remaining 90 per cent is lost. This is why food chains usually have only 3 to 5 trophic levels.

only about 10 percent energy transferred per level

An ecological pyramid is a graphical representation of the relationship between the organisms at the various trophic levels of a food chain, with the producers at the base and the top carnivores at the apex. There are three types: (1) the pyramid of numbers (based on the number of individuals at each level); (2) the pyramid of biomass (based on the total dry weight of organisms at each level); and (3) the pyramid of energy (based on the amount of energy at each level). The pyramid of energy is always upright, whereas the pyramids of number and biomass may sometimes be inverted.

Gross primary productivity (GPP) is the total amount of organic matter (or energy) produced by the producers (green plants) during photosynthesis per unit area per unit time. Net primary productivity (NPP) is the amount of organic matter that is left and available to consumers after the producers have used up some of the gross production in their own respiration. Thus NPP = GPP - respiration (R). NPP represents the biomass available for the herbivores and decomposers.

NPP = GPP - respiration

Decomposition is the process by which decomposers (bacteria and fungi) break down the complex organic matter of dead plants and animals (detritus) into simple inorganic substances such as carbon dioxide, water and nutrients, which are released back into the environment. The main steps in decomposition are: fragmentation (breaking of detritus into smaller pieces by detritivores), leaching (water-soluble nutrients go down into the soil), catabolism (enzymatic breakdown into simpler compounds), humification (formation of dark humus) and mineralisation (release of inorganic nutrients).

Nutrient cycling (biogeochemical cycling) is the movement of nutrient elements through the various components (living and non-living) of an ecosystem in a cyclic manner, so that they are used again and again. In the carbon cycle, carbon (as carbon dioxide) is taken from the atmosphere by green plants during photosynthesis and fixed into organic compounds; it passes along the food chain to animals; and it is returned to the atmosphere as carbon dioxide through respiration by plants and animals, decomposition of dead matter by microbes, and the burning of fuels and wood.

Ecological succession is the gradual and orderly process of change in the species composition of a community in an area over time, until a stable community called the climax community is established. Primary succession begins in an area where no living organisms have existed before, such as bare rock, a newly cooled lava flow or a new pond; it takes a very long time as soil has to form first. Secondary succession begins in an area where a community already existed but was destroyed (for example, an abandoned farmland or a burnt forest); it is faster because the soil is already present.

Biodiversity is the variety and variability of all living organisms (plants, animals and microbes) present on the earth, including the ecosystems in which they occur. It exists at three levels: (1) genetic diversity - the variation of genes within a single species (for example, the different varieties of rice or mango); (2) species diversity - the variety of species in a given region; and (3) ecological (ecosystem) diversity - the variety of ecosystems or habitats such as forests, deserts, wetlands and coral reefs in a region.

The latitudinal gradient is a pattern in which species diversity generally decreases as we move away from the equator towards the poles. In other words, tropical regions (near the equator, at low latitudes) have far more species than temperate and polar regions. For example, the tropical rainforests, especially the Amazon rainforest, harbour the greatest biodiversity on earth. This is because tropical regions have a relatively constant, warm climate, more solar energy and have remained undisturbed for a long time, allowing more species to evolve.

We should conserve biodiversity for the following reasons: (1) narrowly utilitarian reasons - humans derive direct economic benefits, such as food, firewood, fibre, medicines and industrial products, from biodiversity; (2) broadly utilitarian reasons - biodiversity provides important ecosystem services, such as the production of oxygen, pollination, and climate regulation; and (3) ethical (moral) reasons - every species has an intrinsic value and a right to exist, and we have a moral duty to protect them and pass on the biological wealth to future generations.

The four major causes of biodiversity loss, together called the Evil Quartet, are: (1) habitat loss and fragmentation - the most important cause, in which natural habitats such as forests are destroyed or broken up (for example, deforestation of tropical rainforests); (2) over-exploitation - excessive hunting, fishing and harvesting of species for human use; (3) alien (invasive) species invasions - the introduction of foreign species that harm the native species; and (4) co-extinctions - when a species becomes extinct, other species associated with it (such as its parasites or partners) also become extinct.

In-situ (on-site) conservation is the conservation of species in their natural habitat, where the whole ecosystem is protected so that the species continue to live and evolve in their natural surroundings; examples include national parks, wildlife sanctuaries, biosphere reserves and sacred groves. Ex-situ (off-site) conservation is the conservation of species outside their natural habitat, in specially protected places, and is used especially for threatened species; examples include zoological parks (zoos), botanical gardens, seed banks, gene banks and cryopreservation.

Biodiversity hotspots are regions with a very high level of species richness and a high degree of endemism (species found nowhere else), which are also under serious threat of habitat loss and destruction. They are given special priority for conservation. Three biodiversity hotspots extend into India: (1) the Western Ghats and Sri Lanka, (2) the Himalaya, and (3) the Indo-Burma region. Protecting such hotspots can greatly reduce the ongoing loss of species.

An endangered species is a species that is in serious danger of becoming extinct in the near future if the factors threatening it continue (for example, the tiger and the Great Indian Bustard). An endemic species is a species that is naturally found only in a particular geographical area or region and nowhere else (for example, the lion-tailed macaque of the Western Ghats). The IUCN (International Union for Conservation of Nature) Red List (Red Data Book) is a document that keeps a record of all endangered and threatened species of plants and animals.

Sacred groves are patches of forest or natural vegetation that are traditionally protected by local communities because of their religious and cultural beliefs, as they are dedicated to a deity or considered holy. In India they are found in regions such as the Khasi and Jaintia Hills of Meghalaya, the Western Ghats of Karnataka and Maharashtra, and the Aravalli Hills. Because cutting even a single tree is often forbidden, these groves are undisturbed and serve as important refuges for many rare, endemic and threatened species of plants and animals, thus helping in in-situ conservation of biodiversity.

Extinction is the complete disappearance of a species from the earth, so that no living individual of that species remains anywhere. It may occur naturally over a long time, but human activities (such as habitat destruction, over-exploitation and pollution) have greatly increased the rate of extinction. The Red Data Book (maintained by the IUCN) is an official document that contains a record of all the species that are endangered, vulnerable, rare or recently extinct, so that special conservation efforts can be directed towards protecting them.