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.