Introduction: Evolution by Natural Selection

evolution n. the doctrine according to which higher forms of life have gradually arisen out of lower

natural selection evolution by the survival of the fittest with inheritance of their fitness by the next generation

gene, n. one of the units of DNA, arranged in linear fashion on the chromosomes, responsible for passing on specific characteristics from parents to offspring. –adj. genic of or relating to a gene. –ns. genome the full set of chromosomes of an individual: the total number of genes in such a set; genotype genetic or factorial constitution of an individual: group of individuals all of which possess the same genetic constitution. Chambers 20th Century Dictionary

A gene is life's way of remembering how to perpetuate itself. Genethics, Suzuki & Knudtson (1990)

A gene is any portion of chromosomal material that potentially lasts for enough generations to serve as a unit of natural selection. The Selfish Gene, Dawkins (1976).

DNA and RNA

Genes are the basic vehicle of biological inheritance. They have a chemical memory which is recorded in the internal structures of a family of biological molecules, the nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

A human cell contains 46 strands of DNA, called chromosomes, in its nucleus, 22 matched pairs and two sex chromosomes. This DNA contains the information necessary for synthesising proteins and for regulating cellular processes.

DNA molecules have two distinct strands which are held together by weak hydrogen bonds to form a double helix. Each strand is made up of a series of small molecules called nucleotides. Only four nucleotides are used, and they are the same in all animals and plants. They each contain a deoxyribose sugar, a phosphate, and one of four kinds of nitrogen-containing bases, which are usually denoted by the first letters of their names: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C).

Each base has distinctive chemical characteristics which allow it to bond with only one of the other three bases. Thymine will combine only with Adenine and Cytosine will combine only with Guanine. Thus there are four possible combinations; (T-A), (A-T), (C-G), (G-C). This is extremely important in that it causes the two strands to be complementary to each other - when they separate, each strand acts as a template to form its complement.

DNA encodes the genetic information for producing proteins in the order of the bases along the DNA. The proteins themselves are made up of amino acids. A three-base sequence along the DNA contains the instructions to make one amino acid. RNA provides the mechanism though which DNA transmits its message to the cytoplasm of the cell.

RNA differs from DNA in that it has ribose sugars and it has another base, Uracil, which replaces T alongside A, G and C. Uracil bonds with adenine forming (U-A) and (A-U) combinations. RNA is usually a single-stranded molecule forming a single helix.

Some Definitions

A protein is any member of a group of complex nitrogenous substances that play an important part in the bodies of plants and animals. Proteins are compounds of carbon,