Chapter Chapter-by-Chapter Anaylsis of The Selfish Gene (1989 edition) - Study Guide by Richard Dawkins

A DNA molecule is a long chain of building blocks - small molecules called nucleotides - arranged in an alpha helical formation (a pair of nucleotide chains twisted together in an elegant spiral). There are four different kinds of nucleotides, whose names are shortened to A, T, C and G. These are the same in all animals and plants differing simply by the order in which they are strung together.

Our DNA lives inside our bodies, distributed among the cells so that, with a few exceptions, each cell contains a complete copy of that body’s DNA. The DNA is present in each cell in the form of chromosomes. These are visible under a microscope as long threads. Each chromosome consists of two identical chromatids which are held together at a narrow region called the centromere. Each chromatid contains one DNA molecule.

Each DNA molecule is made up of a series of genes, where Dawkins defines a gene as "any portion of chromosomal material that potentially lasts for enough generations to serve as a unit of natural selection". This may be confused with a cistron, which is a short portion of the DNA molecule made up of the four- letter nucleotide alphabet coding for one protein. And in some cases the word cistron can be used interchangeably with the word gene, but there are some cases where the cistron cannot fit Dawkins definition of a gene in the sense that they can last for several generations because cistrons can be broken up by the process of crossing over which will be discussed later. Each gene for a particular characteristic can be said to occupy the same position on a chromosome, called the locus. In the gene pool (all the genes of the population in general available) there are several different possible genes for each locus, which are called alleles. They can be considered rivals for same slot on one chromosome.

The number of chromosomes an organism has is characteristic of the species. For example, in humans, there are 46 and in fruit flies there are 8. These chromosomes consist of matching pairs, so humans in fact have two sets of 23 chromosomes, one set coming from the mother and the other from the father. Each chromosome in the pair can be regarded as direct alternatives to the other, with the gene "for" eye colour occupying the same loci on both chromosomes. These genes may be the same (i.e. both coding "for" brown eyes), and therefore considered homozygous, or they may be different, and therefore called heterozygous, depending on the parents. Therefore, each cell contains 23 pairs of chromosomes in its nucleus which make up a complete copy of the bodies DNA.

DNA molecules do two important things: replicate (make copies of themselves) and indirectly supervises the manufacture of the body’s proteins. The former is how we develop from a single cell at conception to the thousand million million cells that make up our body as an adult. The process that takes place is called mitosis. This is when the DNA replicates before the cell divides into two so that each daughter cell receives the same number and type of chromosomes as the original. DNA also holds a central role in protein synthesis because the coded message of the DNA, written in the four-letter nucleotide alphabet, is translated into the alphabet of amino acids which then form a chain to produce protein molecules.

The cells undergo another kind of cell division, other than mitosis, called meiosis. This occurs only in the production of the sex cells and results in halving the number of chromosomes rather than straight replication. This is so that when the two sex cells fuse at conception, the new egg only receives the norm 46 chromosomes and does not double up every generation. Each sex cell contains one of each pair of chromosomes, but not in exactly the same form as they exist in the parents because during the manufacturing of the sex cells, chunks from one chromosome are detached and swapped with the corresponding chunk from its paired chromosome. Therefore, each chromosome in a sperm or egg would be a patchwork of maternal and paternal genes. This process of swapping bits is called crossing over and it promotes variation because it produces new combinations of genes in the new individual. Other ways of achieving genetic variation is through point mutations (when an error occurs that is analogous to a single misprinted letter in a book) and inversion (when a piece of chromosome detaches itself at both ends, turns head over heels, and reattaches itself in the inverted position).

Therefore, before moving on to the second half of the chapter, Dawkins emphasises what he means when he uses the term "gene". He is using the word "gene" to describe a portion of a chromosome that