Darwin’s theory of natural selection explained how new species come about. But Darwin did not know how parents pass on their characteristics to their offspring. Nor did he know how new characteristics arise. We now know many of the answers.
The units of heredity are genes. They determine everything from eye colour to increased risk of suffering certain diseases. Some inherited characteristics (such as the colour of rats) are determined by a single gene, but most (such as human height, weight and eye colour) are determined by a number of different genes. Genes are packaged in long strings of molecules called chromosomes, and in all cells except sex cells there are two sets of chromosomes.
What was unknown was how the instructions for replicating any given trait in the offspring were coded within the gene. In the 1940s scientists began to suspect that a very large and complex molecule called deoxyribonucleic acid (DNA) was involved. Then in 1953 the American James Watson and the Englishman Francis Crick, working at Cambridge University, announced they had worked out how DNA encodes genetic information.
‘A structure this pretty just had to exist.’
James Watson, The Double Helix (1968), on the DNA molecule
Genetic code, they showed, is embedded in the structure of DNA. The DNA molecule is a double helix – two strands that spiral around each other. Each strand is a sugar-phosphate backbone, and each is joined to the other by sequences of pairs of just four chemical components called nucleotide bases. Each base pairs only with one of the other three bases. This structure explains how DNA can replicate itself, by the separation of the two strands, and so pass on genetic information to offspring.
The structure of DNA also explains how the code is embedded. Each sequence of three nucleotide bases (a codon) contains the instructions for the creation of a particular amino acid. Amino acids are the building blocks of proteins, crucial components of all cells (see here ). A gene consists of a sequence of codons coding for a single protein, followed by a stop codon. Some parts of the DNA do not themselves code for amino acids but are control centres for turning genes or groups of genes on and off.
The way that DNA works also explains how mutation leads to new characteristics – the key driving force in natural selection. A mutation is a change in the sequence of nucleotide bases as the DNA replicates itself. This occurs naturally, but exposure to chemicals or radiation can accelerate the mutation rate. Only those mutations that occur in eggs and sperm will be passed on to offspring, and only these matter for evolution. Many mutations are neutral, but some can damage offspring, while a few can be beneficial. Mutations in control genes may have a particularly big effect on the organism. It is the beneficial mutations that better equip an organism for its environment, and it is these that are likely to be passed on to future generations.