Copying genes - mitosis

 
The earliest single celled organisms, the present day bacteria and yeasts, as well as the cells in our bodies, multiply by a process known as mitosis.

The double helix of DNA is gradually parted by an enzyme, producing two chains with unpaired bases. As it does so, free nucleotides from the surrounding environment attach themselves, the end product being two DNA molecules where before there was one.

In any complex multi-celled organism, replication occurs thousands of times during development and growth, and in the process of repair and replacement. Some cells, such as the majority of neurons, once mature, never replace themselves. Others such as blood cells and sperm, the linings of the lung and nasal passages are being replaced throughout life. How replication is controlled is the subject of intensive study.

Clearly in any copying system, errors are likely to occur, and there are complex checking mechanisms. In a single celled creature, such a change is likely to produce a new strain. In species that use sexual reproduction, the altered cells stay within the individual, and are referred to as somatic mutations.⁽¹⁾

1. Compare with germ line mutations. Eggs and sperm are produced from original cells laid down early in embryonic life. Thus germ line mutations occur only in this single cell line or during meiosis. Also somatic mutations are not passed to future generations.