DNA is not perfect. Problems begin when control over parts of the 'blueprint' is lost. In physical terms, a mutation is an alteration in the sequence of nucleotides that comprise DNA - the instructions to make a protein encoded by the mutated DNA are changed. These changes can cause the cell machinery to act spectacularly differently. An understanding of changes or 'mutations' also helps shed light on how DNA works.
Some mutations have no harmful effect at all - for example if the changed section of DNA does not encode a protein or if the cell in which the mutation occurs does not need to make the affected protein.
However, other mutations can change the function of a protein, either subtly or completely destroying its function - which may cause problems if that protein is, say, an enzyme or hormone. Harmful mutations are more common than harmless ones, but not all mutations are fatal as most cells can repair damaged DNA.
Modern scientists often believe that life originated in the sea. Gradual changes in the DNA meant that some forms of early sea life became adapted to life on land. But there used to be a variety of theories about how life began - have a look at the following scene to find out more.
SCENE: Origins of life
So not all changes are necessarily dangerous. Some mutations have no noticeable effect, and a small proportion of mutations are actually beneficial. Mutation can be a slow, adaptive process which contributes to evolution.
Species variations also arise as a result of DNA mutation of the genes in the germ cells, and inheritance of these variations. Over the long term, mutations have helped some animal and plant species to adapt to changing climates and environments while others died out. If DNA replication was 'perfect' and none of the random mutations had taken place, evolution as we know it could not have taken place, so the 'imperfection' of DNA replication is actually a very important feature.
Plants and animals may evolve protective colouration or behaviour that aids survival. Humans have evolved specialisms that allow them to live in extremes of hot and cold, sunlight and darkness.
Some mutations are induced and others are spontaneous. At any point during the human lifetime, cells in the body could start to mutate. This can happen purely by chance, but is more likely to happen as a result of exposure to toxic material which damages DNA, such as nuclear radiation, ultra violet radiation from the sun and certain chemicals called carcinogens. Some toxic chemicals cause mutations by mistakenly being used as bases, to which they bear a resemblance.
Mutated cells of this type usually start to grow uncontrollably, destroying normal cells and spreading through the body. This is cancer. Cancer is more common later in life, as DNA and cells wear out and become more prone to error. Examples of induced mutations also occur in reproductive cells.
The only mutations that can be inherited are those which occur in reproductive cells - and reproductive cells may exhibit both induced and spontaneous mutations. For example, the children of people exposed to nuclear radiation in the bombings of Hiroshima and Nagasaki were generally thought not to exhibit deformities caused from mutations in the reproductive cells of their parents.
But controversy surrounds this issue: some studies suggest that the children of people exposed to radiation may indeed have inherited mutations.