Before you read this, I suggest you read post 21.8.
In my previous posts (21.11, 21.14 and 21.23), it may appear that genes are constant sources of information that never change. This idea explains most of what we know about why individuals have a different genotype and, therefore, a different phenotype to their parents. But it is not completely true. As I have written before (post 17.25) explaining science is a process of diminishing deception. My deception about genes helped me to explain some basic ideas. But now I am going to discuss a complication.
Like any other molecule, DNA can take part in chemical reactions. If these reactions change the way in which its base sequence is read, during mitosis or meiosis, then the genetic information that it carries will be changed. So, chemical reactions can change genes. This process is called mutation.
Will the mutant genes be passed on to another generation? It depends on the type of cell in which the mutation occurred. Sperm and ova are produced by cells called germ cells. If a mutation occurs in a germ cell (a germline mutation), the mutated genes will appear in the sperm or ova and be passed on to a new generation during fertilisation. But most cells in the body are not germ cells – they could be bone cells, muscle cells, skin cells and so on. These other cells are called somatic cells. Mutations in somatic cells (somatic mutations) cannot be passed on to the next generation.
A phenotype with a mutant gene is called a mutant. But real mutants aren’t like teenage mutant ninja turtles or spiderman or the cat/bird in the picture above. They are usually not very different to their parents. One reason is that a drastic change in a gene would probably be fatal. Another is that most characteristics are the result of information carried by very many genes. The chances that the right mutations would occur in all these genes to produces a new feature, like, for example, wings on a horse, are vanishingly small.
Any chemical changes in the DNA bases that change their ability to transmit or implement genetic information will result in a mutation. They can be caused spontaneously by errors in replication of genetic information (see post 21.14). Or they can be caused by an external agent called a mutagen. Mutagens can be biological, physical or chemical. For example, infection by a virus can lead to incorporation of viral DNA into a host chromosome. Radiation, for example γ-rays (from radioactive decay) or ultra-violet radiation (in sunlight) can provide the activation energy to initiate chemical reactions. And some chemicals can be absorbed by a cell and react with the DNA in its nucleus.
Some mutagens can cause changes that lead to uncontrolled, rapid cell division. These cells form tumours that consume the body’s resources – the resulting diseases are called cancers. Mutagens that cause cancers are called carcinogens and the formation of cancerous tumours is called carcinogenesis.
21.24 The genetic code
21.23 Implementing genetic information
21.14 Copying genetic information
21.12 The DNA double helix
21.11 Genes are made of DNA
21.10 Chromosomes 2
21.9 Chromosomes 1
21.2 Nucleic acids