What are mutations?
A genetic defect may be due to a mutation in a particular individual and therefore not run in families. The word comes from the Latin "mutare", which means:
"To change"
A mutation may involve a change in a single base on a DNA molecule. Causes of mutation include:
- Radiation from the ground, space or X-ray machines
If a mutation takes place in a sex cell, it can be passed down to future generations. If the mutation takes place in any other cell in the body, it will affect only that person - it will not affect his or her descendants.
Mutation is not necessarily a bad thing. Some mutations have no impact at all while others can result in organisms with improved characteristics. For this reason mutations have formed the basis for the development of different living things. However, mutations do often lead to unwanted characteristics - for example, they may be pathogenic (cause illness).
Are there different kinds of mutation?
Yes. Here are some examples:
Example 1:
A series of three bases like AAG (which codes for the amino acid lysine) will change to AGG if the adenine in the middle is replaced with guanine through a mutation. AGG codes for an entirely different amino acid (arginine) and so the lysine in the relevant protein will be replaced with arginine.
This mutation may mean that the protein is no longer able to carry out its function. This may result in illness. In fact it could lead to the death of the cell or even the entire organism.
However, if the substitution of lysine with arginine does not affect the function of the protein, the mutation will have no impact at all.
Example 2:
In the example above the mutation took place in the gene's coding region but mutations can also happen in a gene's regulatory region. In this case the protein in question will be completely unchanged but the mutation may affect the regulation of the production of this protein. Too little or too much of that protein may then be made. Changes like this can also lead to illness.
Example 3:
Mutations don't have to take place in an actual gene: they can also happen in the DNA outside the genes. We don't know what function this DNA has and so it is known as junk DNA.
We all have loads of mutations in this junk DNA. They have no effects at all. But they can be used as markers in genetic research and as a means of identifying criminals (DNA fingerprinting).
Are there other types of mutation in a gene?
Yes. There is a completely different type of pathogenic change in a gene known as a trinucleotide repeat.
Example 4:
We learned earlier that each link in a chain of DNA consists of a sugar molecule to which a phosphate group and an organic base are attached. This unit is known as a nucleotide. A trinucleotide consists of three such nucleotides in a row and so contains the code for one amino acid.
Trinucleotide repeats are where the same type of trinucleotide is repeated over and over. They often sit at the end of the coding region of the gene.
This means that the protein in question gains a tail made up of a chain of identical amino acids. A protein is produced purely on the basis of the code given by the gene and so a protein will always consist of the specified amino acids in the specified order.
It has been discovered that a number of diseases are caused by trinucleotide repeats. It has also been found that the longer the chain of identical amino acids on the protein in question, the earlier these diseases emerge and the more severe the consequences they have. The best-known disease of this type is Huntington's chorea.
How do you identify "disease causing" genes?
We can tell whether a disorder is hereditary by seeing whether it is more prevalent in some families than others.
We can also see whether a disorder is hereditary by seeing whether adopted children suffer from a disorder that is found in their biological family but not in their adopted family.
We can also study whether identical twins both suffer from the same problem because they have identical genes.
Many hereditary disorders appear only if the person in question has inherited the defective gene from both of his or her parents.
Having one gene in a chromosome pair functioning normally is often enough to prevent any problems. Of course a healthy person cannot know whether he or she has a defective gene and so may get together with someone who has the same genetic defect. This increases the risk of their children having a double set of the defective gene.
We are keen to learn which genes are responsible for hereditary problems. One of the purposes of the Human Genome Project to map the human gene pool, is to make it easier to identify pathogenic genes.