There is a huge variety of life on this planet. In this series we will look at how all that variation exists, from the DNA level to the population level. Today we are starting off with a source of variation at the DNA level.
What are mutations?
A genetic mutation occurs when the DNA base sequence changes. Often this is spontaneous, perhaps due to an error in DNA replication. Because DNA codes for proteins (see the genes to protein series), this can also result in a change to the amino acid sequence of a polypeptide. Consequently, the tertiary structure of the protein can be altered because the bonds (e.g. ionic bonds) forming between amino acid R groups might form in different places. As proteins need their specific tertiary structure to carry out their function, a genetic mutation can result in a non-functional protein.
Although mutations can be spontaneous, sometimes they can happen due to increased exposure to a mutagenic agents. These are things such as radiation (e.g. ultraviolet radiation from the sun) or mutagenic chemicals.
Types of genetic mutation
There are several different types of mutation that affect the DNA base sequence:
- A substitution mutation is where one base is substituted for another. This could lead to one amino acid being changed. However, because the genetic code is degenerate (meaning that more than one triplet can code for the same amino acid, revise here), sometimes the amino acid sequence is not affected at all.
- A deletion mutation is where a base is deleted (funnily enough), so all the triplets from that point onwards are different because all the bases have shifted back one place. This is called a frameshift, and means that many amino acids are affected.
- An insertion mutation is where a base is inserted (I know, I’m stating the obvious), so all the bases from that point onwards are shifted forward a place. So it is another type of mutation that causes a frameshift.
Other types of mutation include duplication (where one or more bases are duplicated, resulting in frameshift) and inversion (where one or more bases are reversed, but the total number of bases is not changed).
Are mutations harmful?
We have already said that sometimes mutations do not affect the protein due to the degenerate nature of the genetic code, so the mutation is neutral and does not affect the organism. Also, if a mutation occurs within an intron, it will not affect the polypeptide chain because introns are non-coding regions of DNA.
Some mutations can be disadvantageous to the organism, for example if the active site of an enzyme essential for respiration was affected then the organism could not survive. However, sometimes mutations result in an advantageous change which increases the survival chances of the organism. This is the basis of natural selection and how variation develops. We will explore this more in the coming weeks.
Summary
- Mutations are changes in the DNA base sequence which can sometimes result in a change in protein structure.
- They can be spontaneous or the result of a mutagenic agent.
- Some mutations such as substitution may not change the amino acid sequence. Some, such as deletion, cause a frameshift.
- Mutations can be neutral, advantageous, or disadvantageous.
