What is a Mutation?

A mutation occurs whenever there is a change in the genetic information of an organism, due to a variety of causes. There are two classes of mutation: point mutations, and frameshift mutations (Some texts and professors classify frameshift as point mutations; others see it as such a different event with drastically different

Point mutations are single base changes, that do not affect the reading frame; that is, the mutation only makes a single change in a single codon, and everything else is undisturbed.  There are three types of point mutation:

  • Silent Mutation: There is a base change, but the new codon means exactly the same thing as the old one; this is due to the degeneracy of the codon -> amino acid conversion code.  There is no phenotypic change.
  • Missense Mutation: The mutation alters the meaning of the codon, so that the amino acid coded for is not the one that is supposed to be there. This could have no phenotypic effect, if the substituted amino acid was similar in character to the original; it might be a nonfunctional protein, or it could be a conditional lethal, where the protein works under normal conditions, but not in the same operating range as the original protein.
  • Nonsense Mutation:  This mutation changes the codon to a stop codon, which prematurely ends translation when the mRNA transcript is being read by the ribosomes.  This almost always results in a nonfunctional protein, because the latter chunk of it will be missing.

When talking about point mutations, it is important to remember which bases are  purines (A/G) and which are pyrimidines (C/T).  When a point mutation causes a purine to convert to another purine (for example, C to T), this is known as a transition.  When a point mutation changes a purine to a pyrimidine, or vice versa, (i.e., A to T), this is known as a transversion.

Frameshift mutations alter the reading frame of the DNA. They come in two flavors:

  • Insertion: This mutation inserts a base pair (or more) into the DNA, shifting everything to the right (or left, depending on your point of view) by one base pair.
  • Deletion: This mutation deletes a base pair (or more), shifting everything the opposite direction of the insertion.

In either case, it should be obvious that a shift in the reading frame will create a random mess of a protein (much like reading a sentence will if you chop off the first letter of each word and stick it to the end of the previous word). DNA’s coding is in such a way that an altered reading frame will generate stop codons, to limit the amount of energy expended if such mutation occurs. That way, the cell won’t waste energy building the new protein, if it absolutely makes no sense and has no function.

NOTE: It is important to remember that to have a stable, maintained mutation, there must be replication.