Mutations

Mutation

‣ A change to the base sequence of DNA.

Mutagenic agent

• A factor that increases the mutation rate.

‣ Mutations are spontaneous, only the rate of mutation is affected by the environment.

Addition mutation

• When one or more bases are added to the DNA sequence.

‣ Causes a frameshift.

‣ Changes the entire amino acid sequence downstream of the mutation.

‣ More likely to be harmful (or beneficial) as opposed to neutral

because more likely to change the tertiary structure of the protein.

Deletion mutation

• When one or more bases are deleted from the DNA sequence.

‣ Causes a frameshift.

‣ Changes the entire amino acid sequence downstream of the mutation.

‣ More likely to be harmful (or beneficial) as opposed to neutral

because more likely to change the tertiary structure of the protein.

Substitution mutation

• When one or more bases are swapped for one another.

• More likely to be a neutral mutation due to the degenerate nature of the genetic code.

Neutral / silent mutations

‣ No effect on the phenotype of the organism because the functioning protein is still produced.

‣ Most likely following a substitution mutation because only one triplet code is changed (i.e. there is no frameshift).

‣ Due to the degenerate nature of the genetic code, a substitution mutation may not change the amino acid coded for by the triplet.

‣ Even if the mutation does code for a different amino acid, the

amino acid may not be involved with the protein’s function & so

the protein is still functional.

Harmful/beneficial mutations

• Mutations which cause a frameshift are more likely to result in harmful or beneficial mutations.

‣ Changes the amino acid sequence downstream of the mutation.

‣ So changes the sequence of R groups.

‣ So changes the type & position of bonds.

‣ So changes the tertiary structure of the protein.

‣ Can result in a protein which is non-functional (which generally has

a harmful effect on the organism) OR a protein which now

functions better (generally beneficial to the organism).

Why are errors in DNA replication more damaging than errors

in transcription?

‣ Errors in DNA replication can be passed onto daughter cells / offspring → so can have a permanent effect.

‣ Errors in transcription only affect a small number of mRNA molecules, so only affect a small number of proteins. Most mRNA’s produced will not have the error.