Genetic mutations

Define gene mutations

Any change to the quantity of bases or base sequence in the DNA- results in a change in the amino acid sequence of the polypeptide (the primary structure).

Mutations can arise spontaneously during DNA replication.

What are the 2 main types of gene mutations?

1. Point mutation

2. Frame shift

Describe point mutations

Just affects one nucleotide/base e.g. base substitutions:

• A nucleotide in a DNA molecule is replaced with another nucleotide with a different base

3 outcomes:

1. The new triplet codes for a different amino acid

• The polypeptide produced will differ by one amino acid

• The effect will depend on the precise role/importance of that one amino acid

• May result in a different shape and not function properly

2. The new triplet may still code for the same amino acid (due to degenerate code)

3. The new triplet happens to code for a stop codon- so stopping the production of the polypeptide chain

Describe base deletions (causes framshifts)

• When a nucleotide is lost from the normal DNA sequence

• All the subsequent bases are shifted to the left and so cause all the triplets after the deletion to be read differently

• This can cause all the following amino acids to change

• Frame shifts have a much larger effect on the protein than a point mutation

Define chromosome mutations

Changes in the structure or number of whole chromosomes

• Can arise spontaneously

• 2 forms

What are the 2 forms of chromosome mutation?

1. Changes in whole sets of chromosomes (polyploidy)- when organisms have 3 or more sets of chromosomes rather than the normal 2 (occurs mostly in plants)

2. Changes in the number of individual chromosomes- when an organism has one extra or fewer chromosome- called non-disjunction (due to incomplete separation of chromosomes of chromatids in either division) e.g. Down’s syndrome→ additional 21 chromosome