Quick questions on Types of mutation: point, silent, frameshift and chromosomal: HSC Biology Module 6

A point mutation changes a single base pair in the DNA. There are three structural sub-types.

Substitutions are further classified by what they do to the protein.

A frameshift is caused by an insertion or deletion of a number of bases not divisible by three. Every codon downstream of the mutation is shifted, so the amino acid sequence past that point is essentially random and a premature stop codon usually appears within a few codons. The resulting protein is truncated and non-functional.

A chromosomal mutation changes the structure or number of whole chromosomes. These affect many genes at once.

A mutation in a gamete (egg or sperm) is a germline mutation and is passed to offspring. A mutation in a somatic (body) cell is not inherited but can still cause local effects such as cancer.

One base is replaced by another (e.g. A to G). The reading frame is unchanged; at most one codon is altered.

An extra base is inserted into the sequence.

A base is removed from the sequence.

Sickle cell anaemia is a single substitution (A to T) in the beta-globin gene, changing codon 6 from GAG to GTG. This is a missense mutation: glutamic acid becomes valine. The altered haemoglobin polymerises under low oxygen, deforming red blood cells.

Substitutions can be silent, missense or nonsense. The category depends on the codon's effect.

An insertion or deletion of three bases preserves the reading frame and only adds or removes one amino acid (in-frame indel), not a frameshift.

Translocation moves a segment to a different chromosome; inversion flips a segment within the same chromosome.

Only germline mutations contribute to allele frequencies in a population.