Mutations and mutagens (SACE Stage 2 Biology)

What this dot point is asking

You need to classify mutations, explain how each type affects the resulting protein, and describe what causes mutations. This dot point ties together the genetic code, transcription and translation, and protein structure.

What a mutation is

A mutation is a permanent change in the DNA base sequence. Mutations can occur spontaneously during DNA replication or be caused by mutagens. A mutation in a body (somatic) cell affects only that individual; a mutation in a gamete can be inherited.

Mutations are the ultimate source of new alleles and therefore of genetic variation, which is essential raw material for evolution.

Gene (point) mutations

These affect one or a few bases within a gene.

Substitution

One base is replaced by another. The effect depends on the new codon:

• Silent mutation - the new codon codes for the same amino acid (thanks to the degeneracy of the code), so the protein is unchanged.
• Missense mutation - the new codon codes for a different amino acid, which may change protein shape and function (sickle cell anaemia is a missense example).
• Nonsense mutation - the new codon becomes a stop codon, ending translation early and producing a shortened, usually non-functional protein.

Insertion and deletion

One or more bases are added or removed. Unless the number added or removed is a multiple of three, this shifts the reading frame (a frameshift mutation). Every codon after the change is misread, so the protein from that point is almost always completely altered and non-functional. Frameshifts therefore tend to be more damaging than substitutions.

Chromosomal mutations

These involve larger sections of DNA or whole chromosomes, often arising during meiosis. Examples include deletion (loss of a segment), duplication (a segment is repeated), inversion (a segment is reversed), and translocation (a segment moves to another chromosome). Changes in chromosome number, such as the extra chromosome 21 in Down syndrome (non-disjunction), are also chromosomal mutations.

Mutagens

A mutagen is any agent that increases the rate of mutation. Examples include:

• Ionising and UV radiation (X-rays, gamma rays, ultraviolet light), which damage DNA bases or cause incorrect bonding.
• Chemical mutagens such as those in tobacco smoke and some industrial chemicals.

Many mutagens are also carcinogens, because mutations in genes controlling cell division can lead to cancer.

A worked case: sickle cell anaemia

SACE likes a real example that ties the whole pathway together. In sickle cell anaemia a single base substitution in the beta-globin gene changes one codon so that the amino acid glutamic acid is replaced by valine in the haemoglobin protein. This single missense change alters how haemoglobin molecules fold and interact, causing them to stick together and distort red blood cells into a sickle shape under low oxygen. The chain is therefore: point mutation in DNA, one altered codon in mRNA, one changed amino acid, altered protein shape and function, altered cell shape, altered phenotype. It also shows how one mutation can be both harmful (anaemia) and, in malaria regions, advantageous (resistance to malaria in carriers), linking mutations to natural selection.

Why effect depends on where and what

The same type of mutation can have very different effects depending on its position. A substitution in the third base of a codon is often silent because of the degeneracy of the genetic code, whereas a substitution that changes an amino acid in an enzyme's active site can destroy its function. A frameshift near the start of a gene wrecks almost the entire protein, while one near the very end may affect only a few residues. A mutation in an intron or in non-coding DNA may have no effect at all on the protein. Strong answers always reason about both the type of mutation and where in the gene it falls.