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8.1.2The Control of Gene Expression (A Level only)

The Effect of Genetic Mutations — AQA A Level Biology 7402 Revision Notes & Topical Questions

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📚 AQA A Level Biology 7402 Hub / Topical Revision
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8.1.2 The Effect of Genetic Mutations — AQA A Level Biology 7402 notes: Explain how different types of gene mutation alter the base sequence and how this may change the amino acid sequence and function of the protein produced.A gene mutation is a change in the base sequence of DN…

  • mutagenic agents
  • triplet code
  • degenerate
  • non-overlapping
  • substitution
  • Silent mutation

Section 1 of 9Gene mutations and the genetic code

A gene mutation is a change in the base sequence of DNA.

Mutagenic agents
It arises spontaneously during DNA replication, for example when the wrong base is inserted, but the rate is increased by mutagenic agents such as ultraviolet light, ionising radiation (X-rays, gamma rays), some chemicals (e.g. those in tobacco smoke, benzopyrene) and certain viruses.
Mutations are random: they are not directed towards a need.

To understand the effects, recall that the genetic code is a triplet code: three bases (a codon in mRNA) code for one amino acid.

Degenerate
The code is degenerate (most amino acids are coded by more than one codon), non-overlapping and universal.
There are also start and stop codons.
The sequence of amino acids determines the primary, and hence the tertiary structure of a protein and its function, so a change in DNA may change the properties of the protein.

Section 2 of 9Substitution mutations

In a substitution one base is replaced by another. The effect depends on the codon change:

Silent mutation
Silent mutation the new codon codes for the same amino acid, because of degeneracy of the code (often a change in the third base of a codon). The polypeptide is unchanged; no effect on phenotype.
Missense mutation
Missense mutation the new codon codes for a different amino acid.
  • The effect on the protein may be small (if the new amino acid is similar or far from the active site) or large (if the change affects the tertiary structure, for example by replacing an amino acid forming a hydrogen or ionic bond,
    • or in the active site).
  • Example: sickle cell anaemia, caused by a substitution of a T for an A in the gene for the beta-globin chain,
    • so glutamic acid is replaced by valine;
      • the haemoglobin forms fibres at low oxygen concentration, deforming the red blood cells.
Nonsense mutation
Nonsense mutation the new codon is a stop codon, so translation ends early and a shortened, usually non-functional polypeptide is produced.

Section 3 of 9Insertion and deletion mutations

In an insertion one or more extra nucleotides is added; in a deletion one or more is lost.

Frameshift
If the number of bases added or lost is not a multiple of three, a frameshift occurs: the reading frame changes from that point, so every codon after the mutation is altered.
This usually changes many amino acids and often creates a premature stop codon, so the protein is non-functional.
Frameshift mutations are therefore generally more damaging than substitutions.
If three bases (or a multiple of three) are inserted or deleted, the frame is preserved and only one or a few amino acids are added or lost, which may have a smaller effect.

Worked pattern

  • DNA template TAC GGA CTT codes for mRNA AUG CCU GAA (Met-Pro-Glu).
  • Deleting the first C in the second triplet shifts the reading: TAC GAC TT... giving AUG CUG AA...
  • (Met-Leu-...).
  • All following amino acids change.

Section 4 of 9Other mutations affecting chromosomes

Some mutations involve longer stretches of DNA: duplication of a base sequence, inversion of a section, and translocation where a section moves to a different chromosome. Chromosome mutations include non-disjunction, which produces gametes with an extra or a missing chromosome (e.g. Down syndrome, trisomy 21). Although the specification emphasises gene mutations, these larger changes can disrupt many genes.

MutationChange in DNALikely effect on the protein
Silent substitution1 base replaced; same amino acidNone
Missense1 base replaced; different amino acidVariable; may change shape and function
NonsenseCodon becomes a stop codonTruncated, usually non-functional
Insertion/deletion (not multiple of 3)FrameshiftAll amino acids after the mutation change
Insertion/deletion (multiple of 3)Amino acids added or lostOften smaller effect

Section 5 of 9Consequences: harmful, neutral and beneficial

Many mutations have no effect: they may be silent, occur in non-coding DNA, or give a protein that works equally well.

1Harmful
Some are harmful: examples are cystic fibrosis (often a deletion of three bases removing one amino acid from the CFTR protein), which affects chloride ion transport, and mutations in genes controlling the cell cycle, which cause cancer (see 8.2.10 and 8.2.11).
2Beneficial
A few are beneficial, giving an advantage in a particular environment, for example a mutation conferring antibiotic resistance in bacteria, or lactase persistence in adults in populations that drink milk.
3
These provide the raw material for natural selection (7.3.1).

A mutation only passes to offspring if it occurs in gametes or in cells that give rise to gametes; mutations in somatic cells can cause cancer in the person but are not inherited.

Section 6 of 9Linking structure, function and exam technique

When asked how a mutation affects a protein, follow a chain: change in base sequence, change in mRNA codon(s), change in the order of amino acids (primary structure), changed position of hydrogen, ionic and disulfide bonds, change in tertiary structure, and altered shape of the active site or binding site, so the enzyme or carrier no longer works.

Linking structure, function and exam technique
Always state whether the reading frame has been changed, and give the reason when a change has no effect (degenerate code, or the amino acid is not in the active site).
A question about an enzyme may ask about the substrate not fitting, which prevents enzyme-substrate complexes forming, so reactions are slower.
Link with protein synthesis (4.2 topics on transcription and translation) and with gene expression (8.2).

Section 7 of 9Exam-style questions

  1. A gene mutation changes the sequence of bases from TAC GCT to TAC GCA but the polypeptide produced is unchanged. Explain why. [2]2 marks

  2. Explain why the deletion of a single base from a gene usually has a greater effect than a substitution of a single base. [4]4 marks

  3. Sickle cell anaemia is caused by a substitution mutation in the beta-globin gene. Explain how this single base change alters the haemoglobin molecule. [4]4 marks

Section 8 of 9Key terms

TermMeaning
Gene mutationA change in the base sequence of DNA.
SubstitutionReplacement of one base by another.
InsertionAddition of one or more nucleotides into a DNA sequence.
DeletionLoss of one or more nucleotides from a DNA sequence.
FrameshiftChange in the reading frame caused by insertion or deletion not in multiples of three bases.
Silent mutationA mutation that does not change the amino acid sequence.
Nonsense mutationA mutation that produces a premature stop codon.
MutagenAn agent that increases the mutation rate.

Section 9 of 9Exam technique

1
Saying all mutations are harmful; many are neutral and some are beneficial.
2
Forgetting that the code is degenerate, so a substitution does not always change the amino acid.
3
Describing a frameshift for a substitution; frameshifts come from insertions or deletions that are not multiples of three.
4
Saying mutations occur 'in response to' an environmental need.
5
Not linking the change in amino acid sequence to tertiary structure and function.

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