Skip to main content
SingaporeBiologySyllabus dot point

What types of mutation occur in DNA, and how do they affect the protein produced and the organism?

Describe the types of gene and chromosome mutation and explain their effects on protein structure and phenotype

A focused answer to the H2 Biology Molecular Genetics outcome on mutations. Substitution, insertion and deletion; silent, missense, nonsense and frameshift effects; chromosome mutations; and how mutations drive variation, disease and resistance.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to describe the types of mutation (gene mutations such as substitution, insertion and deletion, and larger chromosome mutations), to explain their effects on the protein and the phenotype (silent, missense, nonsense, frameshift), and to recognise that mutations are the ultimate source of new alleles and hence of variation. It draws on the genetic code and on transcription and translation.

The answer

Gene (point) mutations

A gene mutation is a change in the base sequence of a gene. The main types are:

  • Substitution. One base is replaced by another. This changes at most one codon.
  • Insertion. One or more bases are added.
  • Deletion. One or more bases are removed.

Effects on the protein

The effect depends on how the codons change.

  • Silent. The new codon codes for the same amino acid (thanks to degeneracy); the protein is unchanged.
  • Missense. The new codon codes for a different amino acid; the protein may work less well or differently.
  • Nonsense. The new codon is a stop codon; translation ends early and a shortened, usually non-functional protein results.
  • Frameshift. An insertion or deletion of a number of bases not divisible by three shifts the reading frame, so every codon downstream is misread. This usually destroys the protein.

Chromosome mutations

Larger-scale changes affect whole chromosomes or sections: deletion, duplication, inversion or translocation of a segment, or a change in chromosome number (such as non-disjunction producing an extra chromosome).

Significance

Mutations are usually neutral or harmful, but occasionally beneficial. They are the original source of new alleles and therefore of the genetic variation on which natural selection acts.

Examples in context

Example 1. Sickle cell anaemia. A single substitution in the haemoglobin beta-globin gene changes one amino acid (glutamate to valine), a missense mutation that alters the protein's behaviour and causes red cells to sickle. It also illustrates a heterozygote advantage, since carriers have some resistance to malaria.

Example 2. Antibiotic resistance. A chance mutation in a bacterial gene can alter the target of an antibiotic so the drug no longer binds. In the presence of the antibiotic this beneficial mutation is selected for, linking mutation directly to natural selection and to a major public-health problem.

Try this

Q1. Name the type of point mutation that introduces a premature stop codon. [1 mark]

  • Cue. A nonsense mutation.

Q2. Explain why an insertion of three bases is usually less harmful than an insertion of one base. [2 marks]

  • Cue. Three bases add one whole codon without shifting the reading frame, so only one extra amino acid is added; one base shifts the frame and misreads every codon downstream.

Q3. State one environmental factor that increases the rate of mutation and explain how it acts. [2 marks]

  • Cue. Ultraviolet light (or ionising radiation) damages DNA, for example by causing bonds to form between adjacent bases, increasing the chance of errors during replication.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksDistinguish between a substitution mutation and a deletion mutation, and explain why a deletion within a gene usually has a more serious effect on the protein produced.
Show worked answer →

The answer should define both and then justify the greater effect of deletion.

A substitution replaces one base with another, changing at most one codon. A deletion removes one or more bases from the sequence.

A substitution often changes only a single codon, which may still code for the same amino acid (silent) or for a different one (missense), so the protein may be unchanged or only slightly altered.

A deletion of one or two bases (not a multiple of three) shifts the reading frame, because the code is read in consecutive triplets from a fixed start. Every codon downstream of the deletion is therefore misread, so most of the amino acid sequence is changed and the protein is usually completely non-functional. A premature stop codon may also be introduced.

Markers reward correct definitions, the frameshift consequence of the deletion, and the contrast that a substitution affects at most one codon while a deletion affects all subsequent codons.

Original3 marksExplain how a single base substitution can sometimes have no effect on the phenotype of an organism.
Show worked answer →

The answer draws on the degeneracy of the genetic code.

The genetic code is degenerate, so most amino acids are coded by more than one codon. A substitution, especially in the third base of a codon, may produce a different codon that still codes for the same amino acid.

Because the amino acid sequence of the protein is unchanged, the protein folds and functions normally, and the phenotype is unaffected. Such a change is called a silent mutation.

A complete answer may add that even a missense change in a non-critical region can leave function essentially unchanged. Markers reward the use of degeneracy, the unchanged amino acid sequence, and the term silent mutation.

Related dot points