How do mutations alter the polypeptide a gene codes for, and how can one genome give rise to so many specialised cell types?
Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include addition, deletion, substitution, inversion, duplication and translocation of bases. The degenerate nature of the genetic code means that some substitutions do not change the amino acid coded for. Some gene mutations change only one triplet code; the position of a deletion or addition mutation within a gene is important. Mutagenic agents increase the rate of mutation. Stem cells are unspecialised cells capable of dividing and differentiating, and are described as totipotent, pluripotent, multipotent or unipotent.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on gene mutations and cell specialisation. Covers substitution, deletion, addition, inversion, duplication and translocation, the role of the degenerate code, mutagenic agents, and totipotent, pluripotent, multipotent and unipotent stem cells with their uses.
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What this dot point is asking
AQA wants you to classify gene mutations, explain how each type changes (or fails to change) the polypeptide, link this to the degenerate genetic code, and describe how mutagenic agents raise the mutation rate. You then need to define the four levels of stem cell potency and give realistic uses of stem cells.
Types of gene mutation
A gene mutation is a change in the base sequence of DNA. Most arise spontaneously as random errors during DNA replication.
| Mutation | What happens | Typical effect |
|---|---|---|
| Substitution | One base is replaced by another | Changes at most one triplet; may be silent, missense or nonsense |
| Deletion | One or more bases are removed | Frameshift from the point of deletion |
| Addition (insertion) | One or more bases are added | Frameshift from the point of addition |
| Inversion | A sequence of bases is reversed | Several triplets read in reverse, altering amino acids |
| Duplication | A sequence of bases is copied and repeated | Extra triplets, frameshift if not a multiple of three |
| Translocation | A sequence moves to a different chromosome | Can disrupt genes at both the donor and target site |
Why some substitutions are silent
The genetic code is degenerate: most amino acids are coded for by more than one triplet, because there are triplets but only about 20 amino acids.
A substitution changes a single base, so only one triplet changes. There are three possible outcomes:
- Silent (synonymous). The new triplet still codes for the same amino acid because of the degenerate code, so the polypeptide is unchanged.
- Missense. The new triplet codes for a different amino acid, changing one amino acid in the polypeptide.
- Nonsense. The new triplet becomes a stop codon, producing a shortened (truncated) polypeptide.
Why position matters for deletions and additions
A deletion or addition shifts the reading frame: every triplet after the mutation is read differently. Because of this:
- A deletion or addition near the start of a gene affects almost the whole polypeptide.
- The same mutation near the end of a gene affects far fewer amino acids.
- A deletion or addition of a multiple of three bases does not cause a frameshift, because whole triplets are removed or added.
Mutagenic agents
The natural (spontaneous) mutation rate is increased by mutagenic agents (mutagens), including:
- Ionising radiation (X-rays, gamma rays) and ultraviolet light.
- Chemicals such as benzene, components of tobacco smoke, and some pesticides.
Mutagens increase the mutation rate by damaging DNA or interfering with replication. A mutation in a body (somatic) cell is passed only to that cell's descendants, but a mutation in a gamete can be inherited by offspring.
Stem cells and potency
All the body's cells (with a few exceptions such as mature red blood cells) contain the same genes. Cells become different because they express different genes. Stem cells are classified by how many cell types they can become.
| Potency | Can become | Example source |
|---|---|---|
| Totipotent | Any cell type, including extra-embryonic (placental) tissue | Zygote and first few cells of the embryo |
| Pluripotent | Any cell type of the body, but not extra-embryonic tissue | Embryonic stem cells |
| Multipotent | A limited range of cell types | Adult stem cells, e.g. bone marrow |
| Unipotent | One type of cell only | Cells that make cardiomyocytes; cells of the skin |
Totipotent cells are present only for a short time in the early mammalian embryo. As development proceeds, cells become pluripotent, then multipotent.
Uses of stem cells
- Treating disease and injury. Multipotent stem cells from bone marrow are already used to treat leukaemia and some immune disorders. Research aims to repair damaged tissue, for example in spinal injury, type 1 diabetes, Parkinson's disease and heart disease.
- Induced pluripotent stem cells (iPS cells). Adult body cells (e.g. skin cells) can be reprogrammed by switching on specific genes (transcription factors) so that they become pluripotent again. This avoids the ethical objection to destroying embryos and can produce cells genetically matched to a patient.
- Drug testing and research. Stem cells provide a supply of specialised cells for testing new drugs and studying development.
Try this
Q1. Explain why a deletion of three bases may have less effect on a polypeptide than a deletion of one base. [2 marks]
- Cue. A deletion of three bases removes one whole triplet and does not cause a frameshift, so only one amino acid is lost; a deletion of one base shifts the reading frame, altering every triplet downstream.
Q2. Distinguish between a totipotent and a multipotent stem cell. [2 marks]
- Cue. Totipotent cells can differentiate into any cell type including extra-embryonic tissue; multipotent cells can differentiate into only a limited range of cell types.
Q3. Describe two ways mutagenic agents can affect cells. [2 marks]
- Cue. They increase the rate of mutation by damaging DNA or interfering with replication; mutations in somatic cells may lead to cancer, and mutations in gametes may be inherited.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 AQA Paper 23 marksA substitution mutation in a gene does not always change the structure of the polypeptide the gene codes for. Explain why.Show worked answer →
A 3-mark answer needs the link between the substituted base, the triplet, and the degenerate code.
- A substitution changes only one base and therefore only one triplet (codon).
- The genetic code is degenerate, so more than one triplet codes for the same amino acid.
- If the new triplet still codes for the same amino acid, the primary structure (amino acid sequence) of the polypeptide is unchanged, so its structure and function are unchanged.
Markers reward the explicit link "degenerate code, so the same amino acid is still coded for".
2020 AQA Paper 22 marksExplain why a deletion mutation usually has a greater effect on the polypeptide than a substitution mutation.Show worked answer →
A 2-mark answer must use the idea of a frameshift.
- A deletion causes a frameshift: every triplet after the deletion is read differently (the reading frame shifts by one base).
- This changes many amino acids downstream, whereas a substitution changes at most one, so the polypeptide structure is altered far more.
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