3.2.18Reproduction & Inheritance
Mutation: Advanced — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions
5 min read 4 parts Lesson 3 of 4 Unit quiz
Part 1 of 4Types of Gene Mutation
A mutation is a rare, random change in the sequence of bases in DNA. Each triplet of bases codes for one amino acid.
Substitution
- One base is replaced by a different base, which changes at most the one amino acid coded by that triplet.
Insertion
- An extra base is added to the sequence, which changes the triplet in which it occurs and shifts every triplet after it.
Deletion
- A base is lost from the sequence, which changes that triplet and shifts all the triplets that follow it.
Frameshift
- Insertions and deletions alter the way the rest of the sequence is read, so many amino acids after the change are wrong.
| Mutation | Base change | Effect on later triplets |
|---|---|---|
| Substitution | One base swapped | None |
| Insertion | One base added | Shifted |
| Deletion | One base removed | Shifted |
Part 2 of 4From DNA to Phenotype
A mutation affects the phenotype only if it changes the protein that the gene codes for.
1
A change in the base sequence can change the order of amino acids in the protein made from that gene.
2
A different amino acid sequence can change how the protein folds, and so alter its shape.
3
If the shape of a protein changes, its function may change; for example the active site of an enzyme may no longer fit its substrate.
4
This can alter the phenotype, and if the changed gene is in a gamete or an embryo the mutation can be inherited.
5
Many mutations have no effect on the protein, or only a small effect, and so make no difference to the phenotype.
Part 3 of 4Effects and Causes
Effects and Causes
Harmful
- Some mutations cause disease, such as sickle cell anaemia, which results from a change in the gene for haemoglobin.
Beneficial
- Rarely a mutation makes a new allele with an advantage, and natural selection may then make it more common.
Neutral
- Most mutations have no noticeable effect on survival or reproduction of the organism.
Mutagens
- Ionising radiation such as X-rays, gamma rays and ultraviolet light,
- and some chemicals such as those in tobacco tar, increase the mutation rate.
Cancer
- Higher mutation rates can make cells divide uncontrollably, which is why mutagens are linked to cancer.
Part 4 of 4Exam Checklist & Tips
3.2.18 – Key Points for the Exam
- Mutation A random change in the base sequence of DNA.
- Substitution A mutation in which one base is replaced by another.
- Insertion A mutation in which an extra base is added to a DNA sequence.
- Deletion A mutation in which a base is removed from a DNA sequence.
- Mutagen A factor, such as radiation or a chemical, that increases the rate of mutation.
Common Mistakes
- Saying all mutations are harmful; most have no effect and a few are beneficial.
- Thinking a substitution shifts all later triplets; only insertions and deletions do this.
- Writing that mutations are caused only by radiation; they also occur spontaneously.
- Forgetting that a mutation in body cells, unlike one in gametes, is not passed to offspring.
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