Edexcel IGCSE Biology · Spec 3.34-3.37B
Mutation (Triple)
Mutations and mutagens, and how they can lead to variation and to cancer.
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Mutation (Triple)
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Explained
From a changed base to a changed organism
Change one base in the sequence of DNA and you have a mutation. To see why that can matter, follow the chain forwards.
A gene codes for a protein. The order of bases determines the order of amino acids. The order of amino acids determines how the chain folds. The folded shape determines what the protein does.
So a change to a base can change an amino acid, which can change the shape, which can stop the protein working. Every link in that chain is a can rather than a will, and that is the point of the topic.
Why most mutations do nothing
Three reasons, and questions expect at least one.
Much of the DNA in a cell does not code for a protein at all, so a change there has no effect on any protein.
Several different base triplets code for the same amino acid, so a change can produce a triplet that still calls for the same one and the protein is unaltered.
And even where an amino acid does change, it may be in a part of the protein that does not affect the shape of the active site or the working region, so the protein still functions.
When a mutation does matter
If the changed amino acid falls in a critical position, the folding changes. In an enzyme that means the active site changes shape, so the substrate no longer fits and the enzyme cannot catalyse its reaction.
That is why a mutation in a single gene can produce an inherited condition, and why the effect depends entirely on which gene and which part of it was altered.
A few mutations are beneficial, producing a protein that works better or differently in a way that suits the environment. These are much rarer than the harmful ones, and they are the raw material of evolution.
Mutagens and cancer
Mutations happen spontaneously during DNA replication, and mutagens increase the rate. Ionising radiation, meaning ultraviolet, X-rays and gamma rays, and certain chemicals including those in tobacco smoke are the examples named.
A mutagen increases the number of mutations. It does not decide which ones occur, and it cannot produce a useful one to order.
If a mutation affects a gene controlling cell division, the cell may divide uncontrollably, forming a tumour. That is how mutagens are linked to cancer, and a carcinogen is simply a mutagen that causes cancer this way.
What the examiners say about this topic
A principal examiner report for Edexcel International GCSE Biology describes a four mark question that placed natural selection in an unfamiliar context, and reports that many candidates gained all four marks.
The answers it praises share a structure. One is described as correctly stating that mutation generates variation, and that people with the advantageous characteristic would survive and reproduce. Another stated that those individuals survive, reproduce and pass on the allele, and that a mutation had generated that allele in the first place.
A weaker answer gained three of the four for saying that some survive, reproduce and pass on the allele, without saying where the allele came from.
That missing step is the one this topic supplies. Mutation is the origin of the variation, and starting the chain there rather than at survival is what turns three marks into four.
Somatic and inherited
A mutation in an ordinary body cell affects only that cell and its descendants. It can cause a tumour, but it is not passed to any offspring.
A mutation in a gamete can be inherited, and will then be present in every cell of the resulting organism. Distinguishing the two explains why some conditions run in families and others do not.
Spec 3.34-3.37B
What you need to know
- Recall what a mutation is
- Explain how a mutation can change the phenotype
- Know that most mutations have no effect
Active recall
Quick check
Answer each question before opening the answer.
How can a mutation change a protein?
It changes the base sequence, which may change the amino acid sequence, producing an altered or non-functional protein.
Are most mutations harmful?
No — most are neutral (no effect), some are harmful, and rarely one is beneficial.
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