Question 1
Paper 1A style
Which mutation is most likely to cause a frameshift?
- Substitution of one base
- Deletion of three adjacent bases
- Insertion of one base
- Insertion of six adjacent bases
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Answer: C ;
Question 2
Paper 1A style
A base substitution changes the codon GAA to GAG, but the polypeptide is unchanged. What explains this?
- The genetic code is universal.
- The genetic code is degenerate.
- The mutation occurred in an intron.
- Proofreading removed the change.
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Answer: B ;
Question 3
Paper 1B style
Samples of the same bacterial culture were exposed to ultraviolet (UV) light for different times. After exposure, the frequency of mutants unable to make a particular amino acid was measured (practice data). (a) Describe the relationship between exposure time and mutant frequency. [2] (b) Calculate how many times greater the mutant frequency is at 40 s than at 0 s. [1] (c) Suggest why mutants occur in the sample with no UV exposure. [1] (d) Explain how UV light increases the frequency of mutation. [2]
| Exposure time / s | Mutants per 10⁶ surviving cells |
|---|---|
| 0 | 2 |
| 10 | 15 |
| 20 | 38 |
| 30 | 61 |
| 40 | 80 |
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- (a) mutant frequency increases as exposure time increases / positive correlation;
- (a) increase is roughly linear / steady between 10 and 40 s / rises from 2 to 80 per 10⁶ cells; Accept use of data
- (b) 80 ÷ 2 = 40 (times);
- (c) spontaneous mutations / errors in DNA replication or repair (not caused by UV);
- (d) UV is a mutagen / mutagenic radiation;
- (d) UV changes the structure of DNA / causes adjacent bases to bond together;
- (d) leading to incorrect base pairing / changed base sequence when DNA is replicated or repaired;
- (d) longer exposure causes more DNA damage, so more mutations;
- max 6
Question 4
Paper 2A style
Distinguish between the consequences of a mutation in a germ cell and in a somatic cell.
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- germ-cell mutation can be inherited / passed to offspring, whereas somatic mutation is not inherited;
- germ-cell mutation will be in every cell of the offspring, whereas a somatic mutation is only in cells descended from the mutated cell;
- somatic mutation can cause cancer (in the individual);
- max 2
Question 5
Paper 2B style
Explain the role of gene mutation in evolution by natural selection.
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- mutation is the original source of all genetic variation;
- mutation creates new alleles;
- meiosis/sexual reproduction only recombines existing alleles;
- mutations occur at random / not in response to need;
- most mutations are harmful or neutral (for an individual);
- some mutations give an advantage in a particular environment;
- individuals with advantageous alleles are more likely to survive and reproduce;
- so these alleles increase in frequency in the population over generations;
- without mutation there would be no (new) variation for selection to act on;
- max 5