Mutations, Variation and Natural Selection — Answers
Answer sheet with model answers and success criteria for the Year 11 cells & classification worksheet.
Success TutoringWorksheetsAnswer sheetYear 11 · Science · Cells & ClassificationMutations, Variation and Natural Selection — Answers
Part A: Mutations
A mutation is a change in the base sequence of DNA. In a substitution, one base is replaced by another. In an insertion, an extra base is added. In a deletion, a base is lost. Because DNA is read in triplets (groups of three bases), insertions and deletions shift every triplet after them.
Mutations happen randomly when DNA is copied, but mutagens such as UV light, X-rays and some chemicals make them more likely. Most mutations are neutral, some are harmful and a few are beneficial.
- ASubstitution
- BInsertion
- CDeletion
- DNo mutation
Answer: Substitution
One base (A) has been replaced by another (T); the length of the sequence is unchanged.
- ASubstitution
- BInsertion
- CDeletion
- DNo mutation
Answer: Deletion
The fourth base (G) has been lost, so every base after it shifts one place to the left.
Answer: DNA is read in groups of three bases (triplets). A deletion shifts the reading frame, so every triplet after the mutation changes, and many amino acids may be wrong. A substitution changes only one triplet, so at most one amino acid changes (and sometimes none).
Answer: More than one triplet can code for the same amino acid, so the new triplet may still code for the same amino acid. The mutation may also be in a part of the DNA that does not code for a protein.
Answer: mutagens
Answer: False
Only mutations in the cells that make gametes (in the ovaries or testes) can be inherited. A skin cell mutation affects only that person.
Part B: Natural selection
The theory of evolution by natural selection: individuals in a population show variation. More offspring are produced than can survive, so they compete. Individuals with characteristics best suited to the environment are more likely to survive and reproduce, passing on their alleles. Over many generations, favourable characteristics become more common.
Answer: Any two of: mutations (the original source of new alleles), meiosis (shuffling of chromosomes and crossing over), random fertilisation (any sperm can fuse with any egg).
Answer: B, D, A, E, C
- AIndividuals change during their lifetime to suit their environment
- BPopulations change over many generations as favourable inherited traits become more common
- COrganisms develop new traits because they need them
- DAcquired features, such as large muscles from exercise, are passed to offspring
Answer: Populations change over many generations as favourable inherited traits become more common
Natural selection acts on inherited variation that already exists in a population. Individuals do not evolve; populations do.
Answer: False
A scar is caused by the environment, not by a change in DNA in the gametes, so it cannot be inherited.
Part C: Analysing population data
A species of moth has light and dark forms, controlled by a gene. Birds eat moths that they can see resting on tree trunks. In areas where soot from factories made tree bark dark, dark moths were better camouflaged.
Answer: 90%
- 450 ÷ 500 × 100 = 90%
Answer: 5%
- 30 ÷ 600 × 100 = 5%
Answer: When the tree bark became light again, dark moths were easier for birds to see against it, so more of them were eaten. Light moths were better camouflaged, survived and reproduced more, passing on the allele for light colour, so over many generations light moths became more common.
Part D: Antibiotic resistance
Answer: 50
- 50 000 000 ÷ 1 000 000 = 50
Model answer: Within the bacterial population there is variation: a few bacteria carry a random mutation that makes them resistant to the antibiotic. The antibiotic kills the non-resistant bacteria first, but if the course is stopped early, the resistant bacteria (and some less-affected ones) survive. With less competition, they reproduce rapidly, passing on the resistance gene, so the new population is mostly resistant and the antibiotic no longer works. To slow resistance: always finish the full course of antibiotics; do not use antibiotics for viral infections such as colds; reduce the use of antibiotics in farming; and use good hygiene so infections spread less.
Success criteria:
- Variation exists: some bacteria have a resistance mutation
- The antibiotic kills non-resistant bacteria, so resistant ones survive
- Survivors reproduce and pass on resistance, so it becomes common
- Two valid ways to slow resistance
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.