1. During which stage of the cell cycle are gene mutations most likely to occur ?
Answer
During DNA replication in Interphase.
2. Sometimes a substitution mutation does not cause any change in the primary structure of the protein coded for. Explain why.
Answer
The base sequence may change to a different sequence that codes for the same amino acid. The DNA code is degenerate so many amino acids are coded for by more than one triplet.
3. What is meant by a mutagenic agent ?
Answer
An environmental factor that increases the rate of mutation, e.g. tar in tobacco smoke, UV radiation.
4. The Ames test is used to test whether substances cause mutations. In this test, a strain of the bacterium Salmonella typhimurium is used, which has a mutation causing them to grow only on a medium containing histidine. The test detects whether the substance can cause some of the bacteria to mutate back to the strain that can produce histidine naturally. The test is summarised in the figure below. Explain the purpose of the control plate.
Answer
To show that only mutated bacteria grow without histidine.
5. Explain why some colonies have grown in plate X.
Answer
Chance or random mutation.
6. Explain why colonies of bacteria have grown in plate Y.
Answer
The test chemical has caused mutations, allowing bacteria to grow without histidine. The mutations are due to the test chemical rather than chance, because many more mutations occurred in sample Y than in sample X.
7. Give some examples of aseptic technique that would be necessary when conducting the Ames test.
Answer
Sterilise medium before plating, sterilise instruments before use, clean bench with disinfectant, use sterile agar plates, etc.
8. Name the variables that would need to be controlled during the Ames test.
Answer
Temperature, length of incubation and volume of bacteria added to each plate.