OCR A-Level Biology: Nucleic Acids and Enzymes
6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
Before a cell divides, each molecule of DNA must be copied so that both daughter cells receive a complete and identical genome. This copying is described as semi-conservative.
Describe and explain how a molecule of DNA is replicated semi-conservatively. In your answer you should refer to the enzymes involved, the role of complementary base pairing, and explain why the process is described as semi-conservative.
(6 marks)
A student investigated how temperature affects the activity of the enzyme catalase, which breaks hydrogen peroxide down into water and oxygen. At each temperature they added the same volume of a catalase suspension to the same volume of hydrogen peroxide solution and recorded the total volume of oxygen collected in the first 30 seconds. The results are shown below.
| Temperature / °C | Total volume of O₂ collected in 30 s / cm³ |
|---|---|
| 10 | 6.0 |
| 20 | 12.0 |
| 30 | 21.0 |
| 40 | 27.0 |
| 50 | 9.0 |
| 60 | 0.0 |
(a) Calculate the mean rate of oxygen production at 40 °C, in cm³ s⁻¹. Show your working. (2 marks)
(b) Explain the shape of the trend in rate between 10 °C and 60 °C. (4 marks)
An enzyme was supplied with increasing concentrations of its substrate, and the initial rate of reaction was measured. The experiment was repeated with two different inhibitors, P and Q, each added at the same fixed concentration. The results are shown below.
| Substrate concentration / mmol dm⁻³ | Rate, no inhibitor / µmol min⁻¹ | Rate with P / µmol min⁻¹ | Rate with Q / µmol min⁻¹ |
|---|---|---|---|
| 1 | 20 | 9 | 10 |
| 2 | 34 | 18 | 17 |
| 5 | 56 | 40 | 28 |
| 10 | 72 | 64 | 36 |
| 20 | 80 | 78 | 40 |
| 40 | 80 | 80 | 40 |
(a) State the maximum rate (Vmax) reached with no inhibitor and with inhibitor Q. (1 mark)
(b) Using the data, identify which of P and Q is a competitive inhibitor and which is a non-competitive inhibitor. Explain your answer. (4 marks)
Scientists measured the percentage of each of the four bases in nucleic acid samples taken from three newly studied organisms, R, S and T. The figures are shown below.
| Sample | Adenine / % | Thymine / % | Cytosine / % | Guanine / % |
|---|---|---|---|---|
| R | 30 | 30 | 20 | 20 |
| S | 18 | 30 | 28 | 24 |
| T | 24 | 24 | 26 | 26 |
One of these samples is single-stranded DNA, one is double-stranded DNA with a high proportion of C-G pairs, and one is double-stranded DNA with a high proportion of A-T pairs.
Using the data, identify which description fits each sample and explain your reasoning. (5 marks)
A dairy company makes lactose-free milk using the enzyme lactase, which hydrolyses lactose into glucose and galactose. Instead of mixing the enzyme freely into the milk, the company traps the lactase inside small porous alginate beads packed into a column, and runs the milk slowly through the column. The treated milk that leaves the column contains no free lactase.
Suggest two advantages of trapping the lactase in beads rather than mixing it freely into the milk, and explain each advantage. (4 marks)
Many enzymes require a non-protein cofactor in order to function. Two types of cofactor are coenzymes and prosthetic groups.
Describe one difference between a coenzyme and a prosthetic group, and give one named example of each. (3 marks)