A species of bacterium makes an enzyme that breaks down a nutrient.
Scientists compare an original strain, P, with two mutant strains, Q and R.
DNA analysis gives this information.
| Strain | DNA information |
|---|---|
| P | The triplet at one position in the gene is CTT. |
| Q | The triplet at this position is CAT. The rest of the DNA is the same as in P. |
| R | The complete protein-coding sequence is the same as in P. One base differs outside the protein-coding sequence. |
The Table shows the amino acids coded for by the two triplets.
| DNA base triplet | Amino acid |
|---|---|
| CTT | leucine |
| CAT | histidine |
The scientists use equal numbers of bacterial cells grown under the same conditions. They measure the total rate of the enzyme-controlled reaction in each sample. They then extract the enzyme and measure the rate produced by a 1.0 mg sample under the same conditions.
| Strain | Total rate for the bacterial sample / µmol min⁻¹ | Rate for 1.0 mg of extracted enzyme / µmol min⁻¹ |
|---|---|---|
| P | 64 | 32 |
| Q | 16 | 8 |
| R | 24 | 32 |
Calculate the percentage reduction in the rate for 1.0 mg of enzyme from strain Q compared with strain P.
Two explanations are proposed for the lower total rate in strain Q.
Use the DNA information, your answer to part (a) and both rate measurements to suggest which explanation is better. Explain your answer.
A laboratory needs a strain that makes less enzyme than strain P, but any enzyme that is made must retain its original shape.
Suggest whether strain Q or strain R is more suitable. Use the DNA information and the results to explain your answer.
184 exam-style questions on OCR GCSE Biology 1.2 What happens in cells (and what do cells need)?, covering 1.2.1 Structure of DNA, 1.2.2 Nucleotides and complementary bases, 1.2.3 Protein synthesis, 1.2.4 DNA structure and the proteins made, 1.2.5 Investigating enzymatic reactions, and 1.2.6 Mechanism of enzyme action. Each one has a worked solution and a mark scheme showing where the marks go.