QUESTION 139 (5 marks)
Cells with the same DNA sequence are tested under four conditions. A specific activating transcription factor is either absent or supplied. The promoter is in open or compact chromatin. The table gives relative mRNA abundance; background values of 1–3 are considered similarly low for this experiment.
| Chromatin | Factor absent | Factor supplied |
|---|---|---|
| Open | 2 | 120 |
| Compact | 1 | 3 |
a) Compare the factor’s effect under the two chromatin conditions using the values. [2 marks]
b) Explain why supplying the factor does not necessarily produce high expression. [2 marks]
c) Explain why the observed differences do not demonstrate a mutation. [1 mark]
Practice marking scheme
Answer
The factor strongly increases expression with open chromatin, from 2 to 120, but not with compact chromatin, from 1 to 3. Factor supply alone is insufficient under these conditions.
Working
Open chromatin permits access to the promoter, allowing the factor to support transcription machinery. Compact chromatin can limit access despite factor supply. Regulation of the same sequence changes mRNA production without requiring a change to the gene’s bases; the table does not demonstrate a DNA mutation.
Marking criteria
- Uses open-chromatin increase 2 to 120. [1 mark]
- Uses compact-chromatin values 1 to 3 as similarly low. [1 mark]
- Links compact chromatin to restricted promoter access. [1 mark]
- Explains that factor activity depends on access/regulatory context as well as its presence. [1 mark]
- Distinguishes expression regulation from a change in DNA sequence. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.