QUESTION 73 (5 marks)
Two cells from the same organism have the same DNA sequence. Measurements at one gene are shown. Chemical tags associated with compact chromatin are more abundant in cell A.
a) Compare gene expression in the cells using the data. [1 marks]
b) Explain how chromatin structure could contribute to the difference. [3 marks]
c) Explain why this difference does not require a mutation in the gene sequence. [1 marks]
Practice marking scheme
Answer
a) B expresses more mRNA than A. b) A’s compact heterochromatin limits access by transcription machinery; B’s more open euchromatin permits greater transcription. c) Chemical tags/regulation can change expression without changing base sequence.
Working
B has 100 relative mRNA units compared with 5 in A. Compact chromatin can reduce access to a promoter and the DNA template, suppressing transcription. More open chromatin can be accessible to transcription factors and RNA polymerase. Epigenetic differences regulate use of the same DNA sequence rather than necessarily changing that sequence.
Marking criteria
- Uses the data to identify higher expression in B. [1 mark]
- Identifies compact chromatin in A as heterochromatin or less accessible DNA. [1 mark]
- Links reduced accessibility to less transcription. [1 mark]
- Links open chromatin in B to greater access/transcription. [1 mark]
- Explains regulation without a required base-sequence change. [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.