Question
The diagram shows a nucleosome from the nucleus of a eukaryotic cell.
(a) Identify the protein labelled in the diagram. [1]
(b) Outline how nucleosomes affect the transcription of DNA. [1]
The image shows the regulation of the gene responsible for producing lactase.
(c) Identify:
(i) X, the enzyme which copies a DNA sequence [1]
(ii) Y, non-coding DNA at the start of a gene. [1]
(d) Explain the role of lactose in the expression of the gene for lactase production. [3]
(e) State one reason that identical twins may show different methylation patterns as they grow older. [1]
Answer/Explanation
(a) Identify the protein labelled in the diagram.→ Histone
This protein is part of the nucleosome DNA wraps around histones to form chromatin.
(b) Outline how nucleosomes affect the transcription of DNA.
Nucleosomes compact the DNA and can prevent access to transcription factors and RNA polymerase.
This reduces transcription when DNA is tightly wound.
(c) Identify:
(i) X, the enzyme which copies a DNA sequence → RNA polymerase
It’s the enzyme that synthesises mRNA from a DNA template.
(ii) Y, non-coding DNA at the start of a gene → Promoter
This region helps RNA polymerase know where to begin transcription.
(d) Explain the role of lactose in the expression of the gene for lactase production. [3]
- In the absence of lactose, a repressor protein binds to the operator, blocking RNA polymerase from transcribing the gene.
- When lactose is present, it binds to the repressor protein, changing its shape.
- This causes the repressor to detach from the operator, allowing RNA polymerase to transcribe the gene → lactase is produced.
Lactose acts as an inducer that switches the gene ON.
(e) State one reason that identical twins may show different methylation patterns as they grow older.
Environmental factors, such as diet, stress, or exposure to chemicals, can lead to differences in DNA methylation.