Regulation of transcription and translationAQA A-Level Biology: Subtopic test

1. 1
   Oestrogen is a steroid hormone released by the ovaries. In a target cell of the uterus lining, oestrogen enters the cytoplasm and binds to a specific receptor protein. The oestrogen-receptor complex then moves into the nucleus. Cells that lack this receptor protein, such as most muscle cells, do not respond to oestrogen.

   (a)

   Why can oestrogen enter the target cell without a channel or carrier protein?

   [1 mark]

   • AIt is a large protein taken in by endocytosis
   • BIt is a charged molecule that moves through protein channels
   • CIt is broken down at the surface and rebuilt inside the cell
   • DIt is lipid soluble, so passes through the phospholipid bilayer

   (b)

   What does the oestrogen-receptor complex do in the nucleus?

   [1 mark]

   • AIt acts as a transcription factor, binding to DNA to stimulate transcription of target genes
   • BIt binds to ribosomes to start translation
   • CIt binds to mRNA to stop its breakdown
   • DIt hydrolyses DNA to release nucleotides

   (c)

   Explain why oestrogen changes gene expression in cells of the uterus lining but not in most muscle cells.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Researchers followed a pair of identical twins from childhood to the age of 50. Both twins have the same DNA base sequence. At the age of 3 the pattern of DNA methylation in their cells was almost identical. At 50 one twin, who smoked and had a very different diet, had much more methylation of the promoter region of one gene, and cells from this twin contained far less mRNA from that gene.

   (a)

   What is the effect of increased methylation of the promoter region of a gene?

   [1 mark]

   • ATranscription of the gene increases
   • BTranscription of the gene is inhibited
   • CThe base sequence of the gene is changed
   • DBases are deleted from the promoter

   (b)

   How does decreased acetylation of histones affect gene expression?

   [1 mark]

   • AHistones coil more loosely, so transcription increases
   • BHistones are removed from the DNA
   • CHistones associate more tightly with DNA, so transcription factors cannot reach the gene
   • DMethyl groups are added to the DNA

   (c)

   Explain how the two twins can show different gene expression even though their DNA base sequences are identical.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Researchers are developing a treatment for a disorder in which liver cells translate too much of a harmful protein. They design short double-stranded RNA molecules, with a base sequence matching part of the mRNA for the protein, and add them to cultured human liver cells. The protein production fell by 85%, but the amount of DNA and the rate of transcription of the gene did not change.

   (a)

   Describe how RNA interference (RNAi) can inhibit translation of the mRNA for the harmful protein.

   [3 marks]

   (b)

   Explain why the treatment reduced the amount of protein even though transcription was unchanged, and suggest three problems in using it to treat patients.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A study of type 2 diabetes looked at 300 twin pairs in which at least one twin had the condition. The other twin also had it in 68% of identical twin pairs and in 22% of non-identical twin pairs. In the general population about 5% of people develop the condition. Identical twins share all of their alleles, non-identical twins share about half, and both types of twin pair grew up in the same household.

   (a)

   Evaluate the data for the relative influences of genetic and environmental factors on the development of type 2 diabetes.

   [6 marks]

   (b)

   Explain how epigenetic changes could account for cases where only one identical twin develops a condition, and how understanding epigenetics could help in the treatment of cancer.

   [6 marks]

   Total for question 4: 12 marks

End of questions