1. 1
   Escherichia coli bacteria were grown in a medium containing glycerol but no glucose or lactose. The activity of β-galactosidase, the enzyme that breaks down lactose, was about 1 unit per mg of cells. Ten minutes after lactose was added, the activity had risen to about 1000 units per mg. In E. coli, a repressor protein binds to a specific DNA sequence (the operator) next to the promoter of the genes for lactose metabolism. When the bacteria were then moved to a medium without lactose, the rate of β-galactosidase synthesis fell sharply within about 3 minutes; the mRNA for this enzyme has a half-life of about 2 minutes.

   (a)

   How does adding lactose lead to transcription of the β-galactosidase gene?

   [1 mark]

   • ALactose binds to the promoter and acts as RNA polymerase
   • BA form of lactose binds to the repressor, changing its shape so it releases the operator and RNA polymerase can transcribe the gene
   • CLactose removes methyl groups from the promoter
   • DLactose is translated into β-galactosidase at ribosomes

   (b)

   Why does the rate of β-galactosidase synthesis fall so quickly after lactose is removed?

   [1 mark]

   • AThe β-galactosidase gene is deleted from the DNA
   • BAll the ribosomes in the cell are broken down
   • CThe repressor binds the operator again and the existing mRNA is quickly broken down by nucleases
   • DThe mRNA is converted back into DNA

   (c)

   Explain the advantage to E. coli of controlling β-galactosidase production in this way.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Researchers compared a liver cell and a pancreatic β-cell taken from the same person. The person's genome contains about 20 000 protein-coding genes. In the liver cell, mRNA was detected from about 11 000 of these genes, including the gene for albumin; in the β-cell, mRNA was detected from about 10 500 genes, including the gene for insulin. mRNA for enzymes of glycolysis was found in both cells, but insulin mRNA was absent from the liver cell and albumin mRNA was absent from the β-cell.

   (a)

   Which is the same in the liver cell and the β-cell?

   [1 mark]

   • AThe genome
   • BThe transcriptome
   • CThe proteome
   • DThe set of genes being transcribed

   (b)

   The liver cell contains mRNA from about 11 000 of the 20 000 genes. What does this show?

   [1 mark]

   • AThe liver cell has lost the other 9000 genes
   • BThe liver cell's genome is smaller than the β-cell's
   • CThe other 9000 genes contain mutations
   • DOnly a subset of the cell's genes is being expressed

   (c)

   Explain how the two cells come to produce different proteins even though they contain the same genes.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A study of 200 adults compared people living in a city district with high traffic air pollution with people living in a rural area with low pollution. In white blood cells, the mean level of methylation of cytosine in the promoter of an immune-regulating gene was 28% in the high-pollution group and 19% in the low-pollution group. The quantity of mRNA transcribed from this gene was about 40% lower in the high-pollution group. Sequencing showed no difference in the base sequence of the gene or its promoter between the two groups.

   (a)

   Explain how an increase in methylation of the promoter could reduce expression of the gene.

   [3 marks]

   (b)

   Evaluate whether this study shows that air pollution changes gene expression.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Two findings are described in a review of epigenetics. First, in a study of monozygotic (identical) twins, the patterns of DNA methylation and histone modification were almost identical in pairs of 3-year-old twins, but differed considerably in many pairs of 50-year-old twins. The differences were greatest in twins who had lived apart for longer and had different diets and smoking habits. Second, ligers (offspring of a male lion and a female tiger) grow much larger than either parent species, whereas tigons (offspring of a male tiger and a female lion) are similar in size to or smaller than their parents, although both hybrids inherit one set of lion and one set of tiger chromosomes.

   (a)

   Discuss what monozygotic twin studies, such as the one described, reveal about the effect of the environment on gene expression.

   [6 marks]

   (b)

   Explain the epigenetic origin of the difference in size between ligers and tigons.

   [6 marks]

   Total for question 4: 12 marks

End of questions