AQA A-Level Biology: Control of Gene Expression and Biotechnology

The expression of a gene can be controlled at the level of transcription. This control involves transcription factors and also epigenetic modifications to DNA and to the histone proteins around which DNA is wound.

Describe and explain how the expression of genes is controlled at the level of transcription.

In your answer you should refer to:

• the role of transcription factors binding to the promoter region of a gene, using oestrogen as a specific example of a molecule that switches on transcription
• how epigenetic changes — an increase in DNA methylation and a decrease in histone acetylation — reduce transcription.

(6 marks)

In a paternity investigation, DNA was extracted from a child, the child's mother, and two men who could be the biological father (man 1 and man 2). The same restriction enzyme was used to cut each sample, and the fragments were separated by gel electrophoresis. The DNA fragment sizes detected in each sample are shown below.

Sample	DNA fragment sizes present / bp
Child	2000, 1500, 700, 400
Mother	2000, 1200, 700, 300
Man 1	1500, 900, 400, 250
Man 2	1800, 1200, 600, 250

(a) Explain how gel electrophoresis separates the DNA fragments by size. (3 marks)

(b) Using the data, explain how the banding pattern is used to identify the biological father, and state whether it is man 1 or man 2. (3 marks)

(6 marks)

The polymerase chain reaction (PCR) is used to make many copies of a length of DNA in vitro. The reaction mixture is repeatedly taken through a cycle of three temperatures.

(a) Describe the three temperature stages of one PCR cycle and explain what happens to the DNA at each stage. (3 marks)

(b) A reaction is set up containing 50 copies of a target DNA molecule and is run for 30 complete cycles. Assuming the number of molecules doubles each cycle, calculate the number of DNA molecules present at the end. Use the relationship:

$\text{number of molecules} = \text{starting number} \times 2^{n}$number of molecules=starting number×2n

where $n$n is the number of cycles. (2 marks)

(5 marks)

A biotechnology company wants to manufacture a useful human protein, protein Z, by genetically engineering a population of bacteria so that the bacteria make the protein.

Describe and explain the steps the company would take to produce the recombinant DNA and to get it into the bacteria so that protein Z can be made.

In your answer you should refer to:

• how the gene for protein Z could be obtained
• how the gene is inserted into a vector
• how the vector is taken up by the bacteria
• how cells that have taken up the gene can be identified.

(5 marks)

A tumour-suppressor gene normally produces a protein that helps to control the rate of cell division. In some cells, the promoter region of this tumour-suppressor gene has become hypermethylated (its level of DNA methylation has increased).

Explain how this epigenetic change affects transcription of the gene, and how it could contribute to the development of a tumour.

(4 marks)

In type 1 diabetes, the body's own immune system destroys the insulin-secreting β (beta) cells in the pancreas, so the person can no longer produce enough insulin.

Using your knowledge of stem cells, explain how stem cells could in principle be used to treat type 1 diabetes.

(3 marks)