Edexcel A-Level Biology: Modern Genetics

Before a polypeptide can be made, the base sequence of a gene must first be copied into a molecule of messenger RNA (mRNA) inside the nucleus.

Describe and explain how a molecule of mRNA is produced by transcription, and explain how the mRNA made in a eukaryotic cell is modified before it leaves the nucleus. In your answer you should refer to the role of the enzyme involved, the base-pairing that occurs, and why the modification is necessary.

(6 marks)

A forensic scientist used the polymerase chain reaction (PCR) to amplify a DNA sample recovered from a crime scene. The reaction started with 6 copies of the target DNA sequence. Assuming the amount of target DNA doubles with every cycle, the expected number of copies can be found using:

$N = N_0 \times 2^n$N=N0​×2n

where $N_0$N0​ is the starting number of copies and $n$n is the number of completed cycles.

The number of copies expected after the first few cycles is shown below.

Number of cycles, $n$n	Expected number of copies of target DNA
0	6
2	24
4	96
6	384
8	?

(a) Calculate the expected number of copies of the target DNA after 8 cycles. Show your working. (2 marks)

(b) The scientist needed at least 1 500 000 copies to carry out gel electrophoresis. Using the equation, calculate the minimum number of complete cycles required. Show your working. (2 marks)

(c) Explain why the actual number of copies obtained in a real PCR run is usually lower than the number predicted by this equation. (2 marks)

During gel electrophoresis, DNA fragments of known size (a 'ladder') were run alongside a sample. The distance each fragment migrated from the well was measured. Because the relationship between distance and fragment size is not linear, scientists plot the distance migrated against the base-10 logarithm of the fragment size, which gives a straight calibration line.

Fragment size / base pairs	$\log_{10}(\text{size})$log10​(size)	Distance migrated / mm
5000	3.70	12
2000	3.30	24
1000	3.00	33
500	2.70	42
200	2.30	54

An unknown fragment from the sample migrated 48 mm from the well.

(a) State the relationship between the size of a DNA fragment and the distance it migrates through the gel. (1 mark)

(b) Using the calibration data, estimate the value of $\log_{10}(\text{size})$log10​(size) for the unknown fragment, and hence estimate its size in base pairs. Show your working. (3 marks)

(c) Suggest one reason why estimating the size of a fragment larger than 5000 base pairs from this calibration line would be unreliable. (1 mark)

A child has an inherited disorder caused by a non-functional version of a single gene in the cells lining their airways. The faulty gene means a particular membrane protein is not made, so mucus in the lungs becomes thick and sticky.

Researchers are trialling a gene therapy in which a working copy of the gene is packaged inside a harmless virus. The virus is delivered as an inhaled spray so that it infects the cells lining the airways and inserts the working gene into them.

Suggest and explain why this gene therapy is unlikely to provide a permanent cure and must be repeated regularly, and why it would not prevent the child from passing the disorder on to their own children. (5 marks)

In a long-term study, two groups of genetically identical mice were fed different diets. In the group fed a diet rich in chemical 'methyl donors', many cytosine bases in the promoter region of a particular coat-colour gene became attached to methyl groups. These mice produced almost no of the coat-colour protein, even though the DNA base sequence of the gene was unchanged in both groups.

Explain how the addition of methyl groups to the promoter region can switch off this gene without altering the base sequence of the DNA. (4 marks)

The genetic code is described as degenerate, non-overlapping and universal.

State what is meant by each of these three terms when describing the genetic code. (3 marks)