Question 1

Paper 2Medium16 marks

A researcher follows one replication fork in Escherichia coli. Figure 1 shows the two parental template strands and newly synthesized strands A and B. The arrow indicates the direction in which the replication fork moves.

upper parental templatelower parental template3′5′5′3′5′3′5′3′A3′5′3′5′3′5′Bdirection of fork movementshaded sections = RNA primers; unshaded sections = newly synthesized DNA
(a)

Using Figure 1, state which of A and B is synthesized continuously and which is synthesized discontinuously. Name the short DNA pieces on B.

[3]
(b)

Explain, using the strand orientations in Figure 1, why A requires one RNA primer whereas B requires repeated RNA primers.

[3]
(c)

A newly synthesized lagging-strand region is 2.4 kb long. The mean length of an Okazaki fragment is 200 nucleotides. For this calculation, use 1 kb = 1000 nucleotides. Estimate the number of Okazaki fragments and the number of RNA primers required.

[2]
(d)

In four separate bacterial preparations, the replication fork has already formed and exactly one enzyme is selectively inactivated. All other replication components remain active.

PreparationObservation
PThe existing leading strand continues to extend, but no new RNA primers are detected on the lagging-strand template.
QRNA primers are present, but no DNA is added to their 3′ ends.
RDNA has been extended from the primers, but RNA primer sections remain in the daughter strands.
SRNA primers have been removed and replaced with DNA, but nicks remain between adjacent lagging-strand fragments.

For each preparation, identify the inactive enzyme and explain how its normal function accounts for the observation.

[8]

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