Question

5) The following sequence of bases is present along one chain of a DNA double-helix that has opened up at a replication fork, and synthesis of an RNA primer on this template begins by copying the base in bold. 3' - ... TCT GAT ATC AGT ACG ... - 5'

a) If the RNA primer consists of 8 nucleotides, what is its base sequence?

b) In the intact RNA primer, which nucleotide has a free hydroxyl (-OH) terminus and what is the chemical group on the nucleotide at the other end of the primer?

c) If replication of the other strand of the original DNA double-helix proceeds continuously (with few or no intervening RNA primers), is the replication fork more likely to move in a left-to-right or a right-to-left direction?

d) Based on your answer to c), where would helicase be most likely located?

e) Which strand would ligase be closer to, the one listed above or its complementary strand? Explain why.

Answer 1

5)

a) Since the RNA primer is synthesised in the 5'-3' polarity and the bases uracil is present instead of thymidine in RNA,therefore the base sequence of the RNA primer is as 5' AGACUAUA 3'

b) The 3' endof the primer contains the hydroxyl group and 5' end consist of the phosphate group.Therefore, nucleotide containing adenosione at the 3' end contains free hydroxyl group and the nucleotide containing adenosine close to 5' end contains free phosphate group.

c) The other strand of DNA will be in the 5' to 3' direction so the synthesis of new strand will be in a discontinuous fashion as the DNA polymerase only synthesises new DNA strands in the 5'-3' polarity and the direction will be from left to right in a discontinous manner.

d) Helicase also has the polarity 5'-3'.Therefore helicase is present in the direction of the leading strand that is from right to left.

e) The function of DNA ligase is the ligation or bringing together of the Okazaki fragment of the lagging strand. Since in leading strand fragments are not produced ligase is not required so the ligase enzyme is closer to the lagging strand.