Nucleic Acids and Protein Synthesis: Question 3

Worked solution

Part (a): The role of DNA polymerase

Once a template strand is available, DNA polymerase builds the new complementary strand alongside it. It moves along the template strand and:

• selects free (activated) DNA nucleotides that have paired, by complementary base pairing, with the exposed bases on the template strand,
• catalyses the formation of phosphodiester bonds between the sugar of one nucleotide and the phosphate of the next, joining adjacent nucleotides together to extend the new strand.

DNA polymerase can only add nucleotides to a strand in one direction. As a result, alongside one of the two template strands it builds the new strand continuously, in one unbroken run, while alongside the other template strand it can only work a short distance at a time, so the new strand there is produced as a series of short, separate fragments.

Part (b): The role of DNA ligase

The short fragments produced on one of the two new strands are not yet a complete strand, they need to be joined together. DNA ligase catalyses the formation of phosphodiester bonds between the adjacent fragments of newly made DNA, sealing the gaps between them so that they become linked into a single, continuous new strand, just like the strand built continuously by DNA polymerase alongside the other template.

Part (c): Why replication is semi-conservative

At the end of replication, two DNA double helices exist where there was originally one. In each of these two new (“daughter”) molecules, one strand is an original (parental) strand that has been conserved unchanged from the parent molecule, and the other strand is entirely newly synthesised. Because each daughter molecule conserves exactly half, one of the two strands, of the original molecule, the mechanism is described as semi-conservative (“semi” meaning half). This contrasts with a conservative model (in which one daughter molecule would be entirely old and the other entirely new) and a dispersive model (in which old and new DNA would be mixed together within both strands of both daughter molecules). Neither of which is what actually happens.

Final answers

• (a) DNA polymerase joins complementary free nucleotides to each template strand by forming phosphodiester bonds, building one new strand continuously and the other in fragments.
• (b) DNA ligase joins the fragments of newly made DNA together by forming phosphodiester bonds, producing one continuous new strand.
• (c) Semi-conservative because each daughter molecule contains one original (parental) strand and one newly made strand.