Question

Given below are the diagrammatic representations of the replicating fork of DNA in E. coli. Study the diagrams and answer the questions that follow.

(a) Which one of the three diagrams (i), (ii) or (iii) is the correct representation of the replicating fork of DNA replication? Explain your answer.
(b) Name the enzyme used in E. coli to join the newly synthesised fragments of DNA.

Hint:

In DNA replication, DNA polymerase adds nucleotides only in the 5′ to 3′ direction. DNA ligase seals nicks between Okazaki fragments on the lagging strand.

Answer 1

(a) Step 1: The correct diagram is (ii).
Step 2: DNA replication occurs in the 5′ to 3′ direction. At the replication fork, one strand (leading strand) is synthesized continuously, while the other (lagging strand) is synthesized discontinuously in short fragments. 
Step 3: In diagram (ii), both forks show correct orientation: the leading strand is synthesized towards the fork and the lagging strand away from it, consistent with the antiparallel nature of DNA and enzymatic action of DNA polymerase. 
(b) Step 1: The enzyme used is DNA ligase. 
Step 2: DNA ligase joins Okazaki fragments on the lagging strand by catalyzing the formation of phosphodiester bonds.