Question

In the image shown above, why is one strand continuous while the other shows several discontinuous fragments? Which enzymes are involved in this process and what are their functions? [5]

Answer 1

In the process of replication, synthesis of a new DNA strand always proceeds in the 5’→3’ direction. Since the two strands of DNA run in antiparallel directions, the two templates provide different ends for replication. [1 mark]

The template strand with polarity 3’→5’ synthesizes its complementary strand continuously in the
5’→3’ direction. [1 mark]
Replication is discontinuous on template with polarity 5’→3’ because only a short segment of DNA strand can be built in the 5’→3’ direction due to exposure of a small stretch of template at a time. Short segments of replicated DNA are called Okazaki fragments. [1 mark]

The enzymes that are involved in this process are:
1. DNA-dependent DNA polymerase which uses a DNA strand as the template for the synthesis of a new DNA strand. [½ + ½ mark ]
2. DNA ligase joins the Okazaki fragments of the lagging strand to form a continuous DNA strand. [½ + ½ mark]