Question

A student mixes various molecules needed for DNA replication. When he adds DNA, replication occurs, but each DNA duplex consists of a normal DNA strand paired with numerous segments of DNA that are a few hundred nucleotides long. What has been left out of the mixture

Answer 1

Answer: DNA Ligase

Explanation:

During DNA replication, both the parental DNA strands are separated and act as templates for the synthesis of new strands. The formation of the new strands occurs in only 5'-3' direction and thus both the strands are synthesized in a different manner. One new strand with the template of 3'-5' direction will be synthesized continuously and is called a leading strand. The other strand with the template of 5'-3' direction will be synthesized discontinuously in the form of DNA fragments. These fragments are called Okazaki fragments and the strand is called a lagging strand. DNA ligase connects these fragments later to form a continuous lagging strand.

Therefore, DNA ligase is required for ligation of Okazaki fragments, and in absence of DNA ligase, the ligation of fragments will not occur and the newly synthesized strand will be discontinuous. However, if DNA ligase is not present in the reaction mixture, then the chances are that not all the newly synthesized DNA strands will be discontinuous. Only the DNA molecule with the lagging strand will be in the form of fragments.