Question

The discontinuously synthesized fragments are joined by the enzyme:
(A) DNA ligase
(B) DNA polymerase
(C) Helicase
(D) Lyase

Answer 1

Hint: Replication is a process in which a new DNA strand is synthesised in a semi-conservative manner. During replication one strand is synthesised in the direction of the replication fork and another is synthesised opposite to it, the fragments synthesised in opposite direction are known as okazaki fragments and they need to be joined together.
Step by step solution:
 Process of DNA replication is carried out in the S phase of cell division. Helicase or unwindase is responsible for unwinding or uncoiling of the two complementary DNA strands; the process can be carried out further only when both the strands are separated. Helicase uncoils the strands by breaking hydrogen bonds between the two complementary strands, this process requires ATP or energy. And hence option (C) is incorrect .
DNA polymerase is responsible for synthesis of both lagging and leading strands. It adds nucleotides which are complementary to the template strand in a particular manner. Therefore option (B) is also incorrect.
Lyases are enzymes which are responsible for catalyzing addition and elimination reactions. Therefore option (D) is also incorrect.
DNA ligase is responsible for joining two discontinuous strands together, these discontinuous strands are known as okazaki fragments, these are synthesised in the direction opposite to the direction of unwinding. Therefore option (A) is the correct answer.
Note: during DNA replication DNA-dependent DNA polymerase catalyse polymerisation only in one direction that is 5'\longrightarrow 3' so the template strand with polarity 3'\longrightarrow 5' forms a continuous strand and the template strand with polarity 5'\longrightarrow 3' forms a discontinuous strand.