Q.2
In DNA replication, the Okazaki fragments are joined by
(A) DNA helicase
(B) DNA ligase
(C) DNA polymerase
(D) DNA primase

DNA ligase joins Okazaki fragments during DNA replication. This enzyme seals the nicks between fragments on the lagging strand. The correct answer is (B) DNA ligase.

Option Analysis

DNA helicase (A): Unwinds the DNA double helix at the replication fork by breaking hydrogen bonds, creating single strands for replication. It does not join fragments.

DNA ligase (B): Catalyzes phosphodiester bond formation between the 3′-OH of one Okazaki fragment and the 5′-phosphate of the next, linking them into a continuous strand.

DNA polymerase (C): Synthesizes new DNA strands by adding nucleotides to primers; DNA polymerase I removes RNA primers and fills gaps, but does not seal final nicks.

DNA primase (D): Synthesizes short RNA primers needed for DNA polymerase to start Okazaki fragment synthesis on the lagging strand.

In DNA replication, Okazaki fragments joined by DNA ligase ensure accurate lagging strand synthesis. This process is essential for CSIR NET Life Sciences preparation, covering molecular biology fundamentals.

Lagging Strand Synthesis

DNA replication is semi-discontinuous: the leading strand forms continuously, while the lagging strand produces short Okazaki fragments (100-200 nucleotides in eukaryotes) due to the 5′ to 3′ polymerase directionality. Primase lays RNA primers, DNA polymerase extends them, and gaps form between fragments after primer removal.

Role of DNA Ligase

DNA ligase seals nicks by forming phosphodiester bonds, creating a continuous strand. It uses ATP or NAD+ to activate the 5′-phosphate, essential for genome integrity. Mutations impairing ligase activity lead to replication defects.

Other Enzymes Explained

• DNA helicase unwinds DNA, forming replication forks.

• DNA polymerase builds fragments and replaces primers.

• DNA primase initiates synthesis with RNA primers.