Question

Find the correct sequence of reaction catalyzed by DNA ligase: A. DNA ligase ligates Okazaki fragment to rest of lagging strand. B. Polymerase I eliminates downstream RNA primer by nick translation. C. Unwinding of parental duplex by helicase and elongation of leading strand by polymerase III exposes single-stranded region in front of lagging strand. D. Polymerase III extends DNA Okazaki fragments from primer. E. Primase synthesizes RNA primer.

• A. C, E, D, B, A
• B. E, A, D, C, B
• C. B, A, C, E, D
• D. B, C, A, D, E

Hint:

Lagging strand synthesis sequence: helicase unwinds DNA, primase makes primer, DNA polymerase III extends, DNA polymerase I removes primer, and ligase seals the nick.

Answer 1

The Correct Option is A

Concept:
During DNA replication, the lagging strand is synthesized discontinuously as Okazaki fragments. These fragments must be joined by DNA ligase.

Step 1: Helicase unwinds DNA.
The parental DNA duplex is unwound by helicase, exposing single-stranded templates. \[ \text{First step} = C \]

Step 2: Primase synthesizes primer.
Primase synthesizes short RNA primers on the lagging strand template. \[ \text{Second step} = E \]

Step 3: Polymerase III extends Okazaki fragments.
DNA polymerase III adds deoxyribonucleotides and extends the Okazaki fragments. \[ \text{Third step} = D \]

Step 4: Polymerase I removes RNA primer.
DNA polymerase I removes the RNA primer and replaces it with DNA. \[ \text{Fourth step} = B \]

Step 5: DNA ligase joins fragments.
Finally, DNA ligase seals the nick between adjacent DNA fragments. \[ \text{Fifth step} = A \] Thus, the correct sequence is: \[ C,\ E,\ D,\ B,\ A \] \[ \therefore \text{Correct Answer is (A)} \]