Question

Which of the following is true about E. coli DNA ligase
(A) Catalyzes the formation phosphodiester bond between 3'-OH and 5'-$PO_4^-$ group of a DNA strand
(B) Requires ATP as energy source
(C) Requires NAD+ as energy source
(D) It can ligate two molecules of single-stranded DNA
Choose the correct answer from the options given below:

• A. (A), (B), (C) and (D) only.
• B. (A), (B) and (C) only.
• C. (A) and (B).
• D. (A) and (C).

Hint:

An easy way to distinguish DNA ligases is by their cofactor requirement:
- *E. coli* DNA ligase $\rightarrow$ $NAD^+$ dependent.
- T4 DNA ligase $\rightarrow$ ATP dependent.
T4 DNA ligase can also ligate blunt-ended double-stranded DNA, whereas *E. coli* DNA ligase requires cohesive (sticky) ends.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
DNA ligases are essential molecular enzymes that repair nicks in the sugar-phosphate backbone of DNA.
They function by catalyzing the formation of a covalent phosphodiester bond between adjacent 3'-hydroxyl and 5'-phosphate ends.
The cofactors and substrate requirements vary significantly between DNA ligases isolated from different biological sources.

Step 2: Detailed Explanation:
Let us analyze each statement regarding *Escherichia coli* DNA ligase:
Statement (A) is correct because all DNA ligases catalyze the formation of a phosphodiester bond between a 3'-OH and a 5'-phosphate group.
Statement (B) is incorrect because *E. coli* DNA ligase does not use adenosine triphosphate (ATP) as an energy source.
Instead, bacteriophage T4 DNA ligase and eukaryotic DNA ligases are the ones that require ATP.
Statement (C) is correct because *E. coli* DNA ligase specifically utilizes nicotinamide adenine dinucleotide ($NAD^+$) as its cofactor to drive the adenylation of the enzyme active site.
Statement (D) is incorrect because DNA ligases require a double-stranded DNA template to orient the adjacent ends.
They cannot ligate single-stranded DNA molecules.
Evaluating the statements shows that only (A) and (C) are correct.

Step 3: Final Answer:
Statements (A) and (C) are correct.
This corresponds to Option (D).