Question

T4 DNA ligase works best at

• A. $4^{\circ}$C
• B. $15^{\circ}$C
• C. $37^{\circ}$C
• D. 4--$15^{\circ}$C

Hint:

Ligation is a "Balancing Act."
Hot ($37^{\circ}$C) = Fast Enzyme, No Annealing.
Cold ($4^{\circ}$C) = Slow Enzyme, Great Annealing.
Middle ($15$-$16^{\circ}$C) = The Sweet Spot!

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
T4 DNA ligase is an enzyme used to join DNA fragments by catalyzing the formation of phosphodiester bonds. The question asks for the optimal temperature for a ligation reaction in a laboratory setting.

Step 2: Detailed Explanation:

• Enzyme Source: T4 DNA ligase is isolated from the T4 bacteriophage. Like many enzymes from organisms that infect bacteria (like E. coli), its biological catalytic activity is highest at $37^{\circ$C}.

• The Ligation Conflict: A ligation reaction involves two separate processes:
- The chemical activity of the enzyme (joining the ends).
- The physical annealing of the DNA ends (hydrogen bonding between sticky ends).

• Temperature Sensitivity: While the enzyme is fastest at $37^{\circ}$C, the hydrogen bonds between short "sticky" ends (cohesive ends) are extremely unstable at this temperature and will constantly break. At $4^{\circ}$C, the bonds are very stable, but the enzyme works too slowly.

• The Compromise: To maximize the efficiency of the reaction, scientists use an intermediate temperature. $15^{\circ$C to $16^{\circ}$C} is considered the "practical optimum" for standard ligation protocols because it provides enough kinetic energy for the enzyme while maintaining stable annealing of the DNA fragments.

• Contextual note: For "blunt-end" ligation, where there are no sticky ends to anneal, the reaction is often done at lower temperatures for longer periods, but $15^{\circ}$C remains the classic textbook answer for general cloning.

Step 3: Final Answer:
Although the enzyme's maximum catalytic speed is at $37^{\circ}$C, ligation reactions are most successful at $15^{\circ}$C because it balances enzyme activity with DNA strand stability.