Question

Crossing over involves which of the following events:
(A). DNA cleavage
(B). Ligation
(C). DNA degradation
(D). DNA synthesis
Choose the correct answer from the options given below:

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

Hint:

The standard molecular steps of gene recombination are Cut (cleavage), Paste/Repair (synthesis), and Seal (ligation).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during the pachytene stage of prophase I of meiosis.

Step 3: Detailed Explanation:
The molecular mechanism of homologous recombination and crossing over proceeds through highly coordinated enzymatic steps:

• DNA Cleavage: An endonuclease makes double-strand breaks or single-strand cuts in the DNA strands to initiate the exchange. (A is correct)

• DNA Synthesis: DNA polymerase synthesizes new DNA to fill any single-stranded gaps created during strand invasion and migration. (D is correct)

• Ligation: DNA ligase seals the nicks to join the crossed-over strands covalently. (B is correct)

• DNA Degradation: Broad-scale DNA degradation is a destructive process that is avoided to prevent loss of genetic information. Although minor 5' to 3' resection occurs to create single-strand overhangs, general "DNA degradation" is not listed as a primary constructive step.
Therefore, the essential constructive events included are cleavage, synthesis, and ligation.

Step 4: Final Answer
The correct combination is (A), (B) and (D) only.