4. To obtain recombinant products during meiosis, a double-strand break in the DNA yields crossovers needed for chiasmata formation. The progression of the non-crossover and crossover pathways begins with the formation of D loop, however, it may not result in the production of recombinant gametes.
Following statements are made regarding recombination:
A. Expansion of D-loop takes place in non-crossover pathway, but not in the crossover pathway.
B. Expansion of D-loop takes place in crossover pathway, but not in the non-crossover pathway.
C. Ejection of elongating strand takes place in the non- crossover pathway, but not in the crossover pathway.
D. Ejection of elongating strand takes place in the crossover pathway, but not in the non- crossover pathway.
Which one of the following options represents the correct combination of statements that explain the formation of recombinant gametes?
(1) A and B (2) B and C
(3) C and D (4) D and A
The correct answer is (2) B and C, and a detailed stepwise analysis of all options is below. This response also includes an SEO-optimized article featuring a clear title, introduction, key phrase, meta description, keywords, and a slug based on the main search terms regarding D-loop expansion, ejection, and recombinant gamete formation in meiotic recombination.
Introduction
Meiotic homologous recombination is essential for genetic diversity and accurate chromosomal segregation. Central to this process are the repair events triggered by double-strand breaks (DSBs), which may proceed by either crossover or non-crossover pathways. Understanding how D-loop expansion and strand ejection dictate the fate of these repair events is crucial for comprehending recombinant gamete formation in meiosis.
Option-by-Option Analysis
Option A
Expansion of D-loop takes place in non-crossover pathway, but not in the crossover pathway.
This statement is incorrect. D-loop expansion is characteristic of the crossover pathway, where the D-loop is stabilized and extended to facilitate double Holliday junction formation, not typically in the non-crossover pathway where the D-loop is quickly dismantled after short DNA synthesis.
Option B
Expansion of D-loop takes place in crossover pathway, but not in the non-crossover pathway.
This statement is correct. In the crossover pathway, D-loop expansion allows for extensive strand invasion, branch migration, and ultimately formation of Holliday junctions leading to crossovers and recombinant gametes.
Option C
Ejection of elongating strand takes place in the non-crossover pathway, but not in the crossover pathway.
This statement is correct. The non-crossover pathway (synthesis-dependent strand annealing, SDSA) involves the displacement (ejection) of the elongating strand from the D-loop, which then anneals back to the original DSB site without forming a Holliday junction, resulting in gene conversion without crossover.
Option D
Ejection of elongating strand takes place in the crossover pathway, but not in the non-crossover pathway.
This statement is incorrect. Ejection of the elongating strand is a hallmark of the non-crossover SDSA pathway and not the crossover (Holliday junction-forming) pathway.
Correct Combination Table
| Statement | Crossover Pathway | Non-Crossover Pathway | Correct? |
|---|---|---|---|
| A | No | No | Incorrect |
| B | Yes | No | Correct |
| C | No | Yes | Correct |
| D | Yes | No | Incorrect |
Thus, option (2) B and C correctly describe the molecular steps leading to recombinant gamete formation in meiotic recombination.
Keywords
D-loop expansion, strand ejection, crossover pathway, non-crossover pathway, recombinant gametes, meiotic recombination, double-strand break, Holliday junction
Summary:
To obtain recombinant products during meiosis, statements B and C are correct since D-loop expansion happens in crossovers while ejection of the elongating strand defines non-crossover outcomes.
