This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “Molecular Basis of Homologous Recombination”.
1. What is the other name of DSB repair pathway?
a) RecBAD pathway
b) RecBCD pathway
c) RecABD pathway
d) RecDCB pathway
Explanation: The major DSB repair pathway is also known as the RecBCD pathway. Homologous recombination in eukaryotic cells and the proteins are involved in this pathway.
2. The RecBCD pathway involves the use of ligases.
Explanation: Many recombination proteins are involved in the RecBCD pathway. Beside these proteins topoisomerases and ligases are also recruited to play a critical role in the process of gene exchange.
3. What is branch migration?
a) Break and reformation of identical base pairs
b) Formation of lesion
c) Formation of heteroduplex DNA
d) Dissolution occurs
Explanation: Each time holiday junction moves, base pairs are broken in the parental DNA molecules and identical base pairs are created in the recombinants. This process of sliding of the holiday junction forming identical base pairs in the heteroduplex DNA is known as branch migration.
4. What is resolution?
a) Cleavage of holiday junction
b) Regeneration of duplex DNA molecule
c) Exchange of DNA fragments
d) Heterochromatin structure formation
Explanation: In the process of homologous recombination, DNA molecules are regenerated by the process of genetic exchange. This is known as the process of resolution.
5. How many processes are present to achieve resolution?
Explanation: Resolution can be achieved by two different processes. They are, cleavage of the holiday junction or by the process of dissolution.
6. Which of the following promotes strand exchange?
a) DBS formation
b) Heteroduplex formation
c) Strand invasion protein
d) Branch migration
Explanation: Strand invasion initiates the exchange of DNA strands between the two parental DNA. The enzyme that catalyses strand invasion are called strand exchange proteins.
7. Which type of DNA is connected by a holiday junction?
a) Homologous DNA duplex
b) Heteroduplex DNA
c) Mutated DNA
d) Asymmetric DNA
Explanation: Finishing of recombination usually requires resolution of the holiday junction by cutting the DNA strand. Two homologous DNA duplex is connected by a single holiday junction.
8. From what the splice recombination product is formed?
a) Mutated DNA
b) Cross over DNA
c) Spliced DNA
d) Recombined DNA
Explanation: If the two DNA strands are cut and then covalently joined the resulting DNA molecules are referred to as the splice recombination product. Thus the splice recombination product is formed by two spliced DNA parents.
9. What is a crossover product?
a) Mutated DNA
b) Reassorted DNA
c) Crossover DNA
d) Spliced DNA
Explanation: The splice products result in the reassortment of genes that flank the site of recombination. This type of recombinant product is known as the crossover product.
10. What is a patch product?
a) Regions of hybrid DNA
b) Crossover DNA
c) Mutated DNA
d) Recombinant DNA
Explanation: After resolution and covalent joining of the strands the resulting DNA contains a region of hybrid DNA. This is known as a patch thus the DNA produced is known as the patch product.
11. Patch products are non-crossover products.
Explanation: In patch product formation recombination does not result in the reassortment of the genes flanking the site of initial cleavage. These molecules are therefore known as the non-crossover products.
12. Single strand break repair pathway involves genetic exchange.
Explanation: Homologous recombination is involved in the pathway to repair double strand breaks. This pathway is often initiated by the presence of double strand breaks in the DNA.
13. The double strand breaks, the DNA cleaving enzyme generates ____________
a) Double stranded DNA
c) Single stranded DNA
d) Single stranded over hangs
Explanation: DNA cleaving enzyme sequentially degrades the broken DNA on one strand. This produces a stretch of single stranded DNA known as overhanging DNA molecule.
14. In the DSB pathway the 5’-terminal of the broken strand are degraded.
Explanation: In some cases both strands at the double strand break region are processed. Although only the 5’-terminal ends undergo degradation causing the formation of a 3’-overhanging region.
15. What is gene conversion?
a) Change in the DNA sequence
b) Non-reciprocal step in the DSB repair pathway
c) Break of DNA strands
d) Branch migration
Explanation: If the two original DNA duplex were not identical in sequence near the site of break it leads to gene conversion. In the recombination event if the sequence information is lost from the DNA molecule the resultant DNA sequence is replaced to maintain the integrity of the DNA. This non-reciprocal step in DSB repair system is known as gene conversion.
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