Cytogenetics Questions and Answers – Site-Specific Recombination


This set of Cytogenetics Multiple Choice Questions & Answers (MCQs) focuses on “Site-Specific Recombination”.

1. What is the level of sequence homology required for site specific recombination?
a) Only homologous chromosomes participate
b) A homologous stretch of 3-5 kb
c) A homologous stretch of 20-200bp
d) No sequence homology necessary
View Answer

Answer: c
Explanation: Site specific recombination takes place between regions of very short sequence homology about 20-200bp. It can occur between non-homologous chromosomes as well.

2. Which of the following is a must for site-specific recombination reaction?
a) Resolvase
b) Ligase
c) Rejuvinase
d) Recombinase
View Answer

Answer: d
Explanation: Recombinase is a tetrameric protein that binds to the two chromosomes undergoing site-specific recombination reaction. It is a must for the reaction.

3. Site specific reaction between two circular plasmids could result in__________________
a) A single circular chromosome
b) Two recombinant circular chromosomes
c) One recombinant linear chromosome and another circular
d) Two recombinant linear chromosomes
View Answer

Answer: a
Explanation: Site specific recombination in bacterial DNA and viral DNA results in a single circular chromosome. This is used by viruses to integrate into bacterial genome.

4. In recombinase the activated ______________ residue performs the catalytic attack.
a) Serine
b) Threonine
c) Tyrosine
d) Cystine
View Answer

Answer: b
Explanation: The activated threonine residues of one pair opposite recombinase molecules first attack creating the Holliday intermediate. Then the other opposite threonine pair resolves it.

5. Site-specific recombination can be called an economic process. State whether it is true or false.
a) True
b) False
View Answer

Answer: a
Explanation: The transient DNA protein linkage in this recombination process preserves the phosphodiester backbone. Thus no ATP or other high energy molecule is necessary.

6. Which one of these is a cis factor in site specific recombination?
a) Cre
b) Int
c) LoxP
d) Flp
View Answer

Answer: c
Explanation: While Cre, Flp or Int are the trans factors, LoxP is a cis factor or DNA element that is recognized by these trans acting factors.

7. If there is 8 bp sequence flanking oppositely oriented 13bp symmetry elements like LoxP, what will be the fate of it after recombination?
a) It will be excised out
b) It will be inverted
c) It will be broken in the middle
d) It will be inserted in another region
View Answer

Answer: b
Explanation: A flanking sequence between two oppositely oriented symmetry elements will be inverted on site specific recombination.

8. Which of the following is an example of site specific recombination?
a) Conjugation in bacteria
b) Lysogeny in lambda phage
c) Transformation in bacteria
d) Lytic cycle in phage
View Answer

Answer: b
Explanation: The phage particles attach to the bacterial genome using site-specific recombination reaction. This is via the means of att sites.

9. If there is a flanking region between two similarly oriented LoxP sequence, what will be its fate on site-specific recombination?
a) It will remain as it is
b) It will be inserted
c) It will be excised out
d) The regions surrounding it will be exchanged
View Answer

Answer: c
Explanation: There will be excision of the flanking region in this case. This is used by lysogenic phages to excise out from the host cell.

10. Integration of the phage into bacterial genome requires__________________
a) Integrase and XIS
b) Integrase and IHF
c) XIS and IHF
d) XIS and FIS
View Answer

Answer: b
Explanation: Integration is brought along by the integrase-which is the recombinase coded by the lambda phage. It also requires IHF or Integration Host Factor coded by the host.

11. Excision of the lamda phage genome from bacterial genome requires IHF and _______________
a) Integrase from phage and XIS from bacteria
b) FIS from phage and XIS from bacteria
c) FIS from phage and INT from bacteria
d) XIS from phage and FIS from bacteria
View Answer

Answer: d
Explanation: Excision of the genome of lamda phage from the bacterial genome requires IHF and XIS that is coded by the bacteriophage. It also requires the FIS coded by the bacterium.

If the DNA segments are named a, b, c and d respectively; and d is in opposite orientation to a and b, which of the following is dispensable for recombianse action?
a) a
b) b
c) c
d) d
View Answer

Answer: a
Explanation: Segment a is dispensable for the recombination purpose. However segment b and d are not indispensable. Also part c is the actual segment that is inverted.

13. Which of the following is wrong?
a) Site specific recombination between two linear chromosomes result in exchange of segment
b) Site specific recombination between two circular chromosomes will result in insertion
c) Site specific mutation in one linear molecule will lead to excision
d) For recombination to take place recombinase is a must
View Answer

Answer: c
Explanation: Although site specific mutation in one linear molecule can lead to excision, this is not the only case. It can also lead to inversion of the flanking sequence if the surrounding segments are in opposite symmetry.

14. Site specific recombination is not used in__________________
a) Life cycle for lamda phage
b) Expression of specific genes during specific stage of development
c) Promotion of programmed DNA rearrangements in embryonic development
d) Recombination repair
View Answer

Answer: d
Explanation: Homologous DNA recombination can be used for recombination repair; site specific mutation can be used to insert or delete DNA segments and also to express specific genes.

15. In mice you place are protein under an inducible promoter and you insert the gene under study in the loxP flanking region between opposite segments. If this tissue is induced which of the following will you see?
a) There will be no visible result
b) The gene under study will be deleted from specific induced cell
c) The gene under study will be deleted from all other than the induced cell
d) The gene will be inserted in the Cre gene sequence
View Answer

Answer: b
Explanation: This method is used to selectively delete some genes from some tissues in vivo. In this case the Cre gene is activated when an induction signal is provided and the Cre protein so formed acts as a trans factor on LoxP sequence to remove the flanking gene of interest.

Sanfoundry Global Education & Learning Series – Cytogenetics.

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