Bacteriophage Lambda – 2 Multiple Choice Questions
1. Which of the promoter requires a low level of cI gene for its activation?
a) PR
b) PL
c) PRE
d) PRM
Answer: d
Explanation: The promoter requiring low level of cI gene for its activation is PRM (Promoter for repressor maintenance). It is inactivated by high levels of the cI gene.
2. Choose the correct statement with respect to lysogen and prophage.
a) The integrated phage is called as lysogen
b) The infected bacterial cell is called a prophage
c) If there is only growth of few lysogens, the E. coli lawn would be turbid
d) If lysogens won’t be formed, the plaques won’t be clear
Answer: c
Explanation: The integrated phage is called as a prophage and the infected bacterial cell is known as lysogen. Lysogens are immune to further infection by the same phage. If only growth of few lysogens would take place, the lawn would be turbid. In the acse, if lysogens aren’t formed, the plaques would be clear.
3. Choose the incorrect statement for cro and Q proteins.
a) They are required for the lytic life cycle to continue
b) Cro protein inactivates the PRM promoter
c) The Q protein is responsible for the expression of late genes
d) cII protein is required for the lytic life cycle to continue
Answer: d
Explanation: The cro and Q proteins are required for the lytic life cycle to continue and cII protein is not required for it. Cro protein inactivates the PRM promoter and the Q protein is responsible for the expression of late genes. These late genes are responsible for producing coat proteins, allowing assembly of functional phage and cell lysis.
4. Induction of lysogen takes place because of __________
a) Low levels of cII gene
b) Low level of cI gene
c) Low levels of both cI and cII gene
d) High levels of both cI and cII gene
Answer: b
Explanation: Induction of lysogen takes place because of low levels of the cI gene. It is caused by specific proteolysis under the action of host recA protein.
5. Which of the phage genes are responsible for phage excision?
a) xis gene
b) int gene
c) recA protein
d) both xis and int gene
Answer: d
Explanation: As the level of cI genes fall, the repression of PR and PL genes is lifted up. Now phage genes are expressed and xis and int genes are expressed. These genes are responsible for causing phage excision.
6. How many phases are there for replication of lambda DNA to take place?
a) 1
b) 2
c) 3
d) 4
Answer: b
Explanation: There are two phases for the lambda DNA replication to take place. They include the theta mode and rolling circle mode.
7. The phase generating additional circular DNA molecules in the first phase does it by __________
a) bidirectional theta mode
b) rolling circle mode
c) separation of the two strands
d) copying only one strand
Answer: a
Explanation: The first phase includes circularization of the molecule by annealing of the cos sites and they replicate in a bidirectional theta mode. This leads to the generation of an additional circular DNA molecule.
8. Choose the incorrect statement with respect to rolling circle replication.
a) It is for a single replication fork and yields a concatemeric molecule
b) Staggered cleavage of DNA takes place at the cos sites
c) The cleavage generates 10 nucleotide long overhangs
d) Without the cos sites, the packaging won’t be possible
Answer: c
Explanation: The second phase constitutes of rolling circle mode. It is done with a single replication fork and yields concatemeric molecules. They are required for assembly into mature phage particles. The packaging region should be flanked by the cos sites. Phage DNA is inserted into the phage head and the cos sites are brought adjacent to each other. It is followed by cleavage at cos sites and it leaves 12 nucleotide long staggered ends. The cos sites are required for packaging to take place.