Bacterial and Phage Genetics Questions and Answers

Q: Frederick Griffith used smooth (S) and rough (R) strains of Streptococcus pneumoniae in his classical experiment that showed DNA might be the genetic element. Which ONE of the following observations gave the clue for this discovery?

The clue was that the R strain became S strain when mixed with heat killed S strain.In 1928 Griffith injected mice with live non-virulent rough (R) pneumococci together with heat-killed virulent smooth (S) cells. Neither alone killed the mice, but the mixture did, and live S bacteria were recovered. Something from the dead S cells had permanently transformed the R cells; Avery, MacLeod and McCarty later showed the 'transforming principle' was DNA.R remaining R would show nothing. S becoming R with killed R cells was not the observation, and mixing R with live S cells proves nothing, because live S bacteria are already present.

Q: Entry of λ phage lysogen to lytic phase is triggered by

Induction is triggered by loss of co-operativity in binding of \(\lambda\) repressor.DNA damage activates RecA, which acts as a co-protease and stimulates the cI repressor to cleave itself between its N-terminal DNA-binding domain and C-terminal dimerisation domain. The DNA-binding fragments can no longer dimerise or bind co-operatively to the operators OR and OL, so they fall off, the lytic promoters fire, and the prophage enters the lytic cycle.An increase in repressor would strengthen lysogeny, not end it. A decrease in RecA function prevents induction. A mutation in the phage genome is not the normal trigger.

Q: Which one of the following conjugations will result in formation of merodiploids?

Merodiploids result from an F′ donor × F− recipient cross.An F′ (F-prime) plasmid is an F factor that has picked up a segment of chromosomal DNA. When it is transferred to an F− cell, the recipient gains a second copy of those chromosomal genes on the plasmid, so it is partially diploid, a merodiploid. This is the basis of complementation tests in bacteria.An F+ × F− cross transfers only the F plasmid, with no chromosomal genes. In Hfr × F− crosses, chromosomal DNA does enter but is usually recombined into the recipient chromosome or lost, rather than kept as a stable second copy. An Hfr cell is not a recipient in the usual sense, so the last option is not a productive mating.

Frederick Griffith used smooth (S) and rough (R) strains of Streptococcus pneumoniae in his classical experiment that showed DNA might be the genetic element. Which ONE of the following observations gave the clue for this discovery?

A.R strain became S strain when mixed with heat killed S strain
B.R strain remained R strain when mixed with heat killed S strain
C.S strain became R strain when mixed with heat killed R strain
D.R strain became S strain when mixed with live S strain

Exam Year(s)

GATE Life Sciences - 2012,

Answer: A

The clue was that the R strain became S strain when mixed with heat killed S strain.

In 1928 Griffith injected mice with live non-virulent rough (R) pneumococci together with heat-killed virulent smooth (S) cells. Neither alone killed the mice, but the mixture did, and live S bacteria were recovered. Something from the dead S cells had permanently transformed the R cells; Avery, MacLeod and McCarty later showed the 'transforming principle' was DNA.

R remaining R would show nothing. S becoming R with killed R cells was not the observation, and mixing R with live S cells proves nothing, because live S bacteria are already present.

Discuss in Forum

Entry of λ phage lysogen to lytic phase is triggered by

A.mutation in the \(\lambda\) genome
B.loss of co-operativity in binding of \(\lambda\) repressor
C.increase in the \(\lambda\) repressor concentration
D.decrease in recA function

Exam Year(s)

GATE Life Sciences - 2012,

Answer: B

Induction is triggered by loss of co-operativity in binding of \(\lambda\) repressor.

DNA damage activates RecA, which acts as a co-protease and stimulates the cI repressor to cleave itself between its N-terminal DNA-binding domain and C-terminal dimerisation domain. The DNA-binding fragments can no longer dimerise or bind co-operatively to the operators O_R and O_L, so they fall off, the lytic promoters fire, and the prophage enters the lytic cycle.

An increase in repressor would strengthen lysogeny, not end it. A decrease in RecA function prevents induction. A mutation in the phage genome is not the normal trigger.

Discuss in Forum

Which one of the following conjugations will result in formation of merodiploids?

A.\(F^{+}\) donor × \(F^{-}\) recipient
B.Hfr donor × \(F^{-}\) recipient
C.\(F'\) donor × \(F^{-}\) recipient
D.\(F^{+}\) donor × Hfr recipient

Exam Year(s)

GATE Life Sciences - 2023,

Answer: C

Merodiploids result from an F′ donor × F⁻ recipient cross.

An F′ (F-prime) plasmid is an F factor that has picked up a segment of chromosomal DNA. When it is transferred to an F⁻ cell, the recipient gains a second copy of those chromosomal genes on the plasmid, so it is partially diploid, a merodiploid. This is the basis of complementation tests in bacteria.

An F⁺ × F⁻ cross transfers only the F plasmid, with no chromosomal genes. In Hfr × F⁻ crosses, chromosomal DNA does enter but is usually recombined into the recipient chromosome or lost, rather than kept as a stable second copy. An Hfr cell is not a recipient in the usual sense, so the last option is not a productive mating.

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