AP-Micro Microbiology: Microbial Genetics 3 — Questions and Answers
Question 1: Bacteriophage lambda can undergo lysogeny by integrating into the host genome as a prophage. Which viral protein is primarily responsible for this integration?
- Cro protein
- Integrase (Int) (Correct answer)
- CI repressor
- N protein
Correct answer: Integrase (Int)
Lambda integrase (Int) catalyzes site-specific recombination between the phage attP site and the bacterial attB site to insert the prophage.
Question 2: In generalized transduction, which type of bacteriophage can transfer any portion of the host chromosome?
- Lysogenic phages only
- Virulent phages that package host DNA randomly (Correct answer)
- Temperate phages that excise precisely
- Filamentous phages
Correct answer: Virulent phages that package host DNA randomly
In generalized transduction, during lytic infection some phage heads accidentally package random fragments of host DNA instead of phage DNA, allowing any gene to be transferred.
Question 3: What is the function of the recA gene product in E. coli?
- It synthesizes new Okazaki fragments
- It catalyzes strand exchange during homologous recombination and activates SOS response (Correct answer)
- It methylates adenine residues in GATC sequences
- It cleaves mismatched bases during mismatch repair
Correct answer: It catalyzes strand exchange during homologous recombination and activates SOS response
RecA protein binds single-stranded DNA, facilitates homologous strand exchange during recombination, and acts as a coprotease to induce the SOS response.
Question 4: A transposable element that requires an RNA intermediate for movement is called a:
- DNA transposon
- Retrotransposon (Correct answer)
- Composite transposon
- Insertion sequence
Correct answer: Retrotransposon
Retrotransposons transpose via an RNA intermediate that is reverse-transcribed into DNA before integration, similar to the replication cycle of retroviruses.
Question 5: In the lac operon, what happens in the presence of glucose and absence of lactose?
- The operon is fully induced
- The operon is repressed by the lac repressor and CAP is inactive (Correct answer)
- CAP activates transcription even without cAMP
- The repressor is removed by allolactose
Correct answer: The operon is repressed by the lac repressor and CAP is inactive
When glucose is present, cAMP levels are low so CAP cannot activate transcription; when lactose is absent, the lac repressor binds the operator, doubly repressing the operon.
Question 6: Which of the following mutations would most likely be corrected by intragenic suppression?
- A large chromosomal deletion
- A second frameshift mutation at a nearby site that restores the reading frame (Correct answer)
- A missense mutation in a different gene
- A nonsense mutation early in the coding sequence
Correct answer: A second frameshift mutation at a nearby site that restores the reading frame
Intragenic suppression occurs within the same gene, such as when a second frameshift mutation nearby reverses the shift and restores the correct reading frame.
Question 7: DNA methylation by Dam methylase in E. coli methylates which sequence?
- CpG dinucleotides
- GATC sequences at adenine (Correct answer)
- CCGG sequences at cytosine
- AATT sequences at thymine
Correct answer: GATC sequences at adenine
Dam methylase methylates the adenine in GATC sequences, which is important for mismatch repair and regulation of replication initiation.
AP-Micro Microbiology: Microbial Genetics 3
Bacteriophage lambda can undergo lysogeny by integrating into the host genome as a prophage.
Which viral protein is primarily responsible for this integration?