Microbial Recombination and Gene Transfer Questions and Answers

Answer: B

Correct answer: RecA

RecA is the central protein of homologous recombination in E. coli. It binds single-stranded DNA to form a nucleoprotein filament, searches the genome for a homologous sequence, and catalyses strand invasion - the pairing of the invading strand with its complement in the duplex, which begins the exchange. It has a second role of equal importance. It is the sensor of DNA damage, and when activated by single-stranded DNA it triggers the SOS response by promoting cleavage of the LexA repressor, switching on a set of repair genes. That response is also what induces a prophage to enter the lytic cycle when its host is damaged. Recombination proceeds through a Holliday junction, a four-stranded crossover intermediate resolved by further enzymes. The other proteins named have unrelated functions: RNA polymerase transcribes DNA, sigma factor directs it to promoters, and the Lac repressor regulates the lactose operon.

Answer: C

Correct answer: conjugative plasmids

An R factor carrying both the resistance transfer factor (RTF) and the r determinants is a conjugative plasmid. The RTF portion supplies the transfer genes - the pilus and mating apparatus - while the r determinants carry the resistance genes themselves, so the plasmid can both confer resistance and move itself into a new host.

A plasmid holding only r determinants without an RTF cannot transfer itself and is non-conjugative; it needs a conjugative plasmid in the same cell to mobilise it. The combination of the two components in one molecule is what makes multiple drug resistance spread so rapidly among enteric bacteria.

Answer: C

Correct answer: simultaneous infection of a host cell by two viruses with homologous chromosomes

Viral genomes recombine when a single host cell is infected simultaneously by two related viruses. Both genomes are then replicating in the same compartment, and homologous regions allow crossing over between them, producing progeny with combinations of markers from both parents.

Transduction and transformation are mechanisms by which bacterial DNA moves between cells, not ways in which two viral genomes recombine with each other. Transcription is the synthesis of RNA from a DNA template and has nothing to do with recombination at all.

Answer: B

Correct answer: Transduction

Transduction is the transfer of bacterial DNA from one cell to another inside a bacteriophage particle. During phage assembly a fragment of host DNA is mistakenly packaged into a capsid, and when that particle infects a new cell it injects bacterial rather than viral DNA, which may then recombine into the recipient's chromosome.

Conjugation needs direct cell contact and a mating bridge, and transformation is uptake of naked DNA from the medium; neither involves a phage. Translation is protein synthesis and is not a mode of gene transfer at all.

Answer: C

Correct answer: Transposons

Transposons carry the genes required for their own integration. The transposase encoded within the element recognises the inverted repeats at its ends and catalyses excision and insertion at a new site, so the element supplies the enzyme that moves it.

Plasmids as a class do not: an ordinary plasmid replicates independently and only an episome, or one carrying a transposon or an integration system of its own, can enter the chromosome. A replicon is simply any unit that replicates from a single origin, and "tandon" is not a term in genetics.

Answer: D

Correct answer: amplify a specific DNA sequence into many copies

The polymerase chain reaction amplifies a chosen DNA sequence exponentially, producing millions of copies from a very small starting quantity. It was devised by Kary Mullis in 1983, for which he received the Nobel Prize in 1993. Each cycle has three steps at different temperatures: denaturation at about 95°C separates the strands; annealing at 50 to 65°C lets short primers bind to the flanking sequences; and extension at 72°C lets the polymerase synthesise the new strands. Each cycle doubles the target, so \( n \) cycles give \( 2^{n} \) copies - thirty cycles yield over a billion. The reaction became practical with Taq polymerase, from the thermophile Thermus aquaticus, which survives the denaturation step and so need not be replenished each cycle. Its applications run from diagnosis and forensic identification to cloning and sequencing. Variants include reverse transcription PCR for RNA and quantitative PCR for measuring amount.

Answer: A

Correct answer: F-factor

The bacterial sex factor is the F, or fertility, factor - a conjugative plasmid whose tra genes encode the sex pilus and the mating apparatus. A cell carrying it acts as donor in conjugation and one lacking it as recipient, which is why it is called the sex factor.

The Col factor encodes colicins, bacteriocins that kill related strains, and the R factor carries drug resistance determinants; either may be conjugative, but neither defines mating type. RNA is not a plasmid at all.

Answer: C

Correct answer: conjugation

Transfer of genetic material from a donor to a recipient bacterium through cell contact is conjugation. A conjugative plasmid in the donor encodes the sex pilus that brings the cells together and the machinery that passes a single DNA strand across the mating bridge.

Because the DNA never enters the medium, conjugation is unaffected by nucleases added outside the cell - the standard way of telling it apart from transformation. Transduction requires a phage particle, and recombination is what may happen to the DNA after it arrives rather than the means of transfer.

Answer: C

Correct answer: from a donor to a recipient cell through direct contact

Conjugation transfers DNA from a donor to a recipient through direct cell-to-cell contact, mediated by a sex pilus encoded by the F plasmid. It was discovered by Lederberg and Tatum in 1946. The donor is F+ if it carries the F plasmid free in the cytoplasm, and it transfers a copy of the plasmid alone, converting the recipient to F+ while remaining F+ itself. If the plasmid has integrated into the chromosome the cell is Hfr - high frequency of recombination - and it transfers chromosomal genes in a linear order beginning from the point of integration. Because transfer is sequential and the mating pair usually separates before it completes, the recipient rarely receives the whole chromosome, and the order in which markers appear is used to map genes by interrupted mating. The other options describe the two remaining mechanisms of gene transfer: transduction by a phage, and transformation by uptake of free DNA.

Answer: A

Correct answer: conjugation

The F, or fertility, factor is the plasmid that makes conjugation possible. Its tra region encodes the sex pilus and the rest of the mating apparatus, and its transfer origin is where the DNA strand is nicked before being passed to the recipient, so a cell carrying it is a donor and one without it is a recipient.

When the F factor integrates into the chromosome the cell becomes Hfr and transfers chromosomal genes in a fixed order - the basis of interrupted mating maps. Replication, transduction and transcription involve no F factor.