Microbial Recombination and Gene Transfer Questions and Answers

Answer: B

Correct answer: metabolic plasmid

Plasmids carrying genes for enzymes that degrade aromatic compounds, pesticides or unusual sugars are metabolic, or degradative, plasmids. The best known are the catabolic plasmids of Pseudomonas - TOL for toluene and xylene, CAM for camphor, OCT for octane - which let the host grow on substrates most bacteria cannot use.

They matter in bioremediation, since the capability can be transferred to other organisms. An F factor is concerned with conjugation and a virulence plasmid with pathogenicity; neither extends the cell's metabolic range.

Answer: A

Correct answer: plant proteins

In plant-microbe interactions the R factors are the products of plant resistance (R) genes, and so are plant proteins. Under the gene-for-gene model each R protein recognises the corresponding avirulence product of the pathogen, directly or through the host protein it modifies, and triggers the hypersensitive response that confines the infection.

They are therefore made by the host and not by the microbe, and they play no part in moving DNA into plant cells - that is the work of the vir genes of the Agrobacterium Ti plasmid. Opines are the modified amino acids that transformed plant tissue makes for the bacterium to consume, an entirely separate idea. Note that in bacterial genetics "R factor" means something quite different: a resistance plasmid.

Answer: D

Correct answer: All of these

Bacteriophages, plasmids and transposable elements are all standard tools of microbial genetics. Phages are used for transduction, for building genomic libraries and as cloning vectors; plasmids are the everyday vectors for cloning and expression, carrying selectable markers and controllable promoters.

Transposable elements are used for insertional mutagenesis and for delivering reporters, since a transposon that lands in a gene disrupts it and simultaneously tags the site so it can be recovered. Each was first studied as a natural agent of gene transfer and then turned into an instrument.

Answer: C

Correct answer: Replicative recombination

Replicative recombination is the mechanism used by mobile genetic elements moving about the chromosome. In replicative transposition the element is copied as it moves through a cointegrate intermediate, leaving one copy at the original site and placing another at the new one, and the reaction requires no homology between element and target.

General recombination is the homology-dependent exchange carried out by RecA, and site-specific recombination acts at defined short sequences such as the phage attachment sites. Transposition is distinguished from both by copying the element as part of the movement.

Answer: A

Correct answer: Lysogeny

Lysogeny is the relationship in which the viral genome is maintained and replicated along with the host chromosome while no new virions are produced. The integrated phage genome is called a prophage, and the bacterium carrying it is a lysogen, which passes it on to every daughter cell.

The prophage represses its own lytic genes and can confer new properties on the host - diphtheria and cholera toxins are both prophage-encoded. Lysis is the opposite outcome, in which progeny virions are made and the cell bursts; transformation and conjugation are bacterial gene-transfer mechanisms, not host-virus relationships.

Answer: B

Correct answer: Insertion element

An insertion sequence is the simplest transposable element: inverted repeats at each end and a single transposase gene between them, with nothing else. It carries only what it needs to move itself, typically about 750 to 1600 base pairs in all.

A composite transposon is larger, being two insertion sequences flanking additional genes such as antibiotic resistance determinants, so it does not answer a description that mentions only the transposase. Viruses and plasmids are not defined by terminal inverted repeats at all.

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.

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: B

Correct answer: F prime factor

An F prime factor is an F factor that carries chromosomal genes. It arises when an integrated F factor in an Hfr cell excises imprecisely, taking a piece of the adjacent bacterial chromosome with it, so the resulting plasmid holds both the fertility functions and those host genes.

Transferring an F prime into a recipient produces a partial diploid, merodiploid for the genes it carries - the classic way of testing dominance and complementation in bacteria. An Hfr cell has the F factor still integrated, and a plain F factor carries no chromosomal genes; an R factor confers drug resistance.

Answer: B

Correct answer: The transmission of genetic information from one independent, mature organism to another

Horizontal gene transfer is the movement of genetic material between organisms that are not parent and offspring. In bacteria it happens by transformation, transduction and conjugation, and it can carry genes across species and even genus boundaries.

Vertical transfer is the other option offered - transmission from parent to offspring during reproduction. The distinction matters because horizontal transfer lets a trait such as antibiotic resistance appear in a lineage that never evolved it. Synthesis of RNA from DNA is transcription, and neither of the synthesis options describes transfer between organisms.