Microbial Recombination and Gene Transfer Questions and Answers

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: Electroporation

Electroporation applies a brief high-voltage pulse that transiently opens pores in the cell membrane, allowing DNA to enter. The membrane reseals within moments, and the cell recovers. It is one of the standard artificial transformation methods, and it achieves considerably higher efficiency than chemical treatment. It also works on a wide range of cell types - bacteria, yeast, plant protoplasts and mammalian cells - where the chemical methods are more restricted. The alternative for bacteria is the calcium chloride method: incubating cells in cold calcium chloride and then applying a brief heat shock at 42°C, which is simpler and needs no equipment but gives lower yields. Other delivery methods have their own niches: the gene gun or biolistic method fires DNA-coated particles into plant cells; microinjection introduces DNA directly with a needle; Agrobacterium transfers DNA naturally into plants; and viral vectors are used for gene therapy. Conjugation and transduction are natural processes rather than techniques.

Answer: C

Correct answer: antibiotic resistance

R plasmids carry genes conferring resistance to antibiotics, and their spread is the principal mechanism by which resistance moves through bacterial populations. Two features make them so effective. They are usually conjugative, carrying their own transfer genes, so they move from cell to cell directly - and across species boundaries, so resistance acquired in a harmless gut organism can pass to a pathogen. And a single R plasmid often carries several resistance determinants together, frequently assembled by transposons and integrons, so exposure to one antibiotic selects for resistance to all of them at once. That is why multiple drug resistance can appear far faster than mutation alone would allow, and why the misuse of any antibiotic contributes to resistance against others. The mechanisms encoded vary: enzymes that destroy the drug, such as beta-lactamase; efflux pumps that expel it; and altered targets that no longer bind it.

Answer: B

Correct answer: carries genes encoding the mating apparatus

A self-transmissible, or conjugative, plasmid carries the complete tra region - the genes for the sex pilus and the rest of the mating apparatus, together with the transfer origin - so it can organise its own conjugation from start to finish.

A mobilizable plasmid carries the transfer origin but not the mating genes, so it can only move when a conjugative plasmid in the same cell supplies the apparatus. Both types transfer a single strand of DNA, and both may or may not carry resistance genes or transposons, so those options do not distinguish them.

Answer: C

Correct answer: site-specific integration of transposable elements

The transposase gene encodes the enzyme that carries out transposition. Transposase recognises the inverted repeats at the ends of its own element, cuts there, and inserts the element at a target site, generating the characteristic short direct repeats of target DNA on either side.

The reaction needs no extensive homology between element and target, which is what distinguishes it from general recombination - a RecA-dependent exchange between homologous sequences that transposase plays no part in. Viral replication within a genome is unrelated to the enzyme.

Answer: B

Correct answer: is able to take up DNA from its surroundings

Competence is the physiological state in which a bacterial cell can take up naked DNA from its environment - the requirement for transformation. It arises naturally in a number of species, including Streptococcus pneumoniae, Bacillus subtilis, Haemophilus influenzae and Neisseria. In these it is a regulated developmental state, usually appearing at a particular point in growth and often controlled by quorum sensing, with dedicated surface machinery for binding and importing DNA. It can also be induced artificially, which is what makes routine cloning possible. Treatment with calcium chloride at low temperature followed by a brief heat shock renders E. coli competent, and electroporation - a short high-voltage pulse that transiently permeabilises the membrane - achieves higher efficiency still. Competence is unrelated to conjugation, which requires the F plasmid, and to lysogeny, which requires an integrated prophage.

Answer: C

Correct answer: Small, circular DNA molecules that can exist independently of chromosomes commonly found in bacteria

A plasmid is a small, usually circular DNA molecule that replicates independently of the bacterial chromosome. It has its own origin of replication, so it is a replicon in its own right, and it carries accessory genes that are useful in some environments but not required for growth.

A protoplast is a cell stripped of its wall, and the membrane structure covering the bacterial chromosome describes the nucleoid or the mesosome rather than a plasmid. The defining features to remember are independent replication and dispensability.

Answer: D

Correct answer: a small circular DNA molecule that replicates independently of the chromosome

A plasmid is an extrachromosomal DNA molecule, usually circular and double-stranded, carrying its own origin of replication so that it replicates independently of the host chromosome. It is called a replicon for that reason. Plasmids are generally dispensable for growth but carry genes conferring advantage in particular conditions: R plasmids confer antibiotic resistance, F plasmids confer the ability to conjugate, Col plasmids encode bacteriocins, and virulence plasmids encode toxins. Their spread by conjugation is the principal route by which antibiotic resistance moves between bacteria, including between species, which is why resistance can appear far faster than mutation alone would allow. In the laboratory they are the standard cloning vectors: a plasmid with a selectable marker, an origin of replication and a multiple cloning site accepts foreign DNA and carries it into a host for replication and expression. A plasmid that can also integrate into the chromosome is an episome.

Answer: A

Correct answer: Mutation

Drug resistance in Mycobacterium tuberculosis arises by chromosomal mutation. Resistance to rifampicin comes from mutations in rpoB, to isoniazid from mutations in katG or inhA, and to streptomycin from mutations in rpsL - each altering the drug target or the enzyme that activates the drug.

No plasmids or natural gene-transfer systems of clinical significance are known in M. tuberculosis, so transduction, transformation and conjugation play no part. This is why multidrug resistance in tuberculosis is built up stepwise under inadequate treatment, and why combination therapy - which makes simultaneous mutation to several drugs vanishingly unlikely - is the standard regimen.

Answer: B

Correct answer: Transposon

A transposon is a segment of DNA carrying one or more genes flanked at both ends by inverted repeat sequences. The inverted repeats are the sites recognised by transposase, the enzyme that cuts the element out and inserts it elsewhere, and the central genes may include drug resistance or metabolic functions in addition to the transposase itself.

An insertion sequence also has terminal inverted repeats but carries only the transposase gene and no additional genes, which is exactly what the phrase "one or more genes in the centre" excludes. A plasmid is a separate replicating DNA molecule, not a mobile segment defined by inverted repeats.