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: D

Correct answer: two IS elements integrate into a chromosome with only a small distance separating them

A composite transposon is formed when two insertion sequences land close together and the DNA between them becomes mobile as a unit. The transposases act on the outermost inverted repeats, so everything lying between the two IS elements - commonly antibiotic resistance genes - is cut out and moved along with them.

Tn5 and Tn10 are built exactly this way, which is how resistance genes acquire mobility they did not originally have. No deletion in the transposase gene is required, and no plasmid is needed for the structure to form.

Answer: C

Correct answer: Non-conjugative plasmids

Non-conjugative plasmids lack the tra genes and so carry no information for their own transfer. Having no way to build a sex pilus or a mating bridge, such a plasmid can move to a new cell only if a conjugative plasmid in the same cell supplies the apparatus, or by transformation or transduction.

Conjugative plasmids are the opposite case, carrying the full transfer region. Cryptic plasmids are defined by having no identifiable phenotype rather than by whether they can transfer, so that term answers a different question.

Answer: A

Correct answer: transformation

Transformation is the uptake of naked DNA from the environment by a competent bacterium and its incorporation into the genome, usually by homologous recombination. It was the first mechanism of bacterial gene transfer discovered, in Griffith's 1928 pneumococcal experiment, and the one Avery, MacLeod and McCarty used in 1944 to identify DNA as the genetic material.

Conjugation requires cell-to-cell contact and transduction requires a phage vector. Lysogenic conversion is the change of phenotype caused by genes carried on a resident prophage, which is a consequence of lysogeny rather than a route of DNA uptake.

Answer: C

Correct answer: Competent

A bacterial cell able to take up naked DNA from its surroundings is described as competent. In naturally transformable species such as Streptococcus pneumoniae, Bacillus subtilis and Haemophilus influenzae, competence is a physiological state that develops at a particular point in growth and involves specific DNA-binding and uptake proteins.

Competence can also be induced artificially in the laboratory, for instance by treating Escherichia coli with calcium chloride and a heat shock, or by electroporation. The other three words have no such technical meaning here.

Answer: D

Correct answer: transduction

Penicillin resistance in staphylococci is classically acquired by transduction. The penicillinase gene lies on a plasmid, and staphylococcal phages package that plasmid and carry it into new cells, which is the principal route by which the resistance spread through hospital strains.

Staphylococci have no efficient natural conjugation or transformation system comparable with that of the enteric bacteria, so those routes are not the usual explanation. Mutation can alter a chromosomal target, but the beta-lactamase responsible here is an acquired plasmid-borne enzyme rather than a mutated host gene.

Answer: C

Correct answer: Virulence plasmid

A virulence plasmid carries genes that make its host more pathogenic - adhesins, invasion determinants, toxins or iron-scavenging systems. The Ent plasmids encoding enterotoxins in Escherichia coli, the pXO1 and pXO2 plasmids of Bacillus anthracis and the invasion plasmids of Shigella are all examples.

An F factor is a fertility plasmid concerned with conjugation and confers no virulence, while a metabolic plasmid encodes degradative or biosynthetic enzymes that widen the range of substrates the cell can use. The classification is by the function the accessory genes serve.

Answer: A

Correct answer: have Px but not the rest of the X coding region

A gene fusion built to monitor gene X joins the promoter Px to a reporter coding sequence such as lacZ. The construct therefore keeps Px, which carries the regulatory information being studied, and discards the X coding region, whose product is not what is being measured.

It must not carry the reporter's own promoter, or the reporter would be transcribed independently of Px and report nothing about gene X. The construct reports on this one promoter, not on every promoter of similar sequence, and it measures promoter activity indirectly through enzyme output rather than giving the direct transcript measurement a microarray provides.

Answer: A

Correct answer: Specialized transduction

In specialised transduction only particular regions of the bacterial genome are transferred. It arises from a lysogenic phage: when the prophage excises imprecisely from its integration site, it takes with it the bacterial genes immediately adjacent to that site - the gal and bio genes in the classic lambda example - and those, and only those, are carried into the next cell.

Generalised transduction can move any region, because it results from random packaging of host DNA fragments during lytic growth. Abortive transduction describes DNA that arrives but is neither integrated nor replicated, rather than which regions are carried.

Answer: A

Correct answer: conjugation

Conjugation is the transfer of genetic material between bacteria in direct physical contact. The donor, carrying a conjugative plasmid, extends a sex pilus that draws the cells together; a single strand of plasmid DNA is nicked at the transfer origin and passed across, and each cell then synthesises the complementary strand.

Transformation uses naked DNA from the medium and transduction uses a phage vector, so neither requires contact between cells. Replication is the copying of DNA within one cell and is not a transfer mechanism.