Microbial Recombination and Gene Transfer Questions and Answers

Answer: D

Correct answer: a random fragment of the host bacterial chromosome

In generalised transduction the phage packaging machinery mistakenly encloses a random fragment of the fragmented host chromosome instead of phage DNA. Any host gene may therefore be transferred, with roughly equal probability. The resulting particle is defective as a phage - it contains no phage genome and cannot replicate - but it can inject its bacterial DNA into a new cell, where it may recombine into the recipient chromosome. The contrast with specialised transduction is the point most often examined. There a temperate phage excises imprecisely from its integration site and carries only the adjacent host genes, so the transfer is restricted to particular loci. The amount of DNA carried is limited by the capsid, typically one to two per cent of the bacterial chromosome, which is why generalised transduction transfers only closely linked genes together - and that limitation is exactly what makes it useful for fine-scale co-transduction mapping.

Answer: D

Correct answer: all of the above

Diagnostic DNA probes exist for all three pathogens. A probe is a labelled nucleic acid fragment complementary to a sequence unique to the organism, and hybridisation reports its presence directly, without the need to culture it.

This matters most where culture is slow, hazardous or impossible: Mycobacterium tuberculosis takes weeks to grow, while hepatitis B virus and the human immunodeficiency virus cannot be cultured routinely at all. Probe and amplification methods give an answer in hours and can also identify the strain or the resistance genotype.

Answer: C

Correct answer: integrates its genome into the host chromosome as a prophage

A lysogenic or temperate phage integrates its genome into the host chromosome as a prophage, where it is replicated passively along with the bacterial DNA and passed to every daughter cell. The host is then a lysogen. The contrast is with the lytic cycle, in which the phage replicates immediately, assembles new particles and lyses the cell. A temperate phage can follow either path, and the choice is governed by a genetic switch - in phage lambda, by the balance between the cI repressor maintaining lysogeny and the Cro protein promoting lysis. DNA damage activates the SOS response, which inactivates the repressor and induces the prophage into the lytic cycle - a sensible strategy, since a damaged host is a poor long-term home. Lysogeny has consequences beyond the phage. Lysogenic conversion gives the host new properties encoded by prophage genes, and the toxins of diphtheria, cholera, botulism and scarlet fever are all prophage-encoded.

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: 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: 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.

Answer: D

Correct answer: all of the above

All three statements hold. A microarray carries DNA segments from many different genes - thousands of spots, potentially a whole genome - whereas a gene fusion reports on a single promoter. It is not built by cloning but by spotting or synthesising oligonucleotides directly on a solid support. And by hybridising labelled cDNA to those spots it measures transcript abundance directly.

A gene fusion does the opposite on each count: one promoter, constructed by cloning, and read out indirectly through the activity of a reporter protein such as beta-galactosidase. The two techniques are complementary, one broad and one deep.

Answer: A

Correct answer: curing

Curing is the loss of a plasmid from a bacterial cell. Because a plasmid is dispensable, its replication can be disturbed without killing the host - by growth at elevated temperature, or with acridine dyes, ethidium bromide or other agents - until daughter cells are produced that no longer carry it.

Curing is a standard tool for deciding whether a trait is plasmid-borne: if the cured strain has lost the antibiotic resistance or toxin production along with the plasmid, the genes lay on the plasmid rather than on the chromosome. The other three words have no defined meaning in this context.

Answer: A

Correct answer: spindle formation

Binary fission involves no spindle. Bacteria are prokaryotes with no nucleus, no centrosome and no mitotic apparatus; the replicated chromosomes are separated instead by attachment to the membrane and by the elongation of the cell itself, with the FtsZ ring driving septum formation.

The other three all occur. The chromosome is duplicated before division, the cell elongates, and cytokinesis divides it into two daughters by ingrowth of septum and wall. Spindle formation is a feature of eukaryotic mitosis and meiosis only.

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.