Microbial Recombination and Gene Transfer Questions and Answers

Answer: D

Correct answer: Episome

An episome is a plasmid that can either replicate autonomously in the cytoplasm or integrate into the host chromosome and be replicated as part of it. The F factor of Escherichia coli is the classic example: free in an F⁺ cell, integrated in an Hfr cell.

A lysogen is a bacterium carrying a prophage, which is a state of the cell rather than a type of plasmid. "Medisome" is not a term in use, and a lysosome is a eukaryotic organelle unrelated to bacterial genetics.

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: 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: C

Correct answer: McClintock

Barbara McClintock discovered transposable elements in maize in the 1940s, working on the unstable pigmentation patterns of kernels. She showed that certain genetic elements - the Ac/Ds system - could move to new chromosomal positions and alter the expression of nearby genes, and she named them controlling elements.

The work was decades ahead of its acceptance and she received the Nobel Prize in Physiology or Medicine in 1983. Griffith is remembered for the 1928 transformation experiment in pneumococcus, and neither Abelson nor Harvey is associated with transposons.

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: A

Correct answer: Conjugation

Transfer of genetic material between bacteria through cell-to-cell contact is conjugation. A 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 the mating bridge, and each cell then synthesises the complementary strand.

Because the DNA never leaves the shelter of the cells, conjugation is unaffected by nucleases in the medium - the standard test distinguishing it from transformation. Transduction requires a phage particle rather than contact.

Answer: B

Correct answer: Bacterial genome maps

Interrupted mating experiments map the bacterial chromosome. In an Hfr × F⁻ cross the chromosome is transferred in a fixed order starting from the point of F integration, so breaking the mating at intervals and scoring which donor markers have arrived reveals the sequence of genes and the time each takes to enter.

Distances on the resulting map are given in minutes of transfer time - the classic 100-minute Escherichia coli map. The technique reports gene order and spacing only; it gives no nucleotide sequence, no measure of DNA homology, and nothing about the cell's proteins.

Answer: D

Correct answer: all of these

All three properties can be carried on plasmids rather than on the bacterial chromosome. Lactose fermentation is encoded by lac plasmids in some enteric bacteria, enterotoxin production by Ent plasmids in enterotoxigenic Escherichia coli, and antibiotic resistance by R plasmids, which often carry several resistance determinants together.

This is the general point about plasmids: they are dispensable replicons carrying accessory functions that are useful in particular environments but not required for growth. Because they can be transferred between cells, traits such as toxin production and drug resistance can spread horizontally through a population far faster than mutation alone would allow.

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.