Microbial Recombination and Gene Transfer Questions and Answers

Answer: B

Correct answer: Lysogenic phages

Specialised transduction is carried out by lysogenic, or temperate, phages. Such a phage integrates its genome into the bacterial chromosome at a specific attachment site, and when the prophage later excises imprecisely it takes the immediately adjacent bacterial genes with it - the gal and bio genes in the classic lambda example.

Because excision always occurs at the same place, only those particular genes can be transferred, which is exactly what makes the process specialised. A strictly lytic phage such as T4 never integrates, so it can only package random fragments of host DNA and gives generalised transduction instead.

Answer: C

Correct answer: natural transformation

Natural transformation is the only one of these in which DNA travels through the medium as naked molecules. A competent cell takes up free DNA from its surroundings, so nucleases in the extracellular medium destroy that DNA before it can be taken up and the process is blocked.

In conjugation the DNA passes directly from donor to recipient through a mating bridge, never entering the medium, and in transduction it is packaged inside a phage capsid that protects it from nucleases. Sensitivity to added DNase is in fact the classic experimental test used to distinguish transformation from the other two mechanisms.

Answer: A

Correct answer: Zinder and Lederberg

Norton Zinder and Joshua Lederberg discovered transduction in 1952, working with Salmonella typhimurium. Using a U-tube with a filter separating the two arms, they found that genetic exchange still occurred even though the strains never touched - ruling out conjugation - and that the agent passing through the filter was a bacteriophage, P22, carrying bacterial DNA.

Griffith demonstrated transformation in 1928 and Avery and colleagues identified its transforming principle as DNA in 1944. Ivanovsky's work on tobacco mosaic disease in 1892 led to the discovery of viruses, not to transduction.

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: An antibiotic resistance gene

A selectable marker is a gene that allows cells carrying the vector to be identified and grown while those without it are killed, and an antibiotic resistance gene is the standard example. Plating transformed cells on medium containing ampicillin, for instance, means only those that took up an ampicillin-resistance plasmid survive. It solves a real problem: transformation is inefficient, and only a small fraction of cells take up the vector, so without selection the successful ones would be lost among the rest. A second stage is often needed to distinguish cells carrying a vector with an insert from those carrying an empty one. Blue-white screening does this: the cloning site sits within the lacZ gene, so an insert disrupts it, and colonies that cannot cleave the substrate X-gal remain white while empty vectors give blue. The other options are also essential parts of a vector - a restriction site to accept the insert, an origin to replicate, a ribosome binding site to express - but none of them selects.

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: 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: 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: 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: B

Correct answer: Bacterial artificial chromosome

A bacterial artificial chromosome, or BAC, is a vector based on the F plasmid replicon and able to carry inserts of 100 to 300 kilobases - far more than an ordinary plasmid. Its stability made it the principal vector of the Human Genome Project. The vectors form a series by capacity, and the order is worth knowing. A plasmid takes up to about 10 kb; a phage lambda vector about 25 kb; a cosmid, which combines plasmid replication with the lambda packaging signal, about 45 kb; a BAC up to 300 kb; and a YAC, a yeast artificial chromosome, over a megabase - though YACs are prone to instability and chimerism, which is why BACs were preferred for genome sequencing. A useful vector needs three things: an origin of replication, a selectable marker such as an antibiotic resistance gene, and a multiple cloning site with unique restriction sites. The other options are not vectors at all.