Microbial Recombination and Gene Transfer Questions and Answers

Answer: C

Correct answer: Transduction

Transduction is the mode of bacterial recombination that uses a viral vector. A bacteriophage packages a fragment of host DNA into its capsid during assembly, and that particle injects bacterial rather than viral DNA into the next cell it infects, where it may recombine into the recipient's chromosome.

The DNA is protected inside the capsid throughout, so the process is unaffected by nucleases in the medium. Transformation uses naked DNA taken from the surroundings and conjugation needs direct cell contact; assimilation is not a mechanism of gene transfer.

Answer: A

Correct answer: Col plasmids

Col plasmids encode colicins - bacteriocins that a bacterium secretes to kill closely related strains competing for the same niche. Colicins act by forming pores in the target's membrane or by cleaving its nucleic acids, and the producing cell protects itself with an immunity protein encoded on the same plasmid. Col plasmids are typically small and present in multiple copies.

R plasmids carry drug resistance determinants, degradative plasmids encode enzymes for unusual substrates such as toluene or camphor, and virulence plasmids carry determinants that make the host pathogenic to a higher organism. Only the Col plasmid is aimed at other bacteria.

Answer: D

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: D

Correct answer: Conjugation

Conjugation is the sharing of genetic material through direct contact between bacterial cells, and it is the mechanism by which advantageous traits such as antibiotic resistance spread through a population. A conjugative plasmid encodes its own transfer machinery, so a single resistant cell can convert its neighbours - and, because plasmid host ranges are often broad, cells of other species too.

The word "sharing" is the clue: the donor keeps a copy while passing one on. Transformation and transduction also move genes horizontally but depend on free DNA or a phage rather than on contact, and fission is ordinary cell division, which passes genes vertically to daughter cells.