Bacterial Genetics: Recombination and Gene Transfer

What biological process involves breaking, rearranging, and combining DNA molecules to produce novel genetic arrangements?

Recombination

What type of recombination involves reciprocal genetic exchange requiring extensive sequence homology between DNA strands?

General recombination (homologous recombination)

During which eukaryotic cell division process does crossing over occur between aligned homologous chromosome pairs?

Meiosis (prophase I)

How does site-specific recombination differ from general homologous recombination regarding sequence requirements?

It occurs between non-homologous DNA molecules lacking extensive sequence similarity and requires a distinct, designated recognition nucleotide sequence recognized by specialized recombinases.

What are the four primary mechanisms of horizontal gene transfer and DNA movement in bacteria?

Transformation, transposition, conjugation, and transduction.

Which researcher discovered the transforming principle in 1928 using Streptococcus pneumoniae?

Fred Griffith

What is cellular competence in bacteria?

The physiological state that allows a bacterial cell to bind and internalize exogenous naked DNA from its surrounding environment.

What distinguishes natural competence in genera like Streptococcus, Acinetobacter, and Haemophilus from non-specific DNA uptake?

Natural competence is an innate, discriminatory process that specifically internalizes DNA released from lysed dead cells belonging to the organism's own genus or species.

How can non-competent bacteria like Escherichia coli be induced to take up foreign DNA in laboratory procedures?

Through artificial competence induced by chemical treatments combined with thermal shifts (alternating hot and cold temperature shocks).

Who discovered mobile genetic elements ('jumping genes') in maize during the 1940s and later received a Nobel Prize?

Barbara McClintock

What structural features define an insertion sequence (IS)?

A short DNA segment bounded on both ends by inverted repeated sequences that encodes the transposase enzyme responsible for excision and insertion.

What structural architecture distinguishes a composite transposon from a simple insertion sequence?

It consists of a central region encoding transposase and accessory genes (such as antibiotic resistance) flanked on both sides by insertion sequence-like elements.

What are bacterial plasmids, and how does their gene content differ from the bacterial chromosome?

Small, circular, self-replicating extrachromosomal DNA molecules with their own origin of replication that encode non-essential accessory traits (such as antibiotic resistance or catabolic pathways) rather than essential housekeeping genes.

Which researchers first documented bacterial conjugation in 1946 as a contact-dependent method of gene transfer?

Joshua Lederberg and Edward Tatum

What did Bernard Davis demonstrate about conjugation using a U-tube partitioned by a semipermeable membrane?

Conjugation strictly requires direct physical cell-to-cell contact, as plasmid transfer failed when cells were separated by a filter that permitted liquid and naked DNA to pass.

What is the structural role of the pilus during conjugative gene transfer?

It tethers the donor cell to the recipient cell and retracts to bring their cell envelopes into direct physical contact; it does not serve as the conduit for DNA transfer.

What are the transfer polarity and final genetic outcome of bacterial conjugation?

Transfer is strictly unidirectional from donor to recipient; following single-strand replication and transfer, both participating cells emerge containing the plasmid.

What viral packaging error during the lytic cycle causes generalized transduction?

During assembly, the viral packaging machinery accidentally encapsidates random fragments of the degraded donor host bacterial chromosome instead of the phage genome.

What viral error during the transition from the lysogenic to lytic cycle causes specialized transduction?

Aberrant or imprecise excision of a prophage from the host chromosome, which accidentally captures adjacent host bacterial genes flanking the viral integration site.

How does the scope of transferred bacterial DNA differ between generalized and specialized transduction?

Generalized transduction can transfer any random segment of the donor bacterial genome, whereas specialized transduction transfers only the specific subset of genes directly flanking the prophage integration site.