Conjugation Practice Questions

Correct answer: The transfer of DNA between cells through direct contact

Conjugation is a horizontal gene transfer mechanism that requires direct cell-to-cell contact. It is often mediated by a specialized structure called a sex pilus produced by the donor cell.

Correct answer: Fertility factor

The Fertility factor, also known as the F factor, is a plasmid that contains the genes necessary for the synthesis of the sex pilus and the transfer of DNA. Cells possessing this plasmid are designated F+ (donors), while those lacking it are F- (recipients).

Correct answer: Sex pilus

The sex pilus, or F pilus, is a protein tube that establishes a bridge between the donor and recipient cells. After attachment, the pilus retracts to bring the two cells into close physical contact for DNA transfer.

Correct answer: A cell with the F factor integrated into its chromosome

Hfr stands for High Frequency of Recombination. These cells have the F factor integrated into their main chromosome, allowing them to transfer chromosomal genes to a recipient cell during conjugation.

Correct answer: It becomes an F+ cell

During F+ x F- conjugation, a copy of the entire F plasmid is usually transferred to the recipient. This converts the F- recipient into a functional F+ donor cell.

Correct answer: Origin of transfer

The oriT is the specific DNA sequence where a specialized enzyme (relaxase) nicks the DNA to begin the transfer process. Only DNA sequences linked to an oriT can be moved through the conjugation bridge.

Correct answer: Relaxase

Relaxase is part of the relaxosome complex that recognizes the oriT. It creates a single-strand break and remains covalently attached to the 5' end of the DNA as it is transferred into the recipient.

Correct answer: A replacement complementary strand is synthesized

Conjugation involves rolling-circle replication. As one strand is displaced and sent to the recipient, the donor cell simultaneously synthesizes a replacement strand so that it retains a complete double-stranded copy.

Correct answer: The recipient stays F- but acquires some chromosomal DNA

In Hfr conjugation, the transfer starts at the integrated oriT and moves into chromosomal DNA. Because the conjugation bridge usually breaks before the entire chromosome and the trailing F factor genes are transferred, the recipient rarely becomes F+ or Hfr.

Correct answer: The F factor excises and carries neighboring chromosomal DNA

F' plasmids result from the imprecise excision of an integrated F factor from an Hfr chromosome. The resulting plasmid carries the original F genes plus a small segment of neighboring chromosomal genes.

Correct answer: Merozygosity

A merozygote is a partially diploid bacterial cell. This occurs when an F' plasmid introduces a second copy of specific genes into a recipient cell that already has those genes on its chromosome.

Correct answer: Surface exclusion

F+ cells express specific proteins (like TraT and TraS) that prevent the formation of stable mating pairs with other F+ cells. This 'surface exclusion' ensures that the energy-intensive conjugation process only occurs with suitable F- recipients.

Correct answer: Minutes

Gene mapping in bacteria is done via 'interrupted mating.' Since the Hfr chromosome is transferred at a constant rate, the time (in minutes) at which specific genes enter the recipient determines their relative position on the circular chromosome.

Correct answer: Gram-negative cells use a sex pilus; Gram-positive cells use surface adhesins

While Gram-negative conjugation is defined by the sex pilus, many Gram-positive species (like Enterococcus) use 'clumping factors.' These are sticky surface proteins induced by pheromones that cause cells to adhere to each other for DNA transfer.

Correct answer: It forms the transmembrane channel for DNA transport

The Type IV Secretion System is a complex molecular machine that spans the inner and outer membranes of the donor. It provides the channel through which the single-stranded DNA and the relaxase enzyme are secreted into the recipient.

Correct answer: Conjugation requires a living donor; transformation uses free environmental DNA

Conjugation requires two living cells to interact physically. Transformation, however, involves the uptake of 'naked' DNA released into the environment by dead or lysed bacteria, requiring no direct contact between donor and recipient.

Correct answer: Origin of transfer (oriT)

The relaxosome is the protein assembly that prepares the plasmid for transfer. It binds to the oriT and facilitates the strand-specific nicking required to initiate the rolling-circle replication and transport.

Correct answer: traA

The traA gene encodes the protein pilin, which is the structural subunit of the sex pilus. Other tra genes are involved in the assembly and processing of the pilus at the cell surface.

Correct answer: Gene A

During chromosomal transfer from an Hfr cell, the genes closest to the oriT are the first to enter the recipient. Therefore, the first gene detected in the recipient (Gene A) is closest to the insertion point of the F factor.

Correct answer: It spreads antibiotic resistance genes rapidly between bacterial species

Conjugation is the primary method for the horizontal spread of antibiotic resistance. Many R-plasmids are conjugative and can move between different genera of bacteria, leading to the rapid emergence of multi-drug resistant 'superbugs' in hospitals.