Which plasmid carries the genetic information necessary to produce a pilus and initiate conjugation?

• A. R Plasmid
• B. Col Plasmid
• C. F Plasmid
• D. Chromosomal Plasmid

Answer: (C)

Conjugation relies on a donor cell making a sex pilus and transferring DNA to a recipient. The genetic instructions for those mating functions are found on conjugative plasmids. The plasmid that carries these essential genes is the F plasmid, the fertility factor. It contains the tra genes and the genes for the F pilus, enabling the donor to contact a recipient and initiate DNA transfer. The transfer often starts with nicking the plasmid and using rolling-circle replication to move a DNA strand into the recipient. This plasmid can exist on its own or integrate into the chromosome to form an Hfr strain, but the key point is that the F factor provides the machinery for pilus formation and DNA transfer. Other plasmids, like those carrying antibiotic resistance (R plasmids) or colicin genes (Col plasmids), do not primarily provide the conjugation machinery. The concept of a “chromosomal plasmid” isn’t a standard category. Therefore, the plasmid carrying the information necessary for pilus production and initiation of conjugation is the F plasmid.