L50: Conjugation

• What are conjugative plasmids?

  Plasmids that can transfer genes to other cells
• What do conjugative plasmids encode?

  Genes that allow transfer to other cells
• How do some plasmids differ in their transfer capabilities?

  Some transfer only to the same species, others are promiscuous
• What do tra genes in plasmids do?

  They are responsible for DNA transfer
• How large are the genes required for plasmid transfer?

  About 33 kbp
• What is the function of the F pilus?

  Connects mating pair for DNA transfer
• What does F stand for in F pilus?

  Fertility factor
• How does the F plasmid spread through F- strains?

  Similar to an infectious agent
• What happens during the transfer of plasmid DNA?

  One strand is nicked and transferred
• What is Rolling Circle Replication (RCR)?

  A method for copying plasmids during transfer
• What initiates replication in RCR?

  RNA primer starts DNA polymerase
• What happens to the new DNA after transfer in HFr strains?

  Most is degraded, some may recombine
• What is a merodiploid strain?

  A haploid strain that is diploid for some genes
• How does the distance from the origin affect gene transfer?

  Further genes take longer and are less likely to transfer
• What is an F' plasmid?

  A plasmid that has some chromosomal genes
• What is the role of plasmids in antibiotic resistance?

  They carry genes that confer resistance
• What is the significance of the Lederberg and Tatum experiment?

  It demonstrated gene transfer via conjugation
• What did Davies' experiment demonstrate about gene transfer?

  It required cell-to-cell contact for transfer
• What is the typical size range of plasmids?

  1 kb to >1 Mbp
• What is the function of episomes?

  They can integrate into the host genome
• How do plasmids affect bacterial virulence?

  They carry virulence factors like toxins
• What are the key messages about plasmids and conjugation?

  • Plasmids exist outside chromosomal DNA
  • Control their own replication and copy number
  • Conjugation requires cell-to-cell contact
  • HFr strains can transfer part of their genome
• What are the roles of plasmids in bacteria?

  • Carry non-essential but useful genes
  • Provide antibiotic resistance
  • Encode virulence factors
  • Produce bacteriocins to inhibit related species
• What evidence supports the mechanism of conjugation?

  • Lederberg and Tatum's experiments showed gene transfer
  • Davies' experiments ruled out other transfer methods
  • Semipermeable filters demonstrated need for contact