Plasmid

• Plasmid


  A small, circular piece of DNA that is different than the chromosomal DNA, which is all the genetic material found in an organism's chromosomes. It replicates independently of chromosomal DNA. Plasmids are mainly found in bacteria, but they can also be found in archaea and multicellular organisms.
• The word Plasmid was first coined by Joshua Lederberg in 1952. The term was first described in a research paper he published describing the experiments carried out by himself and his student Norton Zinder. The experiment was conducted on salmonella bacteria and its virus P22.
• General types of plasmids

  • Conjugative
  • Non-conjugative
• Conjugative plasmids


  Bacteria reproduce by sexual conjugation, which is the transfer of genetic material from one bacterial cell to another, either through direct contact or a bridge between the two cells. Some plasmids contain genes called transfer genes that facilitate the beginning of conjugation.
• Non-conjugative plasmids


  Cannot start the conjugation process, and they can only be transferred through sexual conjugation with the help of conjugative plasmids.
• Specific types of plasmids

  • Fertility F-plasmids
  • Resistance plasmids
  • Virulence plasmids
  • Degradative plasmids
  • Col plasmids
• Fertility F-plasmids


  Contain transfer genes that allow genes to be transferred from one bacteria to another through conjugation. They are episomes, which are plasmids that can be inserted into chromosomal DNA. Bacteria that have the F-plasmid are known as F positive (F+), and bacteria without it are F negative (F-).
• Resistance plasmids


  Contain genes that help a bacterial cell defend against environmental factors such as poisons or antibiotics. Some resistance plasmids can transfer themselves through conjugation, which can make a strain of bacteria resistant to antibiotics.
• Virulence plasmids


  When a virulence plasmid is inside a bacterium, it turns that bacterium into a pathogen, which is an agent of disease. Bacteria that cause disease can be easily spread and replicated among affected individuals.
• Degradative plasmids


  Help the host bacterium to digest compounds that are not commonly found in nature, such as camphor, xylene, toluene, and salicylic acid. These plasmids contain genes for special enzymes that break down specific compounds.
• Col plasmids


  Contain genes that make bacteriocins, which are proteins that kill other bacteria and thus defend the host bacterium.