14.4D: Plasmids and Lysogeny

Learning Objectives

• Distinguish between plasmids and lysogeny in regards to pathogenecity

Plasmids

Plasmids are DNA molecules that are capable of replicating independently from the chromosomal DNA. Plasmids are often characterized by their circular appearance and double-strands; they also vary in size and number. Plasmids are present in the three major domains (Archaea, Bacteria and Eukarya) and are considered to be ‘naked DNA’. ‘Naked DNA’ refers to a specific type of DNA which does not encode for genes promoting the transfer of genetic material to a new host. The plasmids are present within the cells as extra chromosomal genomes and are a common tool used in molecular biology to integrate new DNA into a host. In the field of molecular biology, plasmid DNA is often referred to as ‘ vectors ‘ due to their ability to transfer DNA between organisms. The use of plasmid DNA in molecular biology is considered to be recombinant DNA technology. In addition, plasmid DNA provides a mechanism by which horizontal gene transfer can occur, contributing to antibiotic resistance.

Horizontal gene transfer is a major mechanism promoting bacterial antibiotic resistance, as the plasmid DNA can transfer genes from one species of bacteria to another. The plasmid DNA which is transferred often has developed genes that encode for resistance against antibiotics. The ability to transfer this resistance from one species to another is increasingly becoming an issue in clinics for treatment of bacterial infections. The process of horizontal gene transfer can occur via three mechanisms: transformation, transduction and conjugation. Plasmid DNA transfer is associated with conjugation as the host-to-host transfer requires direct mechanical transfer. The advantages of plasmid DNA transfer allow for survival advantages.

Figure: Horizontal Gene Transfer: There are three mechanisms by which horizontal gene transfer can occur. Specifically, the exchange of plasmid DNA falls under transformation.

Lysogeny

Lysogeny is the process by which a bacteriophageintegrates its nucleic acids into a host bacterium’s genome. Lysogeny is utilized by viruses to ensure the maintenance of viral nucleic acids within the genome of the bacterium host. The virus displays the ability to infect the bacterium host and integrate its own genetic materials into the host bacterium genome. The bacteriophages newly integrated genetic material, called a prophage, is transferred to new bacterial daughter cells upon cell division. The prophage is integrated into the bacterium genome at this point. The lysogenic cycle is key to ensure the transmittance of bacteriophage nucleic acids to host bacterium’s genome. Lysogeny is one of two major methods of viral reproduction utilized by viruses.

Lysogenic cycles are utilized by specific types of viruses to ensure viral reproduction, but they also need the second major method of viral reproduction, the lytic cycle, as well. The lytic cycle, considered the primary method of viral replication, results in the actual destruction of the infected cell. Upon destruction of the infected cell, the new viruses, which have developed after undergoing biosynthesis and maturation, are free to infect other cells. The lytic cycle is characterized by the breakdown of the bacteria cell wall intracellularly. The viruses cause disruption of the bacterial cell by producing enzymes which facilitate this process. An example of a virus which can promote the transformation of bacterium from a nontoxic to toxic strain via lysogeny is the CTXφ virus. Specifically, the bacterium, Vibrio cholerae, is transformed into a toxic strain upon infection with the bacteriophage. This bacterium is then able to produce a cholera toxin, the cause of the disease cholera.

Figure: Lysogenic and lytic cycles: Schematic of lysogenic and lytic cycle utilized by viruses to ensure viral reproduction.

Key Points

• Plasmids are double-stranded circular forms of ‘naked DNA ‘.
• Plasmids are responsible for horizontal gene transfer which promotes the development of antibiotic resistance in bacterium.
• Lysogeny is a major method of viral reproduction characterized by the integration of viral nucleic acids in the bacterium genome.

Key Terms

• bacteriophage: A virus that specifically infects bacteria.
• lysogeny: the process by which a bacteriophage incorporates its nucleic acids into a host bacterium