The Genetics of Viruses and Bacteria

History Knowledge of genetics has been based on work with viruses and bacteria Genetic engineering and recombinant DNA relies on bacteria like E. coli and certain viruses for research and therapy

Genetics of Viruses A virus is a parasite that must live inside another cell to survive It hijacks the cell machinery to transcribe and translate all the proteins needed to create new viruses This process creates thousands of new viruses and destroys the host cell

Genetics of Viruses A virus contains DNA or RNA enclosed in a protein called a capsid Some viruses have a viral envelope derived from the membranes of host cells This disguises the capsid and helps the virus infect the host

The Virus

Genetics of Viruses Each type of virus can infect only ONE specific cell The virus gains entrance into the cell by binding to specific receptors on the cell surface The virus that causes colds in humans infects only the membranes of the respiratory system AIDS infects only one type of WBC

Genetics of Viruses One virus can usually only infect one species The range of organisms a virus can attack is the host range A new virus, like AIDS, may have come from a mutation in the virus that expanded the host range

AIDS Virus

Bacteriophages or Phage Viruses Most complex and best understood viruses Infects bacteria

Bacteriophage Reproduction Lytic Cycle 1) phage enters host cell 2) takes control of cell machinery and replicates itself 3) the host cell bursts, releasing a new generation of infectious phage viruses 4) new viruses infect thousands of cells in the same manner

Bacteriophage Reproduction Lysogenic cycle, in which viruses replicated WITHOUT destroying the host cell 1) phage virus attaches to a specific site in the host’s DNA 2) Remains dormant within the host genome and is called a prophage 3) Host cell divides and the prophage is replicated along with it, so a single infected cell gives rise to a population of infected cells

Bacteriophage Reproduction At some point, an environmental trigger causes the prophage to switch to the lytic phase Viruses capable of both the lytic and lysogenic cycles within a bacterium are called temperate viruses

Retroviruses Viruses that contain RNA instead of DNA and replicate unusually After entering the host cell, the RNA serves as a template for the synthesis of complementary DNA (cDNA) Retroviruses reverse the usual flow of information from DNA to RNA

Retroviruses Reverse transcription occurs under the direction of the reverse transcriptase enzyme Retroviruses enter the host genome, becomes a prophage and can make multiple copies of the viral genome for years Examples are polio and HIV

Retroviruses

Transduction Phage viruses acquire amounts of bacterial DNA as they infect one cell after another This process, leading to genetic recombination, is called transduction Two types: generalized and restricted (specialized)

Generalized Transduction Moves random pieces of bacterial DNA as the phage lyses one cell and infects another during the lytic cycle

Restricted Transduction Involves the transfer of specific pieces of DNA During lysogenic cycle, phages integrate into the host cell at a specific site Later, when the phage ruptures out of the host DNA, it carries a piece of adjacent host DNA with it and inserts this host DNA into the next host it will infect