1. The Genetics of Viruses and Bacteria

2. 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

3. 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

4. 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

5. The Virus

6. 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

8. 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

9. AIDS Virus

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

11. 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

12. 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

13. 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

14. 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

15. 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

16. Retroviruses

17. 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)

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

19. 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