Viruses

1. Viruses

2. Genetics of Viruses • Viruses are parasites of cells • A typical virus: • Penetrates a cell • Commandeers its metabolic machinery • Assembles hundreds of new viruses that are copies of itself • Then leaves the cell to infect other cells • In the process, the host cell is destroyed Molecular Genetics

3. Genetics of Viruses - HIV Molecular Genetics

4. Genetics of Viruses • Viruses are specific for the kinds of cells they will paralyze • Some viruses attack only one kind if cell within a single host species • Others attack similar cells from a range of closely related species • For example the swine flu infects both humans and pigs • Bacteriophages or phages, for example, are viruses that only attack bacteria • Phages are commonly used in research because of how rapidly bacteria can reproduce and because of the simplicity of the structure of a phage Molecular Genetics

5. Bacteriophage Molecular Genetics

6. Structure of Viruses • Viruses consist of the following structures • Nucleic Acid (DNA/RNA) • Capsid • Envelope Molecular Genetics

7. Structure of Viruses • A nucleic acid, either RNA or DNA (but not both), contains the hereditary information of the virus • The DNA or RNA may be double stranded (dsDNA or dsRNA) or single stranded (ssDNA or ssRNA) • A capsid, or protein coat, encloses the nucleic acid • Identical protein subunits, called capsomeres, assemble to form the capsid • An envelope surrounds the capsid of some viruses • Envelopes incorporate phospholipids and proteins obtained from the cell membrane of the host Animation: Hershey and Chase Experiment Molecular Genetics

8. Structural Variations of Viruses Molecular Genetics

9. Viral Replication • Viral replication results in genetic variation, and viral infection can introduce genetic variation into the hosts. • Viruses have highly efficient replicative capabilities that allow for rapid evolution and acquisition of new phenotypes. • Virus replication allows for mutations to occur through usual host pathways.

10. Viral Replication • RNA viruses lack replication error-checking mechanisms, and thus have higher rates of mutation. • Related viruses can combine/recombine information if they infect the same host cell.

11. Viral Replication • So in other words, viruses can mutate very easily giving them the ability to evolve rapidly. • Rapidly evolving viruses make it very difficult for humans to keep up with current vaccines. • When you get a flu vaccine, it is only a vaccine for the most common viruses that year. • There are still others that may effect you. • The person sitting next to you could have the flu and even though you had the vaccine for that strand, it could mutate and in just one sneeze and make you sick!

12. Replication of Viruses • Viruses replicate via a component assembly model allowing one virus to produce many progeny simultaneously via the lytic cycle. • Replication of viruses occurs by one of the two following cycles: • Lytic Cycle • Lysogenic Cycle • The reproductive cycles of viruses facilitate transfer of genetic information. Molecular Genetics

13. Replication of Viruses • Replication of viruses occurs by one of the two following cycles: • Lytic Cycle • Lysogenic Cycle Molecular Genetics

14. Lytic Cycle • In the lytic cycle: • A virus penetrates the cell membrane of the host and uses the enzymes of the host to produce viral nucleic acids and viral proteins • Recall transduction in bacteria!!!! • The nucleic acids and proteins are then assembled into new viruses which subsequently erupts from the host cell, destroying the cell in the process • The new viruses then infect other cells, and the process repeats Animation: Lytic Cycle Molecular Genetics

15. Lytic Cycle Molecular Genetics

16. Variations in the Lytic Cycle • Viruses transmit DNA or RNA when they infect a host cell. • For most DNA viruses, the DNA is replicated to form new viral DNA and the DNA is transcribed to produce viral proteins • The DNA and proteins are assembles into new viruses Molecular Genetics

17. Variations in the Lytic Cycle • For some RNA viruses, the RNA serves as mRNA or as a template to make new mRNA • The mRNA is translated to make proteins and these proteins are assembled with RNA to make new viruses Molecular Genetics

18. Variations in the Lytic Cycle • HIV is a well-studied system where the rapid evolution of a virus within the host contributes to the pathogenicity of viral infection. • Retrovirusesare ssRNA viruses that use an enzyme called reverse transcriptase to make a DNA complement of their RNA • The DNA complement can then be transcribed immediately to manufacture mRNA, or it can begin the lysogenic cycle by becoming incorporated into the DBA if the host • Human immunodeficiency virus (HIV, the cause of AIDS) is a retrovirus Animation: HIV Reproduction Molecular Genetics

19. HIV Molecular Genetics

20. Lysogenic Cycle • Some viruses are able to integrate into the host DNA and establish a latent (lysogenic) infection. These latent viral genomes can result in new properties for the host such as increased pathogenicity in bacteria. • In the lysogenic cycle, the viral DNA is temporarily incorporated into the DNA of the host cell • A virus in this dormant state is called a provirus, (or, if a bacteriophage, a prophage) • The virus remains inactive until some trigger, often an external stimulus (such as radiation or certain chemicals), causes the virus to begin the destructive lytic cycle Animation: Lysogenic Cycle Molecular Genetics

21. Lytic vs. Lysogenic Cycles Molecular Genetics