The Genetics of Viruses and Bacteria

1. The Genetics of Viruses and Bacteria Campbell, Chapter 18

2. General Features • acellular • obligate intracellular parasites • cannot reproduce by themselves • no intrinsic metabolism • no ribosomes • typically quite small

3. Virion, Virus, and Viriod • virion • virus outside of host cell • virus • resides in host cell • viroid • naked, infectious RNA molecules

4. Virus Structure • capsid • protein coat • surrounds inner core of DNA or RNA • protects inner core, determines what kinds of cells the virion can attach to, and responsible for infecting the cell • envelope surrounds the capsid in some animal viruses • derived from plasma membrane • contains virally encoded proteins used to infect host cells

6. Describing a Virus • Is the genome DNA or RNA? • Is the nucleic acid single or double stranded? • Is the virion shape simple or complex? • Is the virion surrounded by a membrane?

7. Bacteriophage Infection • viruses that infect bacteria • recognize host using proteins in the capsid • tail assemblies used to inject genetic material through the bacterium’s cell wall

8. Bacteriophage Infection • once injected, the virus may: • reproduce immediately • integrate into the bacterium’s genome, postponing reproduction • two reproductive cycles • lytic • lysogenic

9. Lytic Cycle • infected cell lyses, releasing phage progeny • virulent viruses only reproduce using the lytic cycle

10. Lytic Cycle • early stage • promoter in viral genome attracts RNA polymerase • early genes code for proteins that terminate host cell transcription, stimulate viral replication and late gene transcription • nuclease enzymes digest host cell’s chromosome

11. Lytic Cycle • late stage • late genes transcribed • code for capsid proteins and proteins to lyse host cell • new viruses are assembled • viruses destroy host cell via lysis

12. Lysogenic Cycle • infected host harbors virus for generations • termed temperate viruses • prophage integrates into host DNA • can exit and undergo lytic cycle • occurs when cell is damaged or stressed

13. Animal Viruses • may directly infect or be vectored • enter cells in 3 ways • naked virion endocytosed by host cell • enveloped virus endocytosed by host cell • host cell membrane and enveloped virus fuse • after reproduction, enveloped viruses bud from host cell by fusing with the host cell’s plasma membrane

17. Lytic vs. Lysogenic • In animal cells, lytic and lysogenic cycles cause differing pathologies • lytic infections of plasma cells by the Epstein-Barr virus (EBV) occur in mononucleosis • latent infections of B cells by EBV predispose the person to lymphoma • lytic infections by human papilloma virus (HPV) cause genital warts • latent infections by some strains of HPV lead to cervical cancer

18. Prions • molecules of a normal body protein that have changed their three-dimensional configuration • PrPC • The normal protein • is called PrPc (for cellular) • is a glycoprotein normally found at the cell surface inserted in the plasma membrane • has its secondary structure dominated by alpha helices • is easily soluble • is easily digested by proteases • is encoded by a gene designated (in humans) PRNP located on our chromosome 20

19. Prions • PrPSC • The abnormal, disease-producing protein • is called PrPSC (for scrapie) • has the same amino acid sequence as the normal protein; that is, their primary structures are identical but • its secondary structure is dominated by beta conformation • is insoluble in all but the strongest solvents • is highly resistant to digestion by proteases • When PrPSC comes in contact with PrPC, it converts the PrPC into more of itself (even in the test tube). • These molecules bind to each other forming aggregates • It is not yet clear if these aggregates are themselves the cause of the cell damage or are simply a side effect of the underlying disease process