Virus ( Virion )

1. Virus (Virion) • Subcellular particle • Subcellular particle • Consists of • DNA or RNA genome • surrounded by protein coat (capsid) • Consists of • DNA or RNA genome • surrounded by protein coat (capsid)

2. Virus Structure • RANDOM FACTS: • Walter Reed discovered the first human virus, yellow fever virus, in 1901. • The name virus was coined from the Latin word meaning slimy liquid or poison. • Viruses are not alive – they are inanimate complex organic matter. They lack any form of energy, carbon metabolism, and cannot replicate or evolve. Viruses are reproduced and evolve only within cells.

3. RNA inside capsid Capsid 0.1 µm Fig. 24-1a, p. 502

4. Capsid with antenna-like fibers DNA inside capsid 0.05 µm Fig. 24-1b, p. 502

5. DNA inside capsid Capsid Tail Tail fibers Emerging DNA 0.1 µm Fig. 24-1c, p. 502

6. Viruses • Cannot metabolize on their own • Contain nucleic acids necessary to make copies of themselves • but must invade and use metabolic machinery of living cells in order to reproduce

7. Origin of Viruses • Viruses may be bits of nucleic acid that originally “escaped” from animal, plant, or bacterial cells RANDOM FACT: There are a million virus particles per milliliter of seawater – for a global total of 1030virions! Lined up end to end, they would stretch 200 million light years into space.

8. Hypothesis • Viruses must have evolved before the three domains diverged • It is unlikely that similar viruses that infect archaea and bacteria evolved twice RANDOM FACT: Some parasitic wasps lay eggs in caterpillars, where they mature into adult wasps. The wasp eggs contain a virus, encoded in the wasp genome, which prevents the caterpillar from rejecting the eggs.

9. Viral Reproductive Cycles • Lytic cycle • virus destroys host cell • Temperate viruses • do not always destroy their hosts • Lysogenic cycle • viral genome replicated along with host DNA

10. Lytic Cycle • 5 steps: • attachment to host cell • penetration of viral nucleic acid into host cell • replication of viral nucleic acid • assembly of components into new viruses • release from host cell

11. Phages 1 Attachment. Phage attaches to cell surface of bacterium. Bacterium Bacterial DNA Penetration. Phage DNA enters bacterial cell. 2 Phage protein Phage DNA Replication and synthesis. Phage DNA is replicated. Phage proteins are synthesized. 3 Fig. 24-2a (1), p. 504

12. 4 Assembly. Phage components are assembled into new viruses. Release. Bacterial cell lyses and releases many phages that can then infect other cells. 5 Fig. 24-2a (2), p. 504

13. 0.25 µm Fig. 24-2b, p. 504

14. Lysogenic Cycle

15. Envelope proteins Virus attaches to specific receptors on plasma membrane of host cell. 1 Envelope Capsid Membrane fusion. Viral envelope fuses with plasma membrane. 2 Nucleic acid Membrane Fusion Receptors Host-cell plasma membrane Virus is released into host-cell cytoplasm. 3 Capsid Cytoplasm Nucleus Viral nucleic acid separates from its capsid. 4 Nucleic acid Ribo-somes 5 Viral nucleic acid enters host-cell nucleus and replicates. ER mRNA 6 Viral nucleic acid is transcribed into mRNA. 7 Host ribosomes are directed by mRNA to synthesize viral proteins. Viruses are released from host cell. 10 8 Vesicles transport glycoproteins to host-cell plasma membrane. New viruses are assembled and enveloped by host-cell plasma membrane. 9 Fig. 24-4b, p. 508

16. Viral Diseases • DNA viruses cause • smallpox, herpes, respiratory infections, gastrointestinal disorders • RNA viruses cause • influenza, upper respiratory infections, AIDS, some types of cancer

17. Rubella • An RNA virus

18. Plant Viruses • Mostly RNA viruses • Spread among plants by insect vectors • Spread through plant via plasmodesmata

19. Retroviruses • Use reverse transcriptase • Transcribe RNA genome into DNA intermediate • becomes integrated into host DNA • Synthesize copies of viral RNA

20. Nucleic acid (RNA) HIV Envelope protein HIV attaches to host-cell plasma membrane. 1 Envelope Capsid Enzymes (reverse transcriptase, ribonuclease, integrase, protease) 2 HIV enters host-cell cytoplasm. CD4 Receptors Viral nucleic acid (RNA) Capsid is removed by enzymes. Reverse transcriptase catalyzes synthesis of single-stranded (ss) DNA that is complementary to viral RNA. 3 Reverse transcriptase Host-cell plasma membrane Cytoplasm ssDNA 4 The DNA strand then serves as template for synthesis of comple- mentary DNA strand, resulting in double-stranded (ds) DNA. Nucleus dsDNA Host chromosome Viral RNA 5 dsDNA is transferred to host nucleus and enzyme integrase integrates DNA into host chromosome. 6 When activated, viral DNA uses host enzymes to transcribe viral RNA. 7 Viral RNA leaves nucleus, viral proteins are synthesized on host ribosomes, and virus is assembled. 8 Virus buds from host cell, using host-cell plasma membrane to make viral envelope. Fig. 24-5, p. 509

21. MORE RANDOM FACTS: The first human influenza virus was isolated in 1933. In 2005, the 1918 pandemic influenza virus strain was constructed from nucleic acid sequence obtained from victims of the disease. • The HIV-1 genome, which is about 10,000 nucleotides long, can exist as 106020 different sequences. To put this number in perspective, there are 1011 stars in the Milky Way galaxy and 1080 protons in the universe. • The smallest known viruses are circoviruses, which are 20 nanometers (0.00002 millimeters) in diameter. The viral genome is 1,700 nucleotides in length and codes for two proteins. • The biggest known viruses are mimiviruses, which are 400 nanometers (0.0004 millimeters) in diameter. The viral genome is 1,200,000 nucleotides in length and codes for over 900 proteins.

22. Our “friend” H1N1(over 61,300 cases confirmed nation-wide in 2009…)