Viruses: a kind of “borrowed life”

1. Viruses: a kind of “borrowed life” HIV infected T-cell

2. Viral Structure • not cells • small infectious particles w/ • DNA or RNA enclosed in a protein coat (capsid) • in some cases, a membranous envelope

3. Viral Genomes • genomes may be either • Double or single stranded DNA, or • Double or single stranded RNA • Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

4. RNA Membranous envelope Head DNA RNA DNA Capsomere Capsid Tail sheath Tail fiber Capsomere of capsid Glycoproteins Glycoprotein 18  250 nm 70–90 nm (diameter) 80  225 nm 80–200 nm (diameter) 20 nm 50 nm 50 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (d) Bacteriophage T4 (c) Influenza viruses

5. Membranous Envelopes • help viruses infect hosts • surround the capsids of influenza viruses and many other viruses found in animals • derived from the host cell’s membrane • contain a combination of viral and host cell molecules

6. Fig. 19-7 Capsid and viral genome enter the cell Capsid RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA Capsid proteins ER Copy of genome (RNA) Glyco- proteins New virus

7. Bacteriophages • AKA phages • are viruses that infect bacteria • most complex capsids found among viruses • w/ elongated capsid head that encloses their DNA • A protein tail piece attaches to host and injects the phage DNA inside

8. Bacteriophages

9. Viral Reproduction • obligate intracellular parasites • can reproduce only within a host cell • Each virus has a host range • a limited number of host species that it can infect

10. VIRUS Entry and uncoating 1 DNA Capsid Transcription and manufacture of capsid proteins 3 Replication 2 • Viruses use the host’s cellular machinery to reproduce itself • enzymes, • ribosomes, • tRNAs, • amino acids, • ATP, • and other molecules HOST CELL Viral DNA mRNA Capsid proteins Viral DNA Self-assembly of new virus particles and their exit from the cell 4

11. Two Reproductive Mechanisms • the lytic cycle • lysogenic cycle

12. The Lytic Cycle • A reproductive cycle that kills the host cell by lysing it or bursting it open with new viral progeny • Bacteriophages and other viruses can reproduce in this way

13. Fig. 19-5-5 Attachment 1 2 Entry of phage DNA and degradation of host DNA 5 Release Phage assembly 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers

14. The Lysogenic Cycle • At the start, this cycle replicates viral genome without destroying the host • The viral DNA molecule is inserted into the host cell’s chromosome • This integrated viral and host DNA is known as a prophage • Every time the host divides, it copies the phage DNA and passes the copies to daughter cells

15. The Lysogenic Cycle • An environmental signal can trigger the virus genome to exit prophage and switch to the lytic mode

16. Fig. 19-6 Daughter cell with prophage Phage DNA The phage injects its DNA. Cell divisions produce population of bacteria infected with the prophage. Phage DNA circularizes. Phage Bacterial chromosome Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Lytic cycle Lysogenic cycle The bacterium reproduces, copying the prophage and transmitting it to daughter cells. The cell lyses, releasing phages. Lysogenic cycle is entered Lytic cycle is induced or Prophage Phage DNA integrates into the bacterial chromosome, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.

17. Viral Classification • two key variables used to classify viruses • DNA or RNA? • Single-stranded or double-stranded?

20. RNA Viruses (Retroviruses) • Retrovirusesuse reverse transcriptase to copy their RNA genome into DNA • Ex. HIV

21. Viral envelope Glycoprotein Capsid RNA (two identical strands) HOST CELL Reverse transcriptase HIV Reverse transcriptase Viral RNA The viral DNA that is permanently integrated into the host genome is called a provirus (unlike prophage) RNA-DNA hybrid DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New virus

22. Membrane of white blood cell HIV 0.25 µm HIV entering a cell New HIV leaving a cell

23. Viral Diseases • Viruses damage or kill cells and tissues: • reproduction • release of hydrolytic enzymes from lysosomes • Some viruses cause infected cells to produce toxins that lead to disease symptoms • Others have envelope proteins that are toxic

24. Vaccines • derivativesof pathogens that stimulate the immune system to mount defenses against the actual pathogen • Vaccines can prevent certain viral illnesses • Viral infections cannot be treated by antibiotics • Antiviral drugs can help to treat, though not cure, viral infections

25. Emerging Viruses • appear suddenly and come to the attention of scientists • Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

26. Flu epidemics come from new strains of influenza virus to which people have little immunity • New viral diseases can emerge when viruses spread from animals to humans • Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity

27. These strains can cause pandemics, global epidemics • The “avian flu” is a virus that recently appeared in humans and originated in wild birds

28. (a) The 1918 flu pandemic 0.5 µm (b) Influenza A H5N1 virus (c) Vaccinating ducks