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Chapter 19~Viruses

Chapter 19~Viruses. Viral structure. Virus : “ poison ” (Latin); infectious particles consisting of a nucleic acid in a protein coat Viruses are not cells Capsid ; (protein shell that encloses the viral genome built from protein subunits called capsomeres. Fig. 19-3.

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Chapter 19~Viruses

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  1. Chapter 19~Viruses

  2. Viral structure • Virus: “poison” (Latin); infectious particles consisting of a nucleic acid in a protein coat • Viruses are not cells • Capsid; (protein shell that encloses the viral genome • built from protein subunits called capsomeres

  3. Fig. 19-3 A capsid can have various structures 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

  4. Viral structure • Viral genomes may consist of either • Double- or single-stranded DNA, or • Double- or single-stranded RNA • Some viruses have membranous envelopes that help them infect hosts • These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals

  5. Concept 19.2: Viruses reproduce only in host cells • Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell • Each virus has a host range, a limited number of host cells that it can infect (receptor molecules on the surface of cells)

  6. General Features of Viral Reproductive Cycles • Once a viral genome has entered a cell, the cell begins to manufacture viral proteins • The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules • Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses

  7. Fig. 19-4 VIRUS Entry and uncoating 1 DNA Capsid Transcription and manufacture of capsid proteins 3 Replication 2 HOST CELL Viral DNA mRNA Capsid proteins Viral DNA Self-assembly of new virus particles and their exit from the cell 4

  8. Viral reproduction: Lytic Cycle • Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle • The lytic cycle: • 1- attachment • 2- injection • 3- hydrolyzation • 4- assembly • 5- release • Results in death of host cell • Virulent virus (phage reproduction only by the lytic cycle) • Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA

  9. Viral reproduction: Lysogenic Cycle • Genome replicated w/o destroying the host cell • Genetic material of virus becomes incorporated into the host cell DNA (prophage DNA) • Temperate virus (phages capable of using the lytic and lysogenic cycles) • May give rise to lytic cycle

  10. Retroviruses: transcribe DNA from an RNA template (RNA--->DNA) Reverse transcriptase (catalyzing enzyme) HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome) RNA viruses

  11. Fig. 19-8b Membrane of white blood cell HIV 0.25 µm HIV entering a cell New HIV leaving a cell

  12. The viral DNA that is integrated into the host genome is called a provirus • Unlike a prophage, a provirus remains a permanent resident of the host cell • The host’s RNA polymerase transcribes the proviral DNA into RNA molecules • The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell

  13. Evolution of Viruses • Viruses do not fit our definition of living organisms • Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid • Candidates for the source of viral genomes are plasmids, circular DNA in bacteria and yeasts, and transposons, small mobile DNA segments • Plasmids, transposons, and viruses are all mobile genetic elements

  14. Viral Diseases in Animals • Viruses may damage or kill cells by causing the 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

  15. Vaccines are harmless derivatives of pathogenic microbes 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

  16. Viruses, viroids, and prions are formidable pathogens in animals and plants • Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide • Smaller, less complex entities called viroids and prions also cause disease in plants and animals, respectively

  17. Viroids and prions • Viroids: tiny, naked circular RNA that infect plants; do not code for proteins, but use cellular enzymes to reproduce; stunt plant growth • Prions:“infectious proteins”; “mad cow disease”; trigger chain reaction conversions; a transmissible protein

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