Download Chapter 19 – Viruses Virus • Obligate intracellular parasite • No

Chapter 19 – Viruses
Virus
 Obligate intracellular parasite
 No metabolic enzymes or ribosomes
 Structure
o Protein coat (capsid)
o Nuclei Acid core (linear or circular)
 DNA viruses – single-stranded or double-stranded
 RNA viruses – single-stranded or double-stranded
 Very small – 4 to 100s of genes
 Capsid
o Gives shape to virus
 Spiral – tobacco mosaic, influenza
 Triangular plats in a polyhedron – adenovirus, bacteriophages
o Contains proteins that assist in recognition of host cell
o Can be surrounded by viral envelope
 Membrane from host + GPs & GLs from virus
Reproduction
 Virus attaches to receptor site on host
 Nucleic Acid enters cell
o Can enter w/o capsid
o Enters w/ capsid & released inside
 Rest of reproduction depends on type of nucleic acid present
 DNA Viruses
o e.g. bacteriophages (bacteria virus)
o Viral DNA replicated by host
o Viral DNA transcribed & translated by host making new viral proteins
o New DNA & capsid assembled
o New Viruses shed from cells
o Lytic Cycle
 Phage causes host DNA to degrade
 e.g. T4 phage
 In last stage, virus directs an enzyme to damage cell wall
 Water floods in & cell lyses
 New viruses released to spread
 Virulent phages only use lytic cycle
o Lysogenic Cycle
 Phage genome replicates w/o destroying host cell
 e.g. Lambda phage
 Viral DNA is inserted into host DNA (prophage)
 DNA synthesis yields daughter cells w/ prophage in genome
 Temperate phage uses both cycles
 RNA Viruses
o ssRNA (class IV) – genome acts as mRNA & is translated directly
o ssRNA (class V) – genome acts as template strand for mRNA & complementary RNA
(gets copied again to make genome RNA)
 RNA replicase used to copy RNA
o Retroviruses (Class VI)
 Deliver reverse transcriptase w/ RNA
 RNA read & cDNA made (no correcting)
 DNA then replicated = doubled stranded
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DNA spliced into host genome by integrase (forming a provirus)
Viral DNA now transcribed & translated
 mRNA also serves as new genome
Protease cleaves/activates polypeptides
HIV leaves & picks up host membrane
Viroids
 Smaller & simpler than viruses, consist of tiny molecules of naked circular RNA that infect plants
 Have several hundred nucleotides that don’t encode for proteins but can be replicated by host’s
cellular enzymes
 RNA molecules can disrupt plant metabolism & stunt plant growth, perhaps by causing errors in
regulatory systems that control plant growth
Prions
 Infectious proteins in mammalian brains
o Scrapie in sheep, mad cow disease, & Creutzfeldt-Jakob
 Slow-acting – up to 10 year incubation
 Virtually indestructible
 Misfolded protein that converts normal proteins into prion version
 Spread by eating meat w/ infected tissue
Bacterial Plasmids
 Extra pieces of DNA, generally good
o Very small (2-30 genes)
 Circular & self-replicating
o Can replicate more or less often than bacterial chromosome
 Extra copies or no copies
 Some can enter & leave genome
 Conjugation
o One way transfer of DNA b/w bacterial cells (Bacteria ‘sex’)
o Sex pilus connects & pulls cells together
o Fertility (F) Factor
 Part of DNA needed for sex pilus
 25 genes in genome or plasmid
 F+ bacteria have it & F- don’t
 If F factor on plasmid, can be transferred via conjugation
 F+ can attach & donate to Fo R Plasmids
 Carry genes for different antibiotic resistances (R)
 Antibiotics kill bacteria w/o R plasmids (& survivors multiply)
 Conjugation can move R
 Transformation
o Cell picks up & integrates foreign DNA from surroundings
 Transduction
o Movement of genes b/w bacteria by bacteriophages
o Generalized – accidental transfer of DNA b/w bacteria after lytic cycle
o Specialized – prophage picks up part of bacteria DNA & moves it (across species?) after
lytic cycle
o Process
 Phage infects bacterium by injecting DNA
 DNA copied & broken up in pieces
 New phages made; A+ bacterial allele gets in one (accidental)
 Phage w/ A+ injects into a new bacteria (doesn’t have A+)
 A+ allele recombines into new bacterial genome