Notes - Virology.docx

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MICB 202
Tracy Kion

Virology Central Dogma - Cells (ds)DNA → mRNA → Protein transcription translation nucleotides → nucleic acids (RNA and DNA) amino acids → proteins (specific functions) Viruses vs Cells 1. Simple structure 2. Genetic information is encoded by DNA or RNA 3. Living cell help replicate Obligate Intracellular Parasite – depend on living host cell for replication  Extracellular  virion/ virus particle  DO NOT  deFRaGMentS  Feed  Respire  Grow  Move  respond to Stimuli  exist primarily as replicating nucleic acid  Intracellular  DO  RA  Replicate  Adapt  Limited genome size ∴ genes allow for:  Replication of viral genome  Production of viral structural proteins  Any required function CANNOT adapt from host cell  Capsid Shape  Icosahedral protein molecules  max internal volume  non-symmetric protein molecules  identical equilateral triangular structures  Helical  Complex  Virion Enzymes  Some mature virion will have virus-encoded enzymes within capsid  Enzymes often polymerases (initiate replication)  Host cell does not provide suitable enzyme  Virus cannot utilize cellular form of enzyme  Cause of Pathological Effects  toxic effects of viral gene products (proteins) on host cell metabolism  host immune system reacts/destroys infected cells expressing viral genes  structural/functional interactions w/ virus gene material modifies host genes  virus replication cycle may destroy host cell  Use of Viruses  enzymes: DNA ligase, reverse transcriptase, RNA polymerases, RNase H  virus use RNA or ss DNA as genome  virus can RNA → DNA  gene therapy Virus Replication Cycle – entire process from attachment to release from cell  enters cell → replicate viral genome → synthesize viral mRNA → make viral proteins → assemble/release progeny virus  takes over cellular metabolism  proteins encoded in genome cannot perform function → use cell enzymes  use own DNA/RNA polymerases or use cell’s  uses cellular ribosomes, ATP, tRNAs, amino acids, etc Virus Infection  Attachment 1. adsorption – random collision between virus particles and host cells  does NOT require energy from host  reversible step 2. binding between virus receptor and anti-receptor  virus receptor – host cell binding site  protein or carbohydrate portion of glycoprotein  involved in normal function of cell  anti-receptor – viral particle binding site *virus CANNOT infect cells without proper viral receptor *host range determined by host cell intracellular factors (transcription factors) -allow viral genome to replicate and direct viral protein synthesis  Entry  susceptible – cell types virus can enter/infect  permissive – cells virus can multiply Virus Attachment Prevention  bind to anti-receptor (virus)  neutralizing antibodies in host  problem: useless against high rates of virus anti-receptor mutations  bind to receptor (host)  neutralizing antibodies in host  problem: can alter cell normal function or disrupt important interactions host cell – cell that supports virus replication cycle of virus infection – process of viral genome introduced into host cell and replicated capsid – protein shell enclosing either nucleic acid only, nucleoprotein complex, or core  capsomer – individual units  helical, icosahedral, or complex nucleocapsid – nucleic acid within icoshedral capsid or helical protein complex  more than one virus protein may be associated with nucleic acid envelope – lipid bilayer and associated glycoproteins that surround many types of nucleocapsids virion – ineffective virus particle (intact/fully assembled) core – internal body containing nucleoprotein complex within another protein shell/capsid  found in some viruses naked virus particle – viruses that do not contain envelopes enveloped viruses – viruses that do contain envelopes virus replication cycle – entire process from attachment to release from cell replication – production of new virus particles infect – viral genome entering cell virus receptor – protein or carbohydrate portion of glycoprotein on surface of host cell  involved in some normal function of cell anti-receptor – proteins on outside of viral particle host range – cell types/species a virus can infect susceptible – cell types virus can infect/enter permissible – cells in which virus can multiply segmented genomes – entire genome contained in several molecules of RNA DNA viruses – replicated DNA using DNA polymerase  some use host cell/some supply pre-made RNA viruses – NOT Retroviruses – encode RNA-dependent RNA polymerases  not found in host cell RNA-dependent RNA polymerases (RDRP) – synthesize complementary strand of RNA  uses viral RNA as template Retroviruses – RNA viruses that use reverse transcriptase  synthesizes single strand of complementary DNA (cDNA)  uses viral RNA genome as template polyprotein – large precursor protein which is enzymatically cleaved into smaller portions monocistronic mRNA – mRNA containing the coding information for one gene only primary cell culture – tissue removed from animal and treated with trypsin to dissociate cells  forms monolayer  NOT immortal, eventually dies tumour cell – grows indefinitely in cell culture  cells transformed  no longer sensitive to primary cell constraints  forms monolayers lysate – suspension in culture medium  unlysed cells/cell debris removed by centrifuge cytopathic effect/ “cell injuries” – morphological changes experienced by infected animal cells syncytia – masses of fused cells containing more than one nucleus in vitro – in glass away from living organism in vivo – in body of living organism pathogenicity – capacity of one organism to cause disease in another  disease is an abnormal situation  infection results in damage to host  alteration in normal host functions lytic/acute infection – host cell killed  usually during release of progeny virus particles  POLIOVIRUS persistent infection – slow release of virus over long period  few infected cell deaths  Hepatitis B latent infection – viral genome present, not replicated, and no harm to cell  no disease symptoms  no free viruses detected in body during latency  can be re-activated → production of virus particles later  HERPES  HIV oncogenic infection – integration of viral genome into host chromosome  viral genome replicated with host DNA  passed onto progeny cells transformation – process of normal cell changing into tumour cell attenuated vaccine – live virus  replication of virus and synthesis of viral proteins  AB-mediated  cell-mediated  amplifies immune response  propagated in cells other than those of normal host or non-physiological temperatures inactivated vaccine – killed virus  no replication of virus or synthesis of viral proteins  AB-mediated  NOT cell-mediated  may NOT result in long-lasting immune memory  produced in quantity → inactivated by chemical  no longer effective virus, but still immunogenic purified viral components/subunit vaccine – subset of purified viral proteins  portion of virus genome used → safe/no contamination of original live pathogenic virus  often fail: not sufficient to protect against viral challenge Case Study: Picornavirus Poliovirus – small RNA virus  Enterovirus – genus  Fecal-oral route  Acid stable ( < pH 3)  Limited replication in oropharynx → stomach acid → extensive replication in lower intestinal tract (natural habitat)  resistant to common lab disinfectants  sensitive to formaldehyde  denatures virus during vaccine production  sensitive to chlorine levels of 0.3 to 0.5 ppm  transmission through water ingestion  flaccid muscular paralysis – weakening or loss of muscle tone  caused by motor neuron death Genome  one single-stranded (+) RNA  genomic RNA is infectious  introduced artificially (micro-injection) → virus progeny  viral protein genome-linked (VPg) – small, basic protein ~23 amino acids long  covalently attached to 5’ and 3’ polyA tail *VPg is NOT the same as a 5’ cap  untranslated region (UTR) in RNA  long at 5’ cap  important in translation of RNA  important to packaging of genome in capsids  short at 3’ tail  important in synthesis of (-) strand for replication  encodes single polyprotein ~2100 to 2400 amino
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