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Lecture 2

Lecture 2 CSB351 Virology

2 Pages

Cell and Systems Biology
Course Code
Mounir Abou Haidar

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Definition of Virus
Viral particles formed of one or several molecules of nucleic acids: RNA OR DNA, but never both
Virus: icosahedral or helical shaped (RNA not in the hole of the loop but is packed inside the helix
Capsid/coat is to protect virus: most viruses have capsid --> some viruses may exist without
Some viruses are parasite of other viruses: Hep D (human) is parasite of HepB --> steals capsid protein
Virus, consisting of genetic material enclosed in a protective coating
No metabolic system, no intrinsic motility, cannot respond to stimuli, do not grow in the usual sense--> EXCEPTION: one
bacteria virus (tail part contracts, virus injects DNA into bacteria)
Ability to maintain genetic continuity, with possibility for mutation, is the only basis for considering viruses to be ALIVE
Viruses: in ever single living organism--> ubiquitous
HIV: retrovirus--> different: take their genetic info (RNA), convert into DNA, which goes into nucleus of the cell,
integrate into chromosome and stay there --> viral genetic information becomes a PART of genetic info of the cell
All other types: don't go near the nucleus--> go into cytoplasm of the cell, makes millions of copies of their own
gene then virus particle and then burst through the cell
Virus integrates piece of DNA near oncogene (normal genes in human cells involved in cell division--> most of
these, proto-oncogenes,usually turned off after embryonic stage)--> viral DNA integration re-activate oncogene-->
continuous/unstoppable cell division--> cancer (i.e. Cervix cancer)
The genes added to cell by the virus cause the infected cell to either produce more virus particles (end result cell death)
or to change characteristics of the cell (cancerous)
Goodheart (1969):
Nature of Virus
Particles formed of one or several molecules of nucleic acids: RNA or DNA but not both
Several= Divided genome--> i.e. Influenza virus (several pieces of RNA)
One= single genome--> i.e. Polio, TMV
CAPSID: genome usually (but not necessarily) covered by one or several types of protein
Envelope: some viruses, on top of having capsid protein, also have envelope
Polio virus, Norwalk
Most ___ plant viruses are not enveloped
Enveloped: HIV, Herpes, Rabis
Not all viruses are enveloped:
Influenza virus: i.e. H1N1
HA = glycoprotein (protein + sugar attached) --> used as receptor-binding for the virus: recognize some molecule (sialic
acid) on the surface of the cell to ATTACH--> virus is endocytosed by the cell --> replicates in the cell, makes lots of
copies, infection--> virumia= lots of virus in the blood--> FEVER: immune system makes Ab
NA = neuraminidase --> exactly like sialic acid, but functions to cut virus and release them from the cell, so they can go
and infect other cells
Virus: before get released from cell, spike into their lipid bilayer the VIRAL NA and HA --> virus then detach and signal
LIPID: viruses take it from the cell they infect --> through budding
Smaller: bacteria 1 micron, viruses 30 000 to make up 1mm
Unable to multiply outside a living cell --> Absolute Parasitism
Viruses are:
Nucleic acid: RNA/DNA carries genetic information, to code for diff genes of the virus
Genome = set of genes
All viruses must code for at least 1 protein: polymerase (few exceptions) --> synthesis of new RNA/DNA of the virus; all
also code for their coat/capsid proteins to cover and protect their genetic material + form the virus particle
Here: must take nucleotides from the cell's machinery --> cannot synthesize their own nucleotides
Protein: i.e. TMV only 1 protein making helical structure
Lipids: not all have this
Cations: K+1, Ca+2, Mg+2
Nucleic acid = sugar + base + phosphate
RNA/DNA: -charged due to phosphate groups (normally have + charged histones to neutralize this)
Virus: grabs some cations from the cell to neutralize
Composition of viruses:
Helical/rod shaped
Lecture 2 Definition of Virus
1:26 PM
CSB351 Course Notes Page 1

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Lecture2 Definition of Virus September-17-09 1:26 PM DefinitionofVirus { Viralparticlesformedofoneorseveralmoleculesofnucleicacids:RNAORDNA,butneverboth { Virus:icosahedralorhelicalshaped(RNAnotintheholeoftheloopbutispackedinsidethehelix { Capsidcoatistoprotectvirus:mostviruseshavecapsid-->somevirusesmayexistwithout { Somevirusesareparasiteofotherviruses:HepD(human)isparasiteofHepB-->stealscapsidprotein { Goodheart(1969): Virus,consistingofgeneticmaterialenclosedinaprotectivecoating Oneofthesimplestentitiestoreproduce Nometabolicsystem,nointrinsicmotility,cannotrespondtostimuli,donotgrowintheusualsense-->EXCEPTION:one bacteriavirus(tailpartcontracts,virusinjectsDNAintobacteria) Abilitytomaintaingeneticcontinuity,withpossibilityformutation,istheonlybasisforconsideringvirusestobeALIVE Viruses:ineversinglelivingorganism-->ubiquitous Thegenesaddedtocellbytheviruscausetheinfectedcelltoeitherproducemorevirusparticles(endresultcelldeath) orto changecharacteristicsofthecell(cancerous) HIV:retrovirus-->different:taketheirgeneticinfo(RNA),convertintoDNA,whichgoesintonucleusofthecell, integrateintochromosomeandstaythere-->viralgeneticinformationbecomesaPARTofgeneticinfoofthecell Allothertypes:dontgonearthenucleus-->gointocytoplasmofthecell,makesmillionsofcopiesoftheirown genethenvirusparticleandthenburstthroughthecell VirusintegratespieceofDNAnearoncogene(normalgenesinhumancellsinvolvedincelldivision-->mostof these,proto-oncogenes,usuallyturnedoffafterembryonicstage)-->viralDNAintegrationre-activateoncogene--> continuousunstoppablecelldivision-->cancer(i.e.Cervixcancer) Natureof Virus { Particlesformedofoneorseveralmoleculesofnucleicacids:RNAorDNAbutnotboth Several=Dividedgenome-->i.e.Influenzavirus(severalpiecesofRNA) One=singlegenome-->i.e.Polio,TMV { CAPSID:genomeusually(butn
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