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Lecture 4

Lecture 4 CSB351 Virology

3 Pages

Cell and Systems Biology
Course Code
Mounir Abou Haidar

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Nucleic Acid Enzymes
Polymerase: enzyme that synthesize N.A.
Pyrophosphate: need triphosphate (cytosine triphosphate CTP, adenine triphosphate ATP, etc )-->triphosphate, broken down by
polymerase, energy from pyrophosphate used to make covalent bond, RNA/DNA synthesized becomes a chain-like molecule
(covalent bonds, very strong)
DNA-dependent RNA polymerase: (DNA viruses)
Transcription of mRNA from nuclear DNA
Need promoter: sequence of DNA that RNA polymerase binds to--> to start transcription
Weak/strong promoter: how much it attracts RNA polymerase
Virus makes their promoters more attractive than that of the cell, so can use cell's machinery to transcribe their own
genes --> thus strong promoters are usually viral
RNA-dependent RNA polymerase:(RNA viruses)
Make new RNA from RNA template (copyase)
RNA Viruses code for their own, for their own replication
ALL RNA viruses encode for this
RNA polymerase: Doesn't need primer
DNA-dependent DNA polymerase: needs primer
Most DNA viruses code for the own, make new DNA copy from RNA template (reverse transcriptase)
P 13:
Modifying Enzymes
Modifying enzymes that add a methyl (-CH3) group --> modify the first nucleotide on the RNA (mRNA or (+) viral RNA)--> first
nucleotide normally is guanosine, at position 7 (m7G)--> add methyl group
Viral (+) RNA: majority of these +RNA viruses have a cap at 5' end as well, like the mRNAs (some don't--come back later:
*MOST (+) RNA viruses have 5' cap on their mRNA!!
*Polio: (+) virus, has no cap! Why? Diff strategy: creates special structure--> cap-independent translation! (majority are cap-
Viral (-) RNAviruses: not translatable, don't need cap, thus not capped
DNA viruses: can't put cap on DNA --> DNA not translatable by ribosome (only translates RNA)
Need RNA polymerase of the cell to read DNA and transcribe in the nucleus into mRNA (which usually transcribe DNA of the
cell)--> this is why DNA needs to go into nucleus for viral replication, because need cell's mechanisms and polymerases
Get special structure at 5' end of all mRNA of cell--> makes ribo recognize them, is a "cap", like signal --> ready for translation!
Repairs "broken" nucleotides
Repairs circular DNA from a linear chain
Don't worry about terminal tranferases
Endonucleases: Restriction enzymes -->enzymes can cut in the middle of the chain
Cuts only double-stranded DNA
Sequences are always symmetrical: 2-fold symmetry
Staggered breaks--> complementary cohesive ends
Exonucleases: cuts from one end of the nucleic acid chain or the other (i.e. 5' end)
Rnase (ribonucleases): digests Rna (*DICER)
Dnase (deoxyribonucleases): digests DNA
E.g. Snake venome phosphdiesterase splits RNA from 3' end sequentially, have same reaction for DNA
Nuclease S1: cuts only single-stranded regions of DNA or RNA (S1 = single-stranded)
RnaseH: Ribonuclease H = hybrid
Only found in RETROVIRUSES (i.e. HIV)
Retroviruses have RNA as genome --> virus first synthesize cDNA from RNA --> now have RNA/DNA hybrid
Rnase H cuts RNA ONLY when it's hybridized with DNA --> after RNA copied into DNA, is degraded [while DNA is copied into
DS DNA (DNA polymerase: DNA-dependent DNA polymerase]
Degrades N.A.
Responsible for unwinding enzymes or proteins
Large majority of viral polymerases (RNA/DNA): have helicase activity
Lecture 4: Pg 12 cont'd
2:10 PM
CSB351 Course Notes Page 1

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Lecture4: Pg12contd September-24-09 2:10 PM NucleicAcidEnzymes { Polymerase:enzymethatsynthesizeN.A. Pyrophosphate:needtriphosphate(cytosinetriphosphateCTP,adeninetriphosphateATP,etc)-->triphosphate,brokendownby polymerase,energyfrompyrophosphateusedtomakecovalentbond,RNADNAsynthesizedbecomesachain-likemolecule (covalentbonds,verystrong) { RNApolymerase:Doesntneedprimer a. DNA-dependentRNApolymerase: (DNAviruses) TranscriptionofmRNAfromnuclearDNA Needpromoter:sequenceofDNAthatRNApolymerasebindsto-->tostarttranscription Weakstrongpromoter:howmuchitattractsRNA polymerase Stronger=transcribingmoreRNA Virusmakestheirpromotersmoreattractivethanthatofthecell,socan usecellsmachinerytotranscribetheirown genes-->thusstrongpromotersareusuallyviral b. RNA-dependentRNApolymerase:(RNAviruses) MakenewRNAfromRNAtemplate(copyase) RNAVirusescodefortheirown,fortheirownreplication ALLRNAvirusesencodeforthis { DNA-dependentDNApolymerase:needsprimer MostDNAvirusescodefortheown,makenewDNA copyfrom RNAtemplate(reversetranscriptase) Retroviruses! P13: ModifyingEnzymes Methylases { Modifyingenzymesthataddamethyl(-CH3)group-->modifythefirstnucleotideontheRNA(mRNAor(+)viralRNA)-->first nucleotidenormallyisguanosine,atposition7(m7G)-->addmethylgroup { Getspecialstructureat5 endofall mRNAofcell-->makesriborecognizethem,isacap,likesignal -->readyfortranslation! Viral(+)RNA: majorityofthese+RNAviruseshaveacap at 5 endas well,likethemRNAs (somedont--comebacklater: polio) *MOST(+)RNAviruseshave5cap on theirmRNA!! *Polio:(+)virus,hasnocap! Why?Diffstr
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