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MBB162 > Bullough (Viruses) > Flashcards

Bullough (Viruses) Flashcards

1
Q

Define virus

A
  • genetic elements that cannot replicate independent of living host cell
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2
Q

Can viruses exist outside host cell?

A
  • can exist as virus particles outside host
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3
Q

Are viruses common?

A
  • most numerous MOs on Earth
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4
Q

What are the intracellular and extracellular forms of a virus?

A
  • intracellular form is replicative state

- extracellular form is virion, NA surrounded by protein and other molecules

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5
Q

Implications of virus size

A
  • small size places great restrictions on genome size (0.5-1000kb)
  • some have less than 5 genes
  • so reliant on host replicative and metabolic machinery
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6
Q

Variations of viral genomes?

A
  • DNA or RNA
  • ss or ds
  • linear or circular
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7
Q

Solution to small size of viruses

A
  • have small no. protein species making up capsid

- capsid proteins often capable of self assembly

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8
Q

Characteristics of spherical animal viruses

A
  • have protein shells built up of many copies of 1 or more identical subunits
  • some also enveloped
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9
Q

Icosahedral shells

A
  • highest poss symmetry

- built from 20 identical modular units, each containing 1 or more protein molecules

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10
Q

Types of viral protein expressed

A
  • timing of protein expression v important
  • early proteins, for rep of viral NA etc., eg enzymes
  • middle proteins
  • late proteins, inc coat proteins
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11
Q

What type of genome do bacteriophages have?

A
  • mainly dsDNA
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12
Q

Characteristics of T4 bacteriophage genome

A
  • genome encodes over 250 protein types
  • also encodes some of own tRNAs
  • DNA circularly permuted w/ terminal repeats at each end
  • uses various mechanisms to ensure own genes and not hosts transcribed
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13
Q

RNA pols and σ factors

A
  • σ recognises promoter and initiation site
  • transcrip begins and σ released
  • RNA chain growth cont to termination site
  • release of pol and RNA
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14
Q

How is viral transcrip ensured? (T4)

A
  • T4 doesn’t encode own RNA pol, uses hosts
  • mod to specifically recognise phage promoters
  • phage encodes anti-σ factor, binds to hosts σ70
  • preventing host transcrip
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15
Q

How is switch to middle proteins achieved?

A
  • some early phage proteins mod host RNA pol α subunits
  • other early proteins bind to RNA pol
  • pol specificity altered to recognise middle promoters
  • MotA recognises seq in middle promoter and guides RNA pol to correct site
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16
Q

Transcrip from late promoters

A
  • req new T4 encoded σ factor

- most late genes encode structural proteins

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17
Q

Lysogenic pathway

A
  • stable genetic relationship w/ host
  • most virus genes not expressed
  • prophage replicated in synchrony w/ host chromosome
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18
Q

Lytic pathway

A
  • produce virions
  • virus genome integrated into host chromosome
  • or exists in plasmid form
  • relies on action of repressor
  • inactivation or prevention of repressor induces prophage, resulting in lysis
  • viral genome ready for packaging and release from host
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19
Q

Bacteriophage λ

A
  • infects E.Coli
  • linear ds DNA
  • viral genome integrates at attachment site attλ
  • req λ integrase
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20
Q

Rolling circle replication

A
  • when λ enters lytic pathway, synthesises long linear concatemers of DNA by rcr
  • 1 strand of circular λ genome nicked
  • making long ss concatemer using unbroken strand as a template
  • 2nd strand made, using ss concatemer as template
  • ds concatemer cut into genome sized lengths at cos sites, giving cohesive ends
21
Q

Key actions in lysogeny

A
  • prevention of late protein prod

- copy of λ genome integrated into host chromosome

22
Q

How is decision between lysis and lysogeny made?

A
  • λ repression (cI) represses synthesis of all other λ proteins
  • Cro repressor stops expression of cII and cIII (inc cI synthesis)
  • commits λ to lysis, depends on v complicated genetic switch
  • final outcome determined by whether Cro protein or cI dominates in given infection
  • if Cro dominates –> lysis
  • if cI dominates –> lysogeny
23
Q

Diversity in animal viruses

A
  • DNA or RNA
  • enveloped or nonenveloped
  • ss or ds
24
Q

Characteristics of animal viruses

A
  • many rep in host nucleus
  • many enveloped, picking up parts of host membrane as leave cell
  • not all infections result in cell lysis/death
  • latent/persistent infections common
  • a few can cause cancer
25
Q

Factors to consider when comparing euk and prok viruses

A
  • DNA rep and transcrip take place in nucleus
  • protein synthesis takes place in cyto
  • mRNAs capped at 5’ end and poly(A) tails at 3’ end
  • polycistronic mRNA cannot be translated
26
Q

(+) strand RNA animal viruses

A
  • typically v small (approx 30nm)
  • linear single ss RNA
  • genomic RNA acts as mRNA (no capping)
  • RNA at 5’ end folds into stem loops and mimics cap w/ protein VPg
  • viral protein synthesised as 1 large polyprotein (common strategy)
  • rep in cyto
  • host RNA and protein synthesis inhibited, as host cap binding protein destroyed
  • eg. Poliovirus
27
Q

(-) strand RNA animal viruses

A
  • eg. rabies, influenza, ebola

- rhabdoviruses

28
Q

Rhabdoviruses

A
  • inc rabies and vesicular stomatitis
  • enveloped
  • viral RNA replicase essential
  • 2 distinct classes of RNA transcribed
29
Q

Influenza genome

A
  • segmented RNA genome
  • enveloped
  • A has 8 linear ssRNA molecules
  • own RNA replicase and RNA endonuclease, cuts primer from hosts capped mRNA precursors
30
Q

Influenza replication

A
  • viral NA rep in host nucleus
  • overall pattern of viral genomic RNA synthesis like rabies
  • (-) copied to (+) to gen (-) progeny
  • transcrip results in viral capped mRNAs, primers cut 5’ end, poly(A) tails added
  • transported to cyto for translation
31
Q

Antigenic shift

A
  • portions of RNA genome from 2 genetically distinct strains both infecting cell, are re-sorted
  • leads to sig diff surface proteins
  • origin of pandemics and epidemics
32
Q

Antigenic drift

A
  • arises from mutations (small changes) in genes for surface proteins
  • surface proteins main immunogenic regions of virus
  • annual vaccine req
33
Q

How do DNA euk viruses replicate?

A
  • in nucleus

- apart from pox

34
Q

Polyomaviruses (eg. SV40)

A
  • some induce tumours in animals
  • SV40 often used as vector for moving genes into euk cells
  • no viral encoded enzymes
  • genome 1 ds DNA
35
Q

SV40 genetic map

A
  • overlapping genes for coat proteins
  • single RNA (1° transcript) made by cellular RNA pol from early region
  • processed into 2 mRNAs, both capped and introns excised out
  • T antigen binds to origin of rep to initiate genome synthesis
  • early and late regions make diff proteins
36
Q

SV40 rep

A
  • genome too small to encode viral DNA pol

- DNA rep in bidirectional way using host cell machinery

37
Q

SV40 coat proteins

A
  • transcrip of late mRNA begins at promoter near origin of rep
  • processed by splicing, capping and polyadenylation to yield mRNA corresponding to 3 coat proteins (VP1, VP2 and VP3 –> genes overlap)
38
Q

SV40 assembly

A
  • coat protein mRNAs to cyto and translated to viral coat proteins
  • transported back to nucleus where virion assembly takes place
39
Q

Permissive and non-permissive cells

A
  • permissive cells allow normal viral rep and lysis

- non-permissive allow DNA integration and can become tumour cells, caused by certain viral proteins being translated

40
Q

Characteristics of retroviruses

A
  • enveloped
  • replicated through DNA intermediate, req reverse transcriptase
  • some carcinogenic
41
Q

Retrovirus genome

A
  • 2 identical ss (+) RNA strands
  • gag region encodes structural proteins
  • pol encodes reverse transcriptase and integrase
  • env encodes envelope proteins that sit in membrane
42
Q

Reverse transcriptase

A
  • DNA pol

- req primer (tRNA packaged into virus from previous host)

43
Q

Gene expression in retroviruses

A
  • provirus (genome expressed or latent)
  • activation of promoters in LTR leads to mRNA transcripts being capped and polyadenylated
  • viral mRNAs encapsidated or translated
  • polyproteins synthesised then processed
44
Q

Hepadnavirus genome and life cycle

A
  • small genomes (approx 3-4 kb)
  • overlapping genes
  • part gDNA ds
  • rep through RNA intermediate
  • v complex life cycle
45
Q

Hepadnavirus rep

A
  • involves transcrip by host RNA pol in nucleus
  • single genome length mRNA made as intermediate
  • viral reverse transcriptase copies this into DNA
46
Q

How is space saved in hepadnavirus genome?

A
  • pol versatile
  • DNA pol and reverse transcriptase activities
  • can complete DNA circle in host and acts as protein primer for 1 of DNA strands
47
Q

Effects of hepadnaviruses on host

A
  • cell transforms into tumour cells
  • lysis
  • persistent infection (slow release w/o death)
  • latent
  • cell fusion (polynuclear fused cells)
48
Q

Characteristics of pox viruses and their genome

A
  • complex and large
  • rep in cyto
  • linear ds DNA
  • vaccinia approx 185kbp and 180 genes
  • 2 strands of double helix cross linked at termini through phosphodiester bonds

MBB162 (6 decks)

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