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Microbiology > Lecture 9: Virology -Viral replication and Taxonomy > Flashcards

Lecture 9: Virology -Viral replication and Taxonomy Flashcards

1) Unique characteristics of viruses 2) Viral morphology 3) Viral replication including mechanism of replication in DNA and RNA viruses

1
Q

Virus Characteristics

  • genome
  • capsid
  • replication
    - >a complete virus particle combining these structural elements is called a _______
A
  • genome consists of DNA or RNA (not both)
  • protein containing structure or coat called the capsid designed to protect the genome
  • replication of the genetic material occurs when the virus takes control of the host cell’s synthetic machinery
  • a complete virus particle combining these structural elements is called a virion
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2
Q

Viral Morphology

-Nucleic Acid

A
  • either DNA or RNA

- strands can be ss, ds, linear or looped, in separate segments or one continuous strand

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3
Q

RNA Viruses

-> (+) vs. (-) stranded

A

-2 types of RNA viruses: positive (+) stranded and negative (-) stranded
[1] (+) strand is just like the mRNA sense strand, which can be used as a template for protein synthesis
[2] (-) strand are not able to begin translation immediately
-must first be transcribed into a (+) strand of RNA, for this the virus must carry the enzyme RNA-dependent RNA polymerase

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4
Q

RNA-dependent RNA polymerase

-aka?

A

transcription of the (-) strand of RNA into (+)
**not found in human cells
aka TRANSCRIPTASE

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5
Q

RNA Viruses

-2 types

A

(1) Retroviruses- unique due to their ability to incorporate into the host genome (e.g. HIV)
(2) Reoviridae- including rotavirus is unique as they are the only viruses with a ds RNA genome

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6
Q

DNA Viruses

  • > (+) vs. (-)
  • exception
A
  • most DNA viruses have both (-) and (+) strands
  • (+) strand (sense) is read, while the (-) strand (antisense) is ignored
  • *parvoviruses are exception and have a ss DNA genome
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7
Q

Capsids

  • 2 types
  • which is only RNA?
  • most assume what shape?
  • exception?
A

2 types: Icosahedral & Helical

  • in helical symmetry, the protein capsomers are bound to RNA (always RNA as only RNA viruses have helical symmetry) into a helical nucleoprotein capsid
  • most of them assume a spherical shape except for rhabdoviruses (rabies virus), which have a bullet-shaped capsid
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8
Q

Envelope

  • what type of membrane?
  • how is it acquired?
  • what are viruses called without the envelope?
A
  • lipid bilayer membrane
  • -acquired by budding through the host cell nuclear or cytoplasmic membrane and tearing off a piece of the membrane as they leave
  • viruses w/o envelope are called naked or nonenveloped, those with membranes are called enveloped
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9
Q

Viral Classification

-4 aspects

A

1) Nucleic Acid
- type: DNA or RNA
- ds or ss
- single or segmented
- (+) or (-) RNA
- complexity of genome
2) Capsid
- Icosahedral
- Helical
3) Envelope
- Naked
- Enveloped
4) Size
- diameter of helical capsid
- # of capsomers in icosahedral

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10
Q

DNA Viruses

-HHAPPPy

A 
Hepadna
Herpes
Adeno
Papova
Parvo
Pox
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11
Q

RNA Viruses

-Everything but HHAPPPy

A 
Arena
Bunya
Corona
Calici
Flavi
Orthomyxo
Paramyxo
Picorna
Reo
Retro
Rhabdo
Toga
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12
Q

Viral Replication

-4 steps

A

1) Adsorption and Penetration
2) Uncoating of the virus
3) Synthesis and assembly of viral products (as well as inhibition of host cell’s DNA, RNA, and protein synthesis)
4) Release of virions from the host cell (either by lysis or budding)

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13
Q

Viral Replication

-Adsorption

A

initial attachment of the virus particle to the host cell involves interaction between specific molecular structures on the virion and receptor molecules on the host cell membrane

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14
Q

Viral Replication

-Attachment sites on the viral surface

A

-specialized attachment surfaces like glycoprotein spikes or unique folding of capsid proteins

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15
Q

Viral Replication

-Host cell receptor molecules

A

-receptor molecules on host cell membrane are specific for each virus family

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16
Q

Viral Replication

-Penetration

A

passage of virion from surface of the cell across the cell membrane and into the cytoplasm

17
Q

Viral Replication

-Receptor mediated endocytosis

A
  • the cell membrane invaginates enclosing the virion in an endocytic vesicle
  • release of virion into the cytoplasm occurs by various routes
18
Q

Viral Replication

  • Membrane fusion
    • what is the end result?
A
  • some enveloped viruses (e.g. HIV) enter a host cell by fusion of their envelope with the plasma membrane of the cell
  • one or more glycoproteins in the viral envelope promotes this fusion
  • the end result of this process is that the nucleocapsid is free in the cytoplasm
  • the viral membrane remains associated with plasma membrane of the host cell
19
Q

Viral Replication

-Uncoating

A

refers to the stepwise process of disassembly of the virion that enables the expression of the viral genes that carry out replication

20
Q

Mechanism of DNA virus genome replication

A

1) Txn of early genes
2) Replication of virus DNA
3) Txn of late genes
4) Assembly of nucleocapsids

21
Q

Mechanism of RNA virus genome replication

-obstacles

A

1) no host cell RNA polymerase that can use the viral parental RNA as a template for synthesis of complementary RNA strands
2) translation of euk. mRNAs begins at only a single initiation site and they are translated into only a single polypeptide
* *RNA viruses which frequently contain only a single molecule of RNA must express the genetic information for at least 2 proteins: RNA dependent RNA polymerase and a minimum of 1 type of capsid protein

22
Q

RNA virus genome replication
-Type I

(slide 19)

A

**RNA viruses with a ssRNA of (+) polarity that replicates via a complementary (-) strand intermediate

-the infecting parental RNA molecule serves both as mRNA and later as a template for synthesis of the complementary (-) strand

23
Q

RNA virus genome replication
-Type II

(slide 20)

A

**Viruses with a ssRNA of (-) polarity that replicates via a complementary (+) strand intermediate

-the (-) strand genomes cannot accomplish replication without prior construction of a complementary (+) strand intermediate

24
Q

RNA virus genome replication
-Type III

(slide 21)

A

**Viruses with a dsRNA genome

  • dsRNA is segmented with each segment coding for one polypeptide, however euk. cells do not have an enzyme capable of transcribing dsRNA
  • type III viral transcripts are produced by virus coded RNA-dependent RNA polymerase (transcriptase)
25
Q

RNA virus genome replication
-Type IV

(slide 22)

A

**VIruses with a ssRNA of (+) polarity that is replicated via a DNA intermediate

  • conversion of a (+) strand RNA to a dsDNA is accomplished by an RNA-dependent DNA polymerase (reverse transcriptase)
  • resulting dsDNA becomes integrated into the cell genome by the action of a viral integrase
  • viral mRNAs and progeny (+) strand RNA genomes are transcribed from this integrated DNA by the host cell RNA polymerase
26
Q

Assembly of nucleocapsids generally takes place in the host cell _______ for most RNA viruses and in the ___________ for most DNA viruses.

A

cytoplasm; nucleus

27
Q

Release of progeny viruses

-Naked virions

A

the cell may lyse and release the virions or the virions are released by reverse phagocytosis (exocytosis)

28
Q

Release of progeny viruses
-Enveloped virions

(see slide 24 for details)

A

the enveloped virions bud through the Golgi apparatus, nuclear membrane or cytoplasmic membrane, tearing off a piece of host cell lipid bilayer as it exits

29
Q

Viral infections in which no progeny virus are produced

A

1) nonpermissive -host cells lacking enzymes, promoters, txn factors req’d for viral replication
2) defective virus -genetically lost ability to replicate
3) death of host cell before viral replication

30
Q

Viral infections -host cell may be altered antigenically but not killed

A

1) permissive -infection is productive, but viral replication and release neither kills the host cell nor interferes with functions
2) persistent infection
3) antigenic specificity of the cell surface may be altered as a result of insertion of viral glycoproteins

31
Q

Viral infections resulting in a latent viral state in a host cell

A
  • persistence of a viral genome inside a host cell with no production of progeny virus
  • latent virus can be reactivated later leading to a productive infection
32
Q

Viral infections resulting in host cell death and production of progeny viruses

A
  • productive infection in which the virus shuts down much of host cell macromolecular synthesis by one or more of the virus
  • lytic cycle

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