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 (32 cards)
Virus Characteristics
- genome
- capsid
- replication
- >a complete virus particle combining these structural elements is called 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
Viral Morphology
-Nucleic Acid
- either DNA or RNA
- strands can be ss, ds, linear or looped, in separate segments or one continuous strand
RNA Viruses
-> (+) vs. (-) stranded
-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
RNA-dependent RNA polymerase
-aka?
transcription of the (-) strand of RNA into (+)
**not found in human cells
aka TRANSCRIPTASE
RNA Viruses
-2 types
(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
DNA Viruses
- > (+) vs. (-)
- exception
- 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
Capsids
- 2 types
- which is only RNA?
- most assume what shape?
- exception?
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
Envelope
- what type of membrane?
- how is it acquired?
- what are viruses called without the envelope?
- 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
Viral Classification
-4 aspects
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
DNA Viruses
-HHAPPPy
Hepadna Herpes Adeno Papova Parvo Pox
RNA Viruses
-Everything but HHAPPPy
Arena Bunya Corona Calici Flavi Orthomyxo Paramyxo Picorna Reo Retro Rhabdo Toga
Viral Replication
-4 steps
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)
Viral Replication
-Adsorption
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
Viral Replication
-Attachment sites on the viral surface
-specialized attachment surfaces like glycoprotein spikes or unique folding of capsid proteins
Viral Replication
-Host cell receptor molecules
-receptor molecules on host cell membrane are specific for each virus family
Viral Replication
-Penetration
passage of virion from surface of the cell across the cell membrane and into the cytoplasm
Viral Replication
-Receptor mediated endocytosis
- the cell membrane invaginates enclosing the virion in an endocytic vesicle
- release of virion into the cytoplasm occurs by various routes
Viral Replication
- Membrane fusion
- what is the end result?
- 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
Viral Replication
-Uncoating
refers to the stepwise process of disassembly of the virion that enables the expression of the viral genes that carry out replication
Mechanism of DNA virus genome replication
1) Txn of early genes
2) Replication of virus DNA
3) Txn of late genes
4) Assembly of nucleocapsids
Mechanism of RNA virus genome replication
-obstacles
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
RNA virus genome replication
-Type I
(slide 19)
**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
RNA virus genome replication
-Type II
(slide 20)
**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
RNA virus genome replication
-Type III
(slide 21)
**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)
