BIOL243 L15 - Virus structure and reproductive cycles

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  • Created by: Katherine
  • Created on: 28-04-17 15:26
What is a virus?
Simple, infectious, non cellular structures that can only reproduce in living cells
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What are the features of a virus?
Miniscule, usually regarded as non living, carry their own genome but lack the capacity for reproduction, rely on host cells for replication, over 200 infectious diseases of humans are caused by viruses.
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How many states do viruses exist in?
2 - Extracellular for transmission and intracellular for replication
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What is extracellular?
Outside the host cell
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What is intracellular?
After infecting the host cell
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The virus genome is usually packeged in...
A protein coat (capsid) to permid movement between hosts
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What is a virion?
The packaged, infectious extracellular form of virus
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What is the structure of virion?
Genome (nucleic acid), protein coat (capsid), lipid membrane? (envelope), Envelope proteins (parts of the virus which interact).
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What are the 3 major morphological types of virus?
Helical, polyhedral, complex
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Give an example of a helical virus
Tobacco mosaic virus (TMV)
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Give an example of polygedral virus
common cold (Rhinovirus)
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Give an example of ac complex virus
Smallpox virus
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What is the nucleic acid of viruses like?
Single stranded DNA (ssDNA), Double stranded DNA (dsDNA), Single stranded RNA (ssRNA - Either negative or positive), double stranded RNA (dsRNA). Genomes can be linear or circlular. May be one or a few individual DNA or RNA molecules
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What is the baltimore classificatino system?
Type I, III, IV, V, VI and VII
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What type of viruses are Type I?
dsDNA viruses
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What type of viruses are Type III?
dsRNA viruses
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What type of viruses are Type IV?
Positive sense ssRNA viruses
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What type of viruses are Type V
Negative sense ssRNA viruses
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What type of viruses are Type VII?
dsDNA viruses wth ssRNA intermediates
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The virion is more than half composed of...
Protein
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What does the viral coat (capsid) contain?
Mutliple copies of individual protein subunits = protomer, capsomer and majority form icosahedral or helical structures.
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What is a protomer?
Oligomeric identical subunits
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What is a capsomer?
3D subunits made up of one or more individual proteins
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What do the polyhedron and helixal structures do?
Provide stable structures for packaging DNA and RNA, using limited numbers of proteins.
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What is the polyhedron structure?
Coat proteins assemble around nucleic acids, multiple copies of one or more proteins, icosahedron (20 faces and 12 corners) is the preferred polyhedral form
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What is the helical structure?
Coat proteins assemble on helical genomic RNA
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How many kinds of proteins are in the capsid of cowpea chlorotic mottle virus?
3
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Are all the proteins structural?
No, the virion also carries proteins which are required for transcription or replication. Other proteins may be required for viral function in the cell but are not are not contained witihn the mature virion (only expressed during replication)
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Many viruses are enveloped in a ...
Phospholipid membrane derived from the host plasma membrane.
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What is the viral life cycle?
Attachment ( cell recognition), entry and uncoating, gene expression and genome replication, transcription, translation, replication. Virion assembly and release - all influenced by virion structure
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How does a virus attach to the surface?
Viral recognition of host cells depends on interactions between virion surface proteins and plasm membrane protein proteins (it uses them as receptor). - The distribution of receptors determines host range (species and cell type)
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Viruses can attach and enter by direct penetration - what is this method?
Virus directly penetrates, where the receptors are. The viral genoe enters the cell.
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If the viruses have a bilipid membrane, how do they enter the cell?
Membrane fusion - the viral glycoprotein attaches to the cell, the envelope opens and the viral gylcoprotein can come out remaining in the cytoplasmic memrane. The viral genome then comes out of the membrane
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If the viruses have a bi lipid membrane, how can they enter thecell?
Endocytosis - where the whole vesicle is engulfed. Once in the cell, the vesicle can do it's own fusion events.
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What are the things that the virus must do?
Produce mRNA for translation into proteins, replicate their genome
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What are the steps of protein formation?
DNA replication, DNA transcription, mRNA translation = protein formation
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The size of the virion is determined by...
Size of genome (selective pressure for small genome size) Viruses have fewer genes, use overlapping open reading frames (ORF)
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How do viruses overcome problems of making multiple proteins from one RNA molecule
Sub genomic RNAs, polyproteins
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How do virses get put back together? Leaky Scanning
To use alternative start codons (leaky scanning). RNA encodes two overlapping ORFs, ribosome enters and scands for first AUG codon. Certain sequences around AUG can cause ribosome to miss start and carry on 'til next AUG
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How do virses get put back together? Read Through
Allows a protein to be produced that has different domains in it. Normal stop codons are UAA, UAG and UGA. In certain contexts, ribosomes can insert an a.a. instead of stopping. Such read-through occurs in 1-10% of translation events.
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What is frame shifting?
RNA encodes two or mre overlapping ORFs, shifting between ORFs allowsdifferent proteins to be made from the same RNA sequence.
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What are sub genomic RNAs
j
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What are polyprotein genes?
RNA is transcribed and translated into a single large polyprotein. The polyprotein contains an internal protease, which is able to chop the molecule into subunits. Different subunits have different functions
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How do most capsids assemble?
Self assemble - Capsids of helical viruses assemble directly around the genom
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Most icosahedral capsids first assemble into an
Immature procapsid - maturation takes place when a viral protease makes specific cleavages to alter the strucutre of capsid proteins. Maturation proteases make good antiviral targets
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Capsid addembly in some larger viruses require...
Scaffold proteins
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What is the release mechanism?
Virus is assembled. It approaches the host membrane and the host membrane, and it pushes it'self up out of the host membrane. As it's leaving, the host membrane sticks to it, forming the bilayer of the capsid.
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How are non-enveloped proteins released?
By exocytosis or by cell lysis.
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