MGY277H1 Study Guide - Midterm Guide: Reverse Transcriptase, Trypsin, Retrovirus
8 Jun 2018
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UNIT 3: THE VIRUSES
• Describe the physical and genomic features of viruses.
Physical:
• A virus consists of genomic DNA surrounded by a protein capsid coat.
• Viruses are very small, 10 – 1000x smaller than the cells they infect.
• The capsid is the entire outside protein coat. This coat is made up of triangles called
capsomeres and these capsomeres are made of capsomeres subunits. A
nucleocapsid is made up of nucleic acid AND the capsid (entire protein coat). It is a
capsid containing a genome.
• A virion is nucleic acid + protein coat
• Viruses can be naked or enveloped. If enveloped the capsid containing nucleic acid is
surrounded by an Envelope. With matrix fluid between the envelope and the capsid.
The envelope is made of a lipid bilayer and the matrix contains proteins + enzymes
with roles in infection. The virus spikes are contained within the envelope.
• Viruses can be: icosahedral (simple flat triangle structure using little energy to
assemble), Helical (helix, some are short and rigid, others are long and filamentous),
or complex (could contain a mixture of the two above structures, like a phage).
Genomic:
• Viruses contain EITHER DNA or RNA. Can be linear or circular, single or double
stranded, or segmented. DNA viruses are larger and more stable due to their less
genetic material.
• Understand how viruses are classified and grouped, both formally and
informally.
• Viruses are broadly classified according to the type of cell they infect.
• Genome structure (type of nucleic acid and strandedness), hosts infected (bacteria,
archaea etc...), Viral shape and structure (enveloped or naked, icosahedral or helical
et…, and Disease symptoms are all characteristics used to classify viruses.
• Families end in viridae (follows no consistent pattern). Genus ends in virus. Species
name is often the name of the disease. Viruses are commonly referred to by species
name.
• Viruses can also be named according to their routes of infection: Enteric, respiratory,
zoonotic, arboviruses.
• Important diseases: West Nile encephalitis, yellow fever, dengue fever (PWN wrote
these so I addig the here.
• Within a virus family, the viruses have similar structures, infect similar host cells, and
cause similar diseases.
• Describe the 5-step infection cycle for enveloped and non-enveloped
viruses. (Bear with me, this one is long and detailed)
• In the latent state of a virus: virus nucleic acid integrates into host genome or
replicates independently like a plasmid.
• Viruses affect many factors in our bodies including pattern recognition receptors
P‘‘ ad our adaptie iuity. These P‘‘s at at the ery iitial stages of
infection. Once they sense structural or genomic viral compounds (A) they initiate
responses (B) through apoptosis, interferon signalling, and interferon-stimulated
genes (ISG). The adaptive immunity then kicks in alter on (C) and kills the virus and
created immunological memory for next times infection. Viruses have such few
genes but can stop very important functions in the body → shows specificity.
find more resources at oneclass.com
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• FIVE STEP REACTION CYCLE: (1) Attachment (2) Penetration and uncoating (3)
Synthesis of viral proteins and genome replication (4) Assembly (5) Release
• (1) Viral spikes bind receptors on plasma membrane (PM) using specific receptors
limiting the range of the virus and accounting for resistance. The receptors which
these spikes bid to are orally reeptors for other thigs oiously e dot
make receptors for viruses to bind to on purpose).
(2) Penetration occurs in one of two ways and depends on whether the virus is
naked or enveloped.
Membrane fusion → Enveloped ONLY: A) Adsorption B) Membrane fusion: the viral
envelope fuses with the PM as both are made of a lipid bilayer. C) Nucleocapsid
release: viral envelope remains with plasma membrane, does not enter cell. D)
Uncoating. Naked viruses do NOT contain an envelope (lipid bilayer) and so cannot
fuse with the PM (lipid bilayer).
Receptor-mediated endocytosis → Enveloped virus AND Naked virus: A) Adsorption:
the binding of the virus to host receptors triggers this process. B) Endocytosis: the
PM surrounds the virus and forms a vesicle. Then the virus envelope fuses with the
membrane and is released from the cell. C) Release from vesicle D) Uncoating. If
you are a naked virus, then you cannot fuses with the host membrane at B) and the
virus damages the host endosome releasing nucelocaspids into the cytoplasm at C).
During penetration, the virion must localize to the site of infection (either the
nucleus or cytoplasm) following Nucelocapsid release in membrane fusion C) and
Release from vesicle in Receptor-mediated endocytosis C).
(2.5) Uncoating occurs prior to the start of replication and is when viral nucleic acid
is released from the capsid. This can occur simultaneously upon entering a cell of
after the final intracellular destination has been reached. The capsid is disassembled
by specific mechanisms.
(3) Once the genome is in the site of replication this process can occur. It requires
viral gene expression (to produce viral structural and catalytic proteins and enzymes
required for replication) and virus replication (to copy its genome). There are three
strategies to doing this: DNA viruses, RNA viruses, and Reverse transcribing viruses.
DNA viruses: These guys usually replicate in the nucleus. They require host
machinery for gene expression & DNA synthesis but encode their own DNA
polymerase. This allows the virus to replicate in cells which are not actively dividing.
This frees the irus fro aiting for certain conditions to replicate.
ds (+/-) DNA → ss (+) RNA (mRNA) → protein
ss (-) DNA → ds (+/-) DNA → ss (+) RNA (mRNA) → protein (a complementary (+)
strand must first be made then its the same as below).
ss (+) DNA → ds (+/-) DNA → ss (+) RNA (mRNA) → protein
RNA viruses: Most of these guys replicate in the cytoplasm and require virally
encoded RNA-dependant, RNA polymerase called replicase which uses RNA as a
template to make DNA.
ss (+) RNA (mRNA) → straight to protein OR ss (+) RNA (mRNA) → ss (-) RNA.
Here (+) sense strand RNA can be made straight into protein or it can be
interconverted to and from ss (-) RNA. Replicase uses ss (+) RNA to make ss (-) RNA.
The ss (-) strand makes more ss (+) RNA which can make more packaged protein or
get into the nucleocapsid of new viruses.
Replicase ss (-) RNA → ss (+) RNA (mRNA) → protein.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Unit 3: the viruses: describe the physical and genomic features of viruses. Physical: a virus consists of genomic dna surrounded by a protein capsid coat, viruses are very small, 10 1000x smaller than the cells they infect, the capsid is the entire outside protein coat. This coat is made up of triangles called capsomeres and these capsomeres are made of capsomeres subunits. A nucleocapsid is made up of nucleic acid and the capsid (entire protein coat). It is a capsid containing a genome: a virion is nucleic acid + protein coat, viruses can be naked or enveloped. If enveloped the capsid containing nucleic acid is surrounded by an envelope. With matrix fluid between the envelope and the capsid. The envelope is made of a lipid bilayer and the matrix contains proteins + enzymes with roles in infection. Genomic: viruses contain either dna or rna. Can be linear or circular, single or double stranded, or segmented.
