1:Structure/Classification/Pathogenesis Flashcards
Four characteristics of Viruses:
- Nucleic acid genome packaged in a protein coat. Protein for protection and genome for initiation and completion of infectious cycle.
- Unable to generate energy or synthesise proteins. Must make mRNA that can be translated by host ribosomes.
- Replication, not growth or division.
- Enter hosts via cellular receptors.
Where do viruses function?
They are obligate intracellular parasites, meaning their genome can only function within a living cell.
Functions of the nucleocapsid?
Protecting the viral genome, transmission and infection.
Characteristics of icosahedral nucleocapsids:
Package DNA or RNA, released by lysis (non-enveloped), or by budding (enveloped viruses).
Characteristics of helical nucleocapsids:.
Protein subunits wrapped around genome, usually packaging RNA, usually enveloped and associated with matrix protein.
Immunogenic components of viruses?
The envelope, if present, or the nucleocapsid. They are immunogenic because they contain antigenic epitopes.
Method of envelope budding
Expression of viral proteins cell membrane, nucleocapsid buds through membrane, forming the envelope.
Pros/Cons of an envelope
Pros: Surrounds the nucleocapsid, aids in transmission and infection.
Cons: Confers instability, may be easily dehydrated.
3 Methods envelopes develop:
- Budding from the plasma membrane
- Bud into cytoplasmic vesicles, released by exocytosis.
- Budding through the nuclear membrane.
Location of viral matrix protein
Located between the the envelope and the nucleocapsid in viruses that bud from the plasma membrane.
Location of tegument protein
Between NC and envelope in viruses that bud from the nuclear membrane.
Characteristics of Virus genomes
RNA or DNA
Single or Double stranded
Segmented, linear or circular.
Contains open reading frames.
Functions of viral genome
Acts as mRNA or template for mRNA.
Template to produce new progeny genomes.
Viral enzymes are…
…contained within the NC, composed of protein and present in the virion only if they are needed prior to mRNA production.
Functions of viral enzymes in the NC
Polymerase: Makes new genomes or mRNA
Integrase: Integrates HIV DNA into chromosome.
Protease: Digests virus polyproteins.
Picornaviridae
+ve ss RNA, icosahedral naked NC, cytophatic (NC released by lysis).
e.g. Poliovirus
Hepadnaviridae
ds DNA, icosahedral NC, enveloped.
e.g. Hepatitis B virus
Flaviviridae
+ve ssRNA, icosahedral NC, enveloped, released by budding in ER->Exocytosis.
e.g. Hepatitis C virus
Retroviridae
+ve ssRNA, 2 gene copies, icosahedral NC, Matrix, enveloped.
e.g. HIV
Orthomyxoviridae
- ve ssRNA, segmented genome, helical NC, Matrix, enveloped, released by budding from plasma membrane.
e. g. Influenza A virus
Herpesviridae
ds DNA genome, icosahedral NC, tegument proteins, enveloped, buds through nucleus.
e.g. Herpes simplex virus, varicella Zoster Virus.
Stage 1 of viral pathogenesis:
- Attachment
- Penetration
- Uncoating
Stage 2 of viral pathogenesis
- Gene expression
5. Gene replication
Stage 3 of viral pathogenesis
- Assembly of nucleocapsid
- Packaging of genome and proteins
- Release from cell
3 methods of viral entry to cells
Receptor mediated endocytosis,
Surface fusion and direct injection (naked viruses)
Start of open reading frame?
5’AUG codon
End of open reading frame?
3’UAG codon
3 coding strategies of genomes
1genome1ORF1Protein
1genome1ORF1+Proteins
1genome1+ORFs1+Proteins
Enzymes carried in Poliovirus?
None! RdelRpol created later for replication
Method of Hep C release?
Budding into ER
Method of Hep B gene expression?
DNA from virus genome is converted from relaxed circular to cccDNA, mRNA is produced which codes for proteins and is then reverse transcribed to produce new genomic DNA
Genome of Hep B?
Relaxed circular (+/-) ss DNA, with 4 open reading frames each coding for multiple proteins
Enzymes contained in HIV?
Reverse transcriptase, integrase, protease.
HIV envelope proteins and human receptor?
gp120 + gp41 bind to CD4 on T lymphocytes
Genome of Influenza A?
8 randomly packaged linear segments each containing one ORF and coding for one protein
Influenza A proteins and human receptors?
Haemaglutanin and neurominidase bind to sialic acid (N cleaves s. acid)
Transport of herpes virus from ECF to nucleus
Enters via surface fusion, travels via dynein to nucleus, Genome (+/-dsDNA) enters though nuclear pore
Transport of Herpes from Nucleus to ECF
Buds through inner nuclear membrane, viral proteins are expressed, buds through outer nuclear membrane into cytoplasm (loses envelope), re-enveloped in the ER and released via exocytosis
