Virus Replication Flashcards
What is the step 0 of viral replication?
Entry into the body
What are 2 examples of entry points into the body (w/ virus)?
- Respiratory; Influenza
- Skin; arboviruses like Dengue
What are the steps of viral replication in order?
(0. Entry into body)
1. Attachment
2. Entry
3. Genome replication
4. Assembly
5. Egress
What is attachment?
Virus must bind to a compatible receptor to initiate infection
What are some examples of host receptors for viruses?
- Proteins (e.g. CD4 for HIV)
- Carbohydrates (e.g. siliac acid for Influenza)
What are some important features of the attachment stage?
- Specificity: viral tropism is determined by presence of compatible receptor
- Essentiality: viruses often bind to essential host molecules (hard for host to mutate away
- Energy free: attachment is due to electrostatic attraction- difficult for cell to realise virus has bound
What are some examples of virus tropism due to attachment?
- HIV-1 has a tropism towards CD4+ T cells, as it uses CD4 as a receptor (and CCR5/CXCR5 co-receptor)
- Influenza has a tropism for respiratory epithelium, as cells there are lined with siliac acid (receptor)
What are 3 main cell entry mechanisms for viruses?
Direct penetration
Membrane fusion
Endocytosis
How does direct penetration work?
DNA is injected into the cell via a tail (e.g. by bacteriophage)
How does membrane fusion work?
Viral envelope fuses with plasma membrane.
Nucleocapsid of virus is released into the cytoplasm
Viral envelope stays as semi-circle on membrane (becomes a patch)
What is an example of a virus that enter via membrane fusion?
HIV
How does endocytosis work?
Virus particle is engulfed by host membrane, and forms endocytic vesicle
Low pH induces membrane fusion through changing structure around vesicle
Nucleocapsid is released into the cytoplasm
Which out of endocytosis and membrane fusion is pH independent?
Membrane fusion is pH independent
What is an example of a virus that enters via endocytosis?
Influenza.
Enters via Cathrin-mediated endocytosis
Does fusion occur in endocytosis?
Yes in some cases.
The difference is that the fusion occurs inside the endoscome, rather than the surface (like with membrane fusion entry).
For endocytosis, what allows fusion inside the endosome to occur?
Acidic endosomal pH triggers conformational change in fusion proteins, allowing fusion
What type of viruses undergo membrane fusion/endocytosis + fusion?
Enveloped viruses
How do non-enveloped viruses typically enter cells?
Endocytosis + pore formation
What is an example of a virus that undergoes endocytosis + pore formation?
Non enveloped viruses like Poliovirus & Adenovirus
What is the main difference between endocytosis w/ fusion and endocytosis w/ pore formation?
In endocytosis w/ pore formation, capsid proteins undergo conformational changes inside the endosome which result in pore formation in the endosomal membrane.
Viral genome is then injected into the cytoplasm
What happens during genome replication?
Various depending on Baltimore classification, however they all have one thing in common:
All viruses must make mRNA that is readable by host ribosomes (this is typically positive sense mRNA)
Typically, what viruses can establish latency in their host?
DNA viruses (like Herpesviruses)
What is the exception to DNA viruses establishing latency?
Retroviruses (like HIV), as they use reverse transcriptase and intergrase to integrate into host genome
What is the assembly stage of viral replication?
Newly synthesised viral proteins and genomes are assembled into immature virions
