What are the steps of viral replication?
- Recognition of a “target“ host cell.
- Internalization of the virus.
- Transcription and Replication.
- Translation.
- Post translational modification.
- Release.
- Particle maturation.
What are the 3 types of viral internalisation?
- Fusion from without
- Receptor-mediated endocytosis
- Receptor-mediated endocytosis with fusion in the endosome
Which viruses demonstrate “Fusion from without”
Measles, mumps, HIV, other membranous viruses
Which viruses demonstrate “Receptor-mediated endocytosis”?
Naked viruses:
■ Adenovirus.
■ Picornaviruses [Polio, hepatitis A]
■ Rotavirus.
Which viruses demonstrate “Receptor-mediated endocytosis with fusion in the endosome”?
Influenza, other membranous viruses
What happens during transcription?
Formation of Viral mRNAs
Describe transcription of RNA viruses
- Positive sense viral RNA used directly as viral
mRNA. - Negative sense viral RNA first transcribed to positive sense RNA
Describe transcription of DNA viruses
mRNA is transcribed from initiation sites
Describe transcription of Retroviruses
○ RNA → DNA.
○ Integrated (proviral DNA).
○ Viral mRNA transcribed from the proviral DNA.
What is the end goal of translation?
Synthesis of Viral Proteins
What happens during translation?
All viruses use cellular ribosomes to translate viral mRNAs, structural proteins and enzymes formed.
● Structural proteins: Building blocks of particles.
● Enzymes: Necessary for viral life cycle.
What happens post-translation?
Modification of Viral Proteins
Name 2 ways viral proteins are modified?
- 3D structure: Folding
- Glycosylation: Carbohydrate addition
How are viruses released?
- Lytic viruses are released on lysis and death of the cell (poliovirus).
- Others (influenza, HIV, measles) escape by budding from the cell surface.
Which viruses require a post-release maturation stage?
HIV and influenza
What happens during the maturation stage?
- HIV proteins cleaved by viral protease.
- Influenza: cellular proteases are needed to cleave the viral HA spike protein.
Name the mechanisms of genetic variation
- mutations
- gene reassortment
- recombination
Define pathogenicity
Compares the severity of disease caused by different organisms
Define virulence
Compares the severity of the disease caused by different strains of the same micro-organism
What effects do viruses have on host cells?
- direct
- indirect
- cytoplasmic
What direct effects do viruses have on host cells?
Some viruses kill cells they infect:
- Normal cellular metabolism comes to a halt.
- Cell becomes a virus factory.
- Some cells only release viruses when they disintegrate.
What indirect effects do viruses have on host cells?
- Virus infected cells produce interferon
- Interferon results in an ‘antiviral state’ BUT this also interferes with normal cell function.
- The immune system attacks and destroys viral infected cells, final result is cell lysis or apoptosis
What 2 things do cytopathic effects impact?
morphology and growth properties
What is the cytopathic effect on morphology?
- Cell fusion (synctia)
- Inclusion bodies: Intracytoplasmic [predominantly RNA viruses], Intranuclear [predominanlty DNA viruses]
