LK - Virus Replication & Life Cycles Flashcards
What is the significance of viruses in terms of their abundance and targets? (4)
- Not a single species knows which is immune to virus infection
- Viruses attacking bacteria, archaea, plants, animals
- Viruses are ubiquitous, important for evolution
- Lots of variations in details wrt replication and life cycles
What are the 5 stages of Virus replication?
- Attachment (adsorption)
- Penetration (injection)
- Synthesis of nucleic acid protein
- Assembly and packaging
- Release (lysis)
Explain the process of attachment in virus replication (3)
- Virus binds to receptor on cell surface (often protein, sometimes sugars, lipids, or complexes of these)
- Wide range of surface receptor used by wide variety of viruses
- Same receptor used by different viruses; some viruses can use more than one receptor
How do viruses attatch in situations like HIV? (2)
- Virus first binds to common, low-affinity receptor
- The virus then ‘rolls’ over cell surface, until it binds to high-affinity receptor
How do viruses bind with CD4? (3)
Binding with CD4 leaves virus too far from cell surface
- Needs co-receptor (CCR5/CXCR4) to pull virus closer to cell surface
- Fusion between virus envelope and cell membrane
How do viruses penetrate animal cells?
Animal cells have mobile lipid bilayer membranes:relatively easy to cross
- Non-enveloped animal viruses enter via endocytosis
- Enveloped animal viruses enter via fusion
2nd uncoating step to release nucleic acid
How do viruses penetrate plant cells?
Plant cells have rigid cell wall: much harder to cross
- Viruses enter via damaged cell walls
- Viruses enter via a vector (animal feeding on plant)
How do viruses penetrate bacterial cells? (5)
Bacterial cells have rigid cell wall: much harder to cross
- Tail fibres bind and flex
- Tail pins bind
- Tail sheath contracts
- Tail core pushed through bacterial wall
- Nucleic acid injected
What happens during the synthesis stage of virus replication? (4)
- Viruses need to replicate their nucleic acid
- Viruses need to produce proteins (for capsid itself and for replication/synthesis)
- Use machinery of host cell (polymerase, ribosomes, etc)
- Order of events (e.g. early or late proteins) tightly regulated
How do DNA-viruses and RNA-viruses synthesise?
DNA-viruses
- Host cell machinery set up to replicate DNA and produce proteins from it
RNA-viruses
- RNA-dependent RNA-polymerase absent from host cell, so need to synthesise it themselves
- In some RNA-viruses, the genomic RNA itself can function directly as mRNA; in others, mRNA needs to be transcribed from the genomic RNA
How do retroviruses replicate? (3)
- ssRNA
- Reverse transcription: ssRNA -> dsDNA (reverse transcriptase)
- dsDNA used as template
What is involved in the assembley of viruses? (3)
Biosynthesis: Phage DNA directs synthesis of viral components by the host cell
Maturation: Viral components are assembled into virions
Release: Host cell lyses and new virions are released
What are the 2 types of release in viruses?
Host cell lyses and new virions are released
1) Budding – host cell may survive
2) Lysis – host cell destroyed
Lytic vs Lysogenic cycles
In the lytic cycle, the phage replicates and lyses the host cell
In the lysogenic cycle, phage DNA is incorporated into the host genome, where it is passed on to subsequent generations
Vertical gene transfer vs Horizontal gene transfer
Vertical gene transfer = transfer of genetic material from parent to offspring (between generations)
Horizontal gene transfer = transfer from one lineage to another (same generation)
