Viruses have an acellular (not cell!) organisation.
Extracellular virus: Virion (metabolically inert)
Living characteristics:
1. Has own nucleic acids.
2. Can reproduce (in host cells).
3. Obtain and use energy (from host).
4. Respond to environment (spontaneous induction).
5. Can adapt to environment (via mutations).
Non-living characteristics:
1. Not cells (no cytoplasm, organelles).
2. Virions metabolically inert.
3. Cannot reproduce independent of host cell.
Virus: OBLIGATE INTRACELLULAR PARASITES (oh i peed)
Nucleocapsid: Capsid + Genome
Comparing between genomes:
1. DNA vs RNA
2. ss vs ds
3. Linear vs circular
4. Single vs segmented molecules
5. +ve sense (can translate) vs -ve sense (cannot translate)
Capsid: Protein coat enclosing nucleic acid genome, built from capsomere subunits.
-Helical (coil)
-Icosahedral (20 triangular facets)
Function:
- Protect from nuclease digestion.
- Attach to host cell membrane.
- Penetrate host cell membrane.
Envelope
Function: Attach via glycoprotein spikes binding to specific receptor sites.
Reproductive Cycle:
1. Adsorption
2. Penetration
3. Synthesis/ Replication
4. Assembly
5. Release
Bacteriophage: Virulent (lytic) vs Temperate phage (lytic and lysogenic)
Virulent: T4
Temperate: Lambda
T4 Virulent Bacteriophage
Structure: Linear, dsDNA in icosahedral capsid.
Helical tail capsid: Tail sheath, baseplate, long tail fibers, baseplate pins
Lytic cycle of T4 Virulent Bacteriophage
1. Adsorption:
Long tail fibers bind to specific receptor sites of bacterial host cell.
Weak chemical bonds are formed, adhering virion to host cell membrane.
Baseplate pin anchors, forming strong irreversible interactions with host cell membrane.
2. Penetration:
Tail sheath contracts, hollow core pierces through cell wall.
Phage dsDNA is injected.
Capsid is left outside bacterium.
3. Synthesis/ Replication:
Phage dsDNA replicates to produce phage dsDNA, transcribes to produce phage mRNA which translates to phage proteins.
Phage proteins:
-Early: Viral nuclease
-Late: Viral components, lysozyme
4. Assembly:
Self-assemble as phage genome packages inside capsid, forming nucleocapsid.
5. Release:
Lysozyme digests peptidoglycan bacterial cell wall, causing osmotic lysis.
T4 virulent bacteriophages are released.
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