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Describe the lytic cycle.
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The lytic cycle is one of the two major reproductive strategies used by bacteriophages (viruses that infect bacteria). It leads to rapid production of progeny viruses and the destruction (lysis) of the infected host cell. Major stages:
1. Attachment (adsorption)
- The phage recognizes and binds to specific receptor molecules on the bacterial surface (e.g., proteins, lipopolysaccharides).
2. Penetration (entry)
- The phage injects its genome (DNA or RNA) into the host; the capsid typically remains outside.
3. Early/host-takeover events
- Early phage genes are expressed. Proteins produced often inactivate host defenses, degrade host DNA, modify host transcription/translation machinery, and prepare the cell for viral replication.
4. Biosynthesis (replication and gene expression)
- The phage genome is replicated and viral proteins (structural proteins, enzymes) are synthesized. Depending on the phage, replication may use host enzymes or phage-encoded polymerases. Gene expression is often temporally regulated (early, middle, late genes).
5. Assembly (maturation)
- New virions are assembled from replicated genomes and newly made structural proteins into complete infectious phage particles.
6. Release (lysis)
- Phage-encoded lytic proteins (e.g., holins that permeabilize the membrane and endolysins/lysozymes that break down the cell wall) cause the bacterial cell to burst, releasing progeny phages to infect new cells.
Key features and terms
- Burst size: number of phage particles produced per infected cell (varies widely).
- Latent period: time between infection and release of progeny.
- Outcome: host cell death and rapid spread of phage.
- Contrast with lysogenic cycle: temperate phages can integrate their genome into the host chromosome (prophage) and replicate passively without lysis until induction triggers the lytic cycle.
Example: T4 bacteriophage follows a lytic cycle—attaches to E. coli, injects DNA, directs rapid synthesis and assembly of ~100–300 new phage particles, then lyses the cell.
1. Attachment (adsorption)
- The phage recognizes and binds to specific receptor molecules on the bacterial surface (e.g., proteins, lipopolysaccharides).
2. Penetration (entry)
- The phage injects its genome (DNA or RNA) into the host; the capsid typically remains outside.
3. Early/host-takeover events
- Early phage genes are expressed. Proteins produced often inactivate host defenses, degrade host DNA, modify host transcription/translation machinery, and prepare the cell for viral replication.
4. Biosynthesis (replication and gene expression)
- The phage genome is replicated and viral proteins (structural proteins, enzymes) are synthesized. Depending on the phage, replication may use host enzymes or phage-encoded polymerases. Gene expression is often temporally regulated (early, middle, late genes).
5. Assembly (maturation)
- New virions are assembled from replicated genomes and newly made structural proteins into complete infectious phage particles.
6. Release (lysis)
- Phage-encoded lytic proteins (e.g., holins that permeabilize the membrane and endolysins/lysozymes that break down the cell wall) cause the bacterial cell to burst, releasing progeny phages to infect new cells.
Key features and terms
- Burst size: number of phage particles produced per infected cell (varies widely).
- Latent period: time between infection and release of progeny.
- Outcome: host cell death and rapid spread of phage.
- Contrast with lysogenic cycle: temperate phages can integrate their genome into the host chromosome (prophage) and replicate passively without lysis until induction triggers the lytic cycle.
Example: T4 bacteriophage follows a lytic cycle—attaches to E. coli, injects DNA, directs rapid synthesis and assembly of ~100–300 new phage particles, then lyses the cell.
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