Viruses Rely Entirely on Host Cells
- Viruses cannot reproduce independently.
- They depend entirely on the host cell’s metabolic machinery.
- The lytic cycle is the most direct reproductive strategy, producing large numbers of viral particles in a short time but at the cost of killing the host.
Definition
The Lytic Cycle
The lytic cycle is a viral replication process during which a virus infects a host cell, takes control of its machinery to produce new viral particles, and ultimately causes the host cell to burst (lyse), releasing the newly formed viruses.
The Lytic Cycle: A Step-by-Step Breakdown
- Step 1: Attachment
- Tail fibers or viral attachment proteins bind to complementary receptor molecules on the host cell surface.
- This specificity determines host specificity (why viruses only infect certain cells).
- Step 2: DNA Entry (Penetration)
- The phage injects its double-stranded DNA into the bacterial cell through a syringe-like tail structure.
- The viral DNA enters, while the capsid remains outside.
- Step 3: Biosynthesis (Replication of Viral Components)
- Viral DNA hijacks the host's enzymes and ribosomes, redirecting host metabolism:
- Early genes produce enzymes for viral DNA replication.
- Late genes encode structural proteins (capsid, tail).
- Normal host functions are suppressed.
- Step 4: Assembly of New Viruses
- Viral proteins and DNA self-assemble into complete bacteriophage particles.
- Each capsid is filled with a copy of the viral genome, forming functional viruses.
- Step 5: Lysis of the Host Cell
- Phage-produced enzymes degrade the bacterial cell wall.
- The host cell bursts (lyses), releasing hundreds of new bacteriophages that spread to infect other cells.
- Self-assembly is a remarkable feature of viruses.
- The repeating protein subunits of the capsid naturally arrange themselves into a symmetrical structure, requiring no additional energy input.
Biological Significance of the Lytic Cycle
- Rapid Amplification: Allows viruses to spread quickly through a host population.
- Host Damage: Destroys host cells, leading to disease symptoms.
- Limitation: If all host cells die, the virus risks extinction unless it spreads to a new host.
- Students often confuse the lytic cycle with the lysogenic cycle.
- Remember, the lytic cycle results in the destruction of the host cell
- The lysogenic cycle involves the integration of viral DNA into the host genome without immediate destruction.
- Why can viruses not reproduce independently, and what cellular machinery do they depend on in the host?
- Define the lytic cycle and explain its biological cost for the host cell.
- During DNA entry, why does only the viral nucleic acid enter the host cell while the capsid remains outside?
- Which enzyme is responsible for degrading the bacterial cell wall during the lysis stage, and why is this necessary for viral release?
