8.3
Réplication virale – Cycle lytique
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Q1: How does T4 bacteriophage attach to and enter E. coli cells?
T4 phage tail fibers bind to specific receptors on the E. coli cell surface, ensuring host specificity. The phage baseplate releases lysozyme, which weakens the bacterial cell wall. The tail sheath then contracts, injecting viral DNA into the cytoplasm while the empty protein capsid remains outside.
Q2: What happens to the host cell machinery during T4 phage infection?
Once inside, T4 DNA rapidly hijacks the host cell. Bacterial transcription and translation are shut down, host DNA is degraded, and cellular resources like RNA and DNA polymerases are redirected entirely toward viral protein synthesis and genome replication using the host's ribosomes and nucleotides.
Q3: How do newly synthesized viral components form complete phage particles?
Viral structural proteins self-assemble into new phage particles through a highly coordinated process. Scaffolding proteins and chaperones assist in the precise attachment of capsids, tails, and fibers, ensuring correct assembly. Fully assembled, infectious virions form within the bacterial cytoplasm, ready to infect new hosts.
Q4: What role do holin proteins play in releasing phage progeny?
Holin proteins create pores in the bacterial cell membrane, facilitating lysozyme action in degrading the cell wall. As enzymatic degradation progresses, the bacterial cell swells before bursting due to osmotic lysis, releasing dozens to hundreds of new phage particles within 25-30 minutes of infection.
Q5: How long does the T4 lytic cycle take from infection to cell lysis?
The entire T4 lytic cycle occurs within approximately 25-30 minutes. During this time, the phage attaches, injects DNA, hijacks host machinery, synthesizes viral components, assembles new virions, and lyses the bacterial cell. The released progeny virions then disperse to infect other susceptible E. coli cells.
Q6: Why is host specificity important for T4 bacteriophage infection?
T4 phage tail fibers specifically recognize and bind to receptor molecules on the E. coli cell surface, ensuring the virus infects only compatible bacterial hosts. This specificity prevents the phage from wasting resources attempting to infect unsuitable cells and allows efficient viral replication in the correct host.
Q7: How does the lytic cycle differ from other viral replication strategies?
The lytic cycle results in rapid viral proliferation and host cell destruction, releasing progeny virions within 25-30 minutes. In contrast, the viral replication lysogenic cycle integrates viral DNA into the host genome without immediate cell lysis, allowing the virus to replicate passively with the host cell.