Question 1

What happens to a bacteriophage genome when it enters a host cell during the lysogenic cycle?

Accepted Answer

Upon infection, the linear phage genome circularizes to prevent degradation by host nucleases. The circular DNA then integrates into the bacterial chromosome via site-specific recombination at a specific attachment site, forming a dormant prophage. This integration allows the viral DNA to persist within the host without triggering immediate cell lysis.

Question 2

How does a prophage remain stable inside a bacterial cell?

Accepted Answer

The cI repressor protein binds to phage operator regions, blocking transcription of genes required for virion assembly and host lysis. This repression maintains prophage dormancy while allowing normal bacterial replication. Each time the bacterium divides, the prophage is duplicated along with the host DNA, ensuring stable vertical transmission across generations.

Question 3

What are the main benefits a lysogenic bacterium gains from carrying a prophage?

Accepted Answer

Lysogenic bacteria acquire immunity against superinfection by the same phage, as the repressor prevents additional infections. Prophages also enable lysogenic conversion, introducing new bacterial traits such as toxin production. Additionally, prophages facilitate horizontal gene transfer, promoting genetic diversity among bacterial populations and enhancing overall fitness.

Question 4

What triggers a prophage to switch from the lysogenic cycle to the lytic cycle?

Accepted Answer

Environmental stress conditions such as UV radiation exposure, starvation, or chemical damage can trigger prophage excision from the bacterial chromosome. Once excised, the prophage enters the lytic cycle, where viral genes are expressed, new virions are assembled, and the host cell ultimately undergoes lysis, releasing progeny phages.

Question 5

How does integrase-mediated recombination enable viral DNA integration into the bacterial chromosome?

Accepted Answer

Phage-encoded integrase catalyzes site-specific recombination between the phage attachment site and the bacterial attachment site (attB in E. coli). This specialized enzyme facilitates precise integration of the viral genome into the host chromosome at predetermined locations, ensuring stable prophage formation and predictable inheritance patterns.

Question 6

Can a prophage spontaneously excise from the bacterial chromosome without external stress?

Accepted Answer

Yes, some prophages can spontaneously excise at low frequency even without external stressors. This spontaneous excision represents a dynamic aspect of lysogeny, where selective pressures and random molecular events influence the long-term persistence of prophages within bacterial populations and their evolutionary impact.

Question 7

Why is temperate bacteriophage lambda considered a model organism for studying lysogeny?

Accepted Answer

Bacteriophage lambda infects Escherichia coli and demonstrates the complete lysogenic cycle, including genome circularization, site-specific integration, repressor-mediated gene silencing, and prophage stability. Its well-characterized molecular mechanisms and regulatory proteins make it an ideal system for understanding how temperate phages balance viral persistence with host cell viability.

Replicazione virale – ciclo lisogenico

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