A2.2 Viruses - Lytic & Lysogenic Cycle

What is the lytic cycle?

A viral replication cycle in which the virus replicates inside a host cell and eventually causes the cell to lyse, releasing new virions.

What are the five stages of the lytic cycle?

Attachment → penetration → replication → assembly → lysis.

What happens during attachment?

Viral surface proteins bind to specific receptors on the host cell.

Why are viruses often host-specific?

Viral attachment proteins must bind to specific receptors on the host cell.

What happens during penetration?

The viral genetic material enters the host cell.

How does a bacteriophage typically penetrate a bacterial cell?

It injects its genetic material into the host cell.

What happens during replication in the lytic cycle?

Host cell machinery is used to replicate viral genetic material and synthesise viral proteins.

Why may host DNA be degraded during viral replication?

To redirect the host cell's machinery towards production of viral components.

What happens during assembly?

New viral genomes and capsid proteins self-assemble into complete virions.

What is a virion?

A complete, fully assembled virus particle.

What happens during lysis?

The host cell membrane or cell wall is broken down and the cell bursts, releasing new virions.

What enzyme can help bacteriophages cause lysis?

Lysozyme.

What happens after lysis?

New virions infect other host cells and repeat the cycle.

What is the lysogenic cycle?

A viral replication pathway in which viral genetic material integrates into the host genome and replicates with it without immediately destroying the host cell.

What are the main stages of the lysogenic cycle?

Attachment and entry → integration → prophage state → replication with host → possible switch to lytic cycle.

What happens during attachment and entry in the lysogenic cycle?

The virus attaches to the host and injects its genetic material.

What happens during integration?

Viral DNA becomes incorporated into the host cell's chromosome.

What is a prophage?

Viral DNA that has integrated into the bacterial chromosome.

What happens while a prophage is inactive?

It remains integrated in the host genome and viral proteins are not produced.

What does temperate mean when describing a virus?

The virus can remain dormant in the host without immediately destroying it.

How is a prophage replicated?

It is copied whenever the host cell replicates its DNA.

Why do daughter cells inherit a prophage?

The integrated viral DNA is replicated along with the host chromosome before cell division.

What can cause a prophage to enter the lytic cycle?

Internal or external stress, such as cellular stress or UV radiation.

What happens when the prophage enters the lytic cycle?

It leaves the host genome, replicates, assembles new virions and eventually causes host-cell lysis.

What is the key difference between the lytic and lysogenic cycles?

The lytic cycle immediately produces new virions and destroys the host cell, whereas the lysogenic cycle initially integrates viral DNA into the host genome without destroying the cell.