in the death or lysis of the infected cell. The cycle begins with the process of at-

tachment to the host cells, following which the viral genome is injected into the host

cell, termed penetration. Then replication of the viral genome and biosynthesis of

viral proteins occurs. This newly formed viral genome is assembled, followed by the

maturation and release of the progeny virions.

During this process of the lytic cycle, the host cell gradually weakens due to viral

enzymes and consequently burst/lyse, releasing the newly formed virions into the

surrounding environment (Figure 2.5).

2.3.2

LYSOGENIC CYCLE

The lysogenic cycle is also known as a non-virulent infection. This does not kill the

host cell; rather, it remains in a dormant state. The lysogenic cycle also starts with

attachment and penetration phases as described in the lytic cycle. But then the

genome integrates itself into the host genome with the help of phage-encoded in-

tegrase enzymes. This integrated genome is termed a “prophage.” The prophage

gets replicated with the host genome and remains there if the host cell is dividing.

But, when the virus following the lysogenic cycle encounters any environmental

stress, the prophage extracts itself from the host genome and enters the lytic cycle,

resulting in the lysis of the host cell. This conversion from the lysogenic to the lytic

cycle is termed “induction.”

Sometimes, during this induction process, the prophage might be left behind, or

take a portion of the host genome when they re-circularize. As a result, during the

next infection they might transport bacterial genes from one strain to another:

known as transduction. It’s one of the reasons resistance virulence traits spread

through the bacterial population (Figure 2.6).

FIGURE 2.5 Lytic cycle: Schematic representation of lytic infection cycle followed by

bacteriophage resulting in subsequent death of the host cell releasing newly formed virions.

Introduction to basic virology

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