Lytic Cycle
The lytic cycle is one of the two primary ways that viruses replicate inside a host cell. In this cycle, the virus actively takes over the host's machinery to reproduce its own viral particles, ultimately leading to the destruction (lysis) of the host cell. The steps of the lytic cycle are as follows:
Attachment: The bacteriophage binds to specific receptors on the surface of the host bacterium.
Entry: The viral genetic material (DNA or RNA) is injected into the host cell, leaving the viral capsid outside. In the case of bacteriophages, the viral DNA is directly injected into the bacterial cytoplasm.
Biosynthesis: Once inside, the viral genome hijacks the host's cellular machinery (such as ribosomes and enzymes) to transcribe and translate viral genes. The host cell begins synthesizing viral components—proteins, enzymes, and copies of the viral genome.
Assembly: The newly synthesized viral components are assembled into complete virus particles (virions). The viral DNA is packed into new capsids, forming mature bacteriophages.
Lysis and Release: After numerous virions are assembled, the host cell becomes full of these viral particles. Eventually, enzymes produced by the virus cause the host cell to break open (lysis), releasing hundreds of new virions to infect other nearby cells.
The lytic cycle is fast, and each cycle results in the death of the infected host cell. It leads to a rapid spread of the virus throughout the host population or tissue.
Lysogenic Cycle
The lysogenic cycle is an alternative viral replication process where the virus integrates its genetic material into the host genome and remains dormant (latent) for an extended period. This cycle does not immediately destroy the host cell. The key stages of the lysogenic cycle are:
Attachment: Like in the lytic cycle, the bacteriophage attaches to the host bacterium and injects its viral DNA into the host cell.
Integration: Instead of immediately hijacking the host cell machinery, the viral DNA integrates into the host's genome. This integrated viral DNA is called a prophage. The bacteriophage essentially "hides" within the host's DNA.
Dormancy: In this stage, the virus remains dormant and does not produce new viral particles. The host cell continues to grow and divide, copying its own DNA along with the prophage. In this way, viral DNA is passed on to daughter cells during cell division without causing harm to the host.
Induction: The viral DNA can remain latent for a long time. However, under certain environmental triggers (such as stress, UV light, or chemical exposure), the prophage can be "reactivated." When this happens, the viral DNA excises itself from the host genome and enters the lytic cycle.
Lytic Cycle Entry: Once reactivated, the virus resumes the lytic cycle, taking over the host cell’s machinery to produce new virions. Eventually, the host cell undergoes lysis, releasing new viral particles.
The lysogenic cycle allows the virus to remain dormant within a host for many generations without killing the host immediately. This form of replication is advantageous for viruses when the conditions are not favorable for active viral replication or when resources are limited.
Key Differences Between Lytic and Lysogenic Cycles:
- Outcome: The lytic cycle leads to the destruction of the host cell, while the lysogenic cycle allows the host cell to survive and continue dividing.
- Viral DNA: In the lytic cycle, the viral DNA is not integrated into the host genome, while in the lysogenic cycle, the viral DNA integrates and becomes a prophage.
- Viral Spread: The lytic cycle produces a large number of new viruses rapidly, whereas the lysogenic cycle can spread viral DNA passively across multiple generations without immediate virion production.
Both cycles are vital for viral propagation, with the lytic cycle being more aggressive and the lysogenic cycle providing a more stealthy, long-term presence within the host.
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