Microbiology: Lysogenic Cycle

• Microbiology involves the study of microorganisms, including bacteria, viruses, fungi, protozoa, and algae.
• It encompasses their structure, function, genetics, ecology, and their interactions with other organisms.
• Microbiology has various sub-disciplines, like bacteriology, virology, mycology, and parasitology.
• It also has practical applications in medicine, agriculture, industry, and environmental science.

Lysogenic Cycle

• The lysogenic cycle is a viral replication cycle where the viral DNA integrates into the host cell's DNA.
• It does not immediately kill the host cell.
• The viral DNA, now called a prophage, is replicated along with the host cell's DNA during cell division.
• The prophage remains dormant within the host cell's genome for an extended period.
• Environmental factors or other triggers can cause the prophage to excise from the host DNA and enter the lytic cycle.
• This switch leads to the production of new viral particles and the eventual destruction of the host cell.
• Lysogeny is common in bacteriophages (viruses that infect bacteria).
• The integrated viral DNA can sometimes confer new properties to the host cell.
• This phenomenon is called lysogenic conversion.
• For example, certain bacteria produce toxins only when they are lysogenized by specific bacteriophages.

Lytic Cycle

• The lytic cycle is a viral replication cycle that results in the destruction of the host cell.
• The virus attaches to the host cell and injects its DNA or RNA.
• Viral DNA replicates using the host cell's machinery.
• Viral components are synthesized and assembled into new viral particles (virions).
• The host cell lyses (bursts open), releasing the newly formed virions.
• These virions can then infect other cells, continuing the cycle.
• The lytic cycle is a rapid process, often completed within hours.
• It is a common mode of replication for many viruses, especially bacteriophages.
• The cycle leads to the death of the infected cell.

Steps of the Lytic Cycle

• Attachment: The virus attaches to specific receptors on the surface of the host cell.
• Penetration: The virus injects its genetic material (DNA or RNA) into the host cell. This can also be referred to as genome entry
• Biosynthesis: Viral DNA directs the host cell's machinery to synthesize viral components (proteins and nucleic acids). This can also be referred to as the synthesis of the phage
• Assembly: The viral components are assembled into new viral particles (virions).
• Lysis: The host cell bursts open (lyses), releasing the newly formed virions.
• Release: The released virions infect other susceptible cells, continuing the cycle.
• Replication of the viral genome occurs within the host cell.
• Transcription and translation of viral genes produce viral proteins.
• The timing of each step can vary depending on the virus and the host cell.

Differences Between Lysogenic and Lytic Cycles

• Outcome for Host Cell: The lytic cycle destroys the host cell. The lysogenic cycle initially preserves the host cell.
• Viral DNA Integration: The lytic cycle does not involve integration of viral DNA into the host DNA. The lysogenic cycle involves integration of viral DNA into the host DNA (forming a prophage).
• Speed of Replication: The lytic cycle is a rapid process that quickly produces many new virions. The lysogenic cycle is a slower process where the viral DNA is replicated along with the host cell's DNA during cell division.
• Dormancy: The lytic cycle does not have a dormant phase. The lysogenic cycle includes a dormant phase where the prophage remains inactive within the host cell's genome.
• Viral Gene Expression: In the lytic cycle, viral genes are immediately expressed to produce viral components. In the lysogenic cycle, most viral genes are repressed, except for those needed to maintain the prophage state.
• Environmental Triggers: The lytic cycle is not typically triggered by environmental factors. The lysogenic cycle can be triggered by environmental factors to switch to the lytic cycle.
• Lysogenic Conversion: Absent in the lytic cycle. Present in the lysogenic cycle, where the prophage can confer new properties to the host cell.
• Cell lysis occurs at the end of the lytic cycle, releasing new virions. Cell lysis is delayed in the lysogenic cycle and only occurs when the prophage enters the lytic cycle.
• The lytic cycle is an immediate and destructive form of viral replication. The lysogenic cycle allows the virus to persist within the host cell for extended periods.