Lysogenic Cycle-Definition, Structure, and Steps


Introduction

Viruses are very minute organisms and are considered a bridge between living and non-living. They are microscopic agents that cause infections in other organisms. Generally, viruses do not have any metabolic machinery of their own to carry out their life processes instead they depend on other organisms called hosts for their survival and multiplication.

It is always a debatable issue whether a virus should be considered a living organism or not. A virus that is outside the body of the host is non-living and is called a virion. In this topic, we will be discussing the replication in viruses, the lysogenic cycle, and its significance.

Fig. Various Types of Viruses

Replication in Viruses - An Overview

Replication is the process by which viruses multiply and produce their own copies inside the host by using the host’s cellular machinery. At the end of the replication, there may or may not be damage to the host cell. There are two pathways by which viruses can multiply in number, they are-

  • Lytic cycle

  • Lysogenic cycle

Some viruses may take up both pathways to multiply themselves, while others may use only the lytic cycle.

In lytic cycle, the virus first holds on to the host cell and injects its genetic material which can be either DNA or RNA. The genetic material then translates protein using host cellular machinery. After that, they assemble all their parts, lyse the host cell, and spread to another host to continue their life cycle.

In the lysogenic cycle, the virus does not kill its host instead incorporates its DNA into the host genome by producing prophage, and as the host genome replicates the viral DNA also multiplies and is transmitted to a daughter host cell and the cycle continues. Now we will be discussing the lysogenic cycle in detail.

The Lysogenic Cycle

  • In this pathway, the bacterial virus better known as bacteriophage attaches itself to the host bacterial cell and injects its genetic material into the bacteria. When bacteria undergo cell division it will undergo replication and so do the viral DNA.

  • The viral DNA replicates by the same chemical reaction that replicates the bacterial DNA. There is no harm caused to the host cell as viral DNA rides on the efforts being spent by the bacteria. The quantity of viral DNA produced is too small by this process and viral genomes do not take over the bacterial machinery like that in the lytic life cycle.

  • By this pathway the virus multiplies its DNA without the expenditure of its own energy and efforts as the viral genome replicates with the genome of the bacteria.

  • Virus remains like that until the approach of favorable conditions, at this time the virus switches to the lytic phase.

  • In this phase viral DNA is transcribed to mRNA which then translates to a protein coat that can carry the viral DNA outside the host cell. A stage comes when the host cell will be full of viruses encapsulated in a viral protein capsid. This overloading will lyse the cell setting the viral particles free to infect other bacteria.

Steps Involved in Lysogenic Cycle

Step I- Attachment and Penetration

  • This step is quite similar to that of the lytic phase of the viral life cycle. In this step, the virus attaches itself to the surface of the bacterial host cell.

  • The attachment is very specific and is mediated by ligands that are present on the surface of the viruses which binds to the specific receptors on the surface of the bacteria.

  • Once attached to the surface the virus injects its genetic material into the cytoplasm of the host cell.

  • A segment of viral DNA is incorporated into the host; this leads to the formation of the prophage. This step converts the infective prophage into a non-infective prophage.

Step II- Replication

  • Once incorporated into the host genome, the prophage replicates with the host genome.

  • Sometimes it happens that the prophage is excluded from the host genome, in that case, the viral enters the lytic cycle and causes the destruction of the host cell

  • Normally as the host cell divides the prophage will be transmitted to the daughter cells from one generation to another.

  • The replication continues and the viral particles remain dormant until some physical, stress factors like chemicals, low nutrients, and UV radiations cause the transition of the lysogenic phase into the lytic phase.

  • Once the transition takes place DNA is transcribed to mRNA and then translated into viral protein. This causes the synthesis and assembly of viral particles which are then released from the host cell by disrupting the host membrane.

  • Now these new viral particles are free to infect other bacteria.

Significance of Lysogenic Cycle or Lysogeny

  • Lysogenic cycle also known as temperate phase act as the medium of gene transfer by a method known as specialized transduction.

  • It helps in recombination as new phage particles releases by lysis transmit the previous bacterial DNA to a new host.

  • It is one of the most common methods of gene transfer from one bacterium to another.

Conclusion

Bacterial viruses also called bacteriophages replicate either by lytic cycle or lysogenic cycle or by both. In the lysogenic cycle, the viruses incorporate their DNA into the host genome and form a structure called a prophage. This prophage replicates with the host genome and is transmitted from one generation to another.

The prophage remains dormant and are temperate as they do not cause harm to the host cell but under favorable conditions switches the lytic phase breaking open the host cell and ready to infect new bacteria. Studying this cycle of replication has shed light on the use and development of certain medicines to block the pathways of viral development.

Updated on: 17-May-2023

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