There are two cycles of viral reproduction. Viral reproduction cycles are the two cycles by which viruses reproduce inside host cells. One is a lytic cycle, while the other one is a lysogenic cycle. The viruses reproducing through this mode of viral reproduction are bacterial viruses or bacteriophages. Bacteriophages that divide via lytic cycle are known as virulent phages, whereas bacteriophages that divide via lysogenic cycle are known as temperate phages. In the lytic cycle, the DNA of the virus is inside the bacterial cell, but it exists as a free molecule and then replicates separately from the host DNA. The viral DNA genome integrates in the host DNA during lysogenic cycle. This is the most important difference between both of these cycles. The rest of them use host machinery to reproduce and multiply.
The lytic cycle is named as such because it is named after the process of lysis, i.e., a process in which a virus infects a cell, it replicates, and bursts inside the cell membrane. Viral cells release viral DNA inside the host genome and then replicate and reproduce inside it, further infecting more cells.
Mechanism and role of the lytic cycle −
The lytic cycle, which is also known as the viral reproductive cycle, has six stages −
Attachment and Penetration − In this stage, the virus injects its nucleic acid into the host cell through its plasma membrane. The virus transfers this by attaching to the receptors present in the host’s cell. Binding is influenced by various factors, like pH and ions. Then, injection of DNA or RNA is done inside the cell, and that cell becomes infected.
Transcription and Biosynthesis − The virus further tries controlling the host’s machinery and then using it for replicating more of itself. DNA viruses lead to transcription forming mRNA, while RNA viruses, reverse transcriptase enzyme transcribes RNA into DNA. Viral DNA hijacks the host’s machinery and starts replicating itself.
Biosynthesis is performed in three steps −
Early phase - RNA polymerase enzyme helps in modifying the transcription of the host’s RNA so that it can transcribe its genetic material.
Middle phase - Nucleic acids (DNA or RNA) is biosynthesized inside the host cell and further replicated by enzymes like reverse transcriptase.
Late phase - Proteins are later formed by the translation of nucleic acids.
Mutation and Lysis − In just half an hour of infection by a viral genome, around 250 new viral bodies or their replicates are produced. After attaining a good number of virions, viral proteins dissolve cell walls by bursting them, i.e., known as lysis. This leads to the release of new virions to other cells or to other organs, where they can further infect many cells as the lytic cycle continues. A phage, causing lysis in any host is known as a virulent phage.
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The lysogenic cycle is the other cycle of viral reproduction, the lytic cycle by which viruses replicate inside the host cell. Lysogeny means the integration of viral nucleic material within a host’s genome.
Steps involved in the lysogenic cycle −
Step 1 − A virus tries to infect the host by injecting its viral DNA into the host’s cell through its cytoplasm. This can also be done by some permeable space between the cell walls.
Step 2 − Then this viral DNA is examined by the host proteins, and in the case of bacteriophage, by the bacteria’s proteins. These are the same proteins that are responsible for the replication of bacterial DNA.
Step 3 − Viral DNA uses the host machinery to replicate and multiply further. In the case of the lysogenic cycle, DNA replication is performed only when host DNA is getting replicated.
Step 4 − Later on, viral DNA gets into the lytic cycle and produces a lot of DNA and capsids.
Step 5 − These capsids come out, and infect many more bacteria, starting the lysogenic cycle once again.
Role of the lysogenic cycle − The purpose of the lysogenic cycle is to allow the cell to replicate and multiply within its host without killing it. A virus genome is in it as a prophage and its viral replication is prevented by a repressor.
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Both lytic and lysogenic cycles are mechanisms involving viral replication. Both of them infect their host to hijack its machinery and then replicate within it.
Both of these cycles can only occur inside the cells of any host.
Both of these cycles can produce a thousand copies in just a few hours inside the host.
Both of them are initiated with the help of a receptor present on the host cell.
| Lytic cycle | Lysogenic cycle |
|---|---|
| It is a type of viral reproduction resulting in the lysis of host cells. | It is a type of viral reproduction mechanism where viral DNA is integrated with the host. |
| It doesn’t have a prophase stage. | It has a prophase stage. |
| Viral DNA replication and the host’s DNA replication are both independent of each other. | Both DNA replications occur simultaneously. |
| Productivity is much higher. | Productivity is lower. |
| Host cells undergo lysis and are lysed. | Host cells aren’t lysed. |
| Cannot follow the lysogenic cycle. | Can follow lytic cycle |
Lytic and lysogenic cycles are the two cycles of viral reproduction happening in viruses like bacteriophages. Both of these cycles infect host cells by injecting their nucleic acids into the host cell through some permeable spaces or through the cell membrane. The viral genome of viruses then replicates inside it, and then it affects other cells subsequently. Though the role of both cycles is to replicate and multiply the number of viral cells inside the host, there are some differences. Cells undergo lysis in the lytic cycle but not in the lysogenic cycle. During the lytic cycle, the viral DNA replicates independently of the host DNA while still inside the bacterial cell. But in the lysogenic cycle, the viral DNA genome is in the host DNA.
Q1. What triggers the lytic cycle?
Ans. Various natural and environmental factors, such as exposure or starvation to toxic chemicals, can also be the reason for prophage to take up the lytic cycle.
Q2. What triggers the lysogenic cycle?
Ans. If a bacterium containing prophages comes in contact with factors such as low nutrient conditions, UV light, or chemicals like mitomycin C, prophages may extract themselves spontaneously from the host genome and enter the lysogenic cycle.
Q3. Does the lytic cycle happen immediately?
Ans. The spread of viral DNA is achieved by prokaryotic reproduction, and the lytic cycle is immediate, resulting in many copies of the virus being created at a rapid rate and then the destruction of these cells.
Q4. Which is faster, the lytic or the lysogenic cycle?
Ans. Lytic cycle is a much faster cycle than the lysogenic cycle.
Q5. What are the advantages of the lytic cycle?
Ans. In the lytic cycle, a virus is able to quickly replicate and can infect many host cells. However, the host is immediately killed, preventing the viral genome from reaching the next generation of the host cells.
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