Lytic Vs. Lysogenic Cycle

Lytic and lysogenic cycles are interchangeable methods of viral multiplication. Similar, and at times, confusing, understanding the difference between both these cycles depends largely on studying each of them individually.

A virus undergoes lytic and lysogenic cycles to reproduce. The lytic cycle is relatively more common, wherein a virus infects a host cell, uses its metabolism to multiply, and then destroys the cell completely.

Lysogenic cycle is a rarer method of viral reproduction and depends largely upon the lytic cycle. Here, the virus integrates its genetic information with that of the host and then becomes dormant, letting the host multiply and continue its normal activities. At some point, the virus is triggered, and it thereafter goes on to multiply and ultimately, destroys the host cell.

In order to understand the differences between these two cycles, we need to individually understand each of them. The first two stages — attachment and penetration — are common for both the cycles.

Stage I: Attachment

A virus requires a host cell for multiplication.
In the first step, a virus attaches itself onto the host cell. The virus may attach itself on the receptor of the cell, or simply any other part on its surface.

Note: In some cases, a small part of the virus attaches to the host, instead of the virus itself.

Stage II: Penetration

Once attached, the virus then gains access to the cell body by breaking through the cell’s plasma membrane and in some cases, the cell wall — tail fibers of the virus and enzymes released by it help in breaking through the same.
The virus then releases its genetic material into the host cell. This genetic material (nucleic acid) could either be a single-stranded RNA, or a double-stranded DNA.
The cell is now said to be infected.

Lytic Cycle

Stage III: Seizure of Cell Mechanism

The injected DNA or RNA then hijacks the host cell’s genetic material and mangles with its gene expression.
If DNA is injected:

The virus’s DNA transcribes itself into messenger RNA molecules.
These molecules can control the host’s ribosomes (protein synthesis powerhouses).
Thus, the virus gains control of the host cell’s mechanism.

If RNA is injected:

An enzyme called reverse transcriptase transcribes the virus’s RNA into DNA.
This DNA is again transcribed into messenger RNA molecules (as explained above).
Thus, it takes over the cell’s mechanism.

The virus takes over the cell’s machinery to produce multiple viral components. Simply put, the virus infects the host to produce more virus parts.
Also, the virus puts the host nucleotides to work and replicates its own genetic material.

Stage IV: Virus Multiplication and Cell Destruction

Now that the genetic material as well as viral parts are replicated, the host metabolism assembles new viruses from scratch.
First, the capsids are assembled, followed by the genetic material in the head, and finally, the tail fiber is attached to the end portion.
Eventually, the host cell is filled with viruses.
Once the assembly is completed, the virus releases an enzyme that disintegrates the cell wall from within, thus destructing the host cell.
The viruses are individual and free now.
Each one of these viruses will go on to infect another host cell.

Lysogenic Cycle

Stage III: Incorporation of Genetic Material

Once the virus’s genetic material is inserted, it attaches itself to that of the host’s.
Some viruses may not immediately start multiplying or replicating after infecting the cell. In case it does not start replicating, the virus is said to be dormant.
A virus can survive for many years in a dormant state without affecting regular bodily activities of the host cell. This is primarily because a virus, in itself, performs no metabolic activities — it is merely genetic information coated in a protein layer.
The virus’s genetic material is known as prophage, while it is in the dormant state.

Stage IV: Replication of Genetic Material

Thereafter, the cell will fall back into its regular metabolic activities and eventually prepare for cell division.
Herein, genetic material or nucleic acid replicates, and the nucleus divides into two parts; each part having the same genetic information.

Note: The genetic information comprises nucleic acid of the virus as well, as discussed in the previous stage.

Similarly, every time, any of the host’s progenies multiply, the virus is replicated into each of them.

Stage V: Cell Division

Once the genetic information is replicated, the remaining stages of cell division follow.
The cell organelles will be replicated (in some cells only) followed by division of the cell body, resulting in two daughter cells — each having the virus’s genetic information incorporated to their genetic material.
The cells have utter normalcy until the virus is triggered. They even grow and go on to multiply into two more daughter cells. These daughter cells carrying the viral genetic matter are known as lysogenic cells.
However, once the virus is triggered, it comes out of its dormant stage and proceeds with the third stage of the lytic cycle, multiplying itself and destroying the host cell.
Though there are several different contributing factors, the exact cause of a trigger is still unknown in eukaryotic cells.
In prokaryotic cells, being exposed to UV radiation is a trigger for the virus.