As mentioned earlier, most viruses used the lytic cycle of replication that results in the eventual death of the host cell. Now, the lytic cycle of replication will be explained in detail.
The following steps are involved in the replication cycle:
1. Attachment
For virus to infect cells, they first have to attach to them. This is done via binding of viral surface proteins to molecules/receptors on the cell membrane or wall of the host cells. This is more or less general for all viruses that go through this cycle. The one thing here that sets ALL viruses apart is the specificity of the surface protein-receptor binding. Viruses have evolved in a way that their surface proteins are only able to bind to receptors on cells that they are able to replicate in.
For example, varicella zoster virus (Chicken Pox), DO NOT infect red blood cells due to the inability of their surface protein to bind to the surface receptors of the red blood cells. This evolution is due to the requirement of the varicella zoster virus as a DNA virus for the host cell to have a nucleus.
2. Penetration
After attaching to the host cell, viruses have 4 ways to enter the cell.
The first is Receptor mediated endocytosis. This occurs for virus that have an envelope. When the surface proteins of the virus bind with the receptors on the cell surface, a signal is produced that causes the cell to engulf the whole virion (A single virus particle), allowing the virion to enter the cell.
The second is Membrane fusion. This also for virus that possess an envelope. In this case, due to the similarities in membrane and envelope composition, they are able to fuse and the virion minus the envelope is able to enter the cell.
The third is Clathrin endocytosis. This method is for naked viruses, or viruses without an envelope. The clathrin induces the formation of vesicles within the cytoplasm that connect with the cell membrane, transporting the virion into the cell.
The final and and also the most uncommon of the 4 is Genome injection. This, as the name suggests is the injection of one or more tubes through the cell membrane then secreting ONLY the viral genome and necessary enzymes through. The rest of the virion remains outside the cell.
3. Uncoating
Only those virus that enter by endocytosis or fusion go through this step. This step involves the activation of proteins and enzymes within the virion that cause its capsid (Shell) to degrade, exposing the genetic material of the virion to the cytoplasm. These genetic material could then be transported to the nucleus if required.
4. Expression
This stage is the conversion of genome (If needed) and then letting ribosomes read it. Depending on the type of virus, this the conversion of genome occurs in different places. DNA conversion occurs in the nucleus whereas RNA conversion occurs in the cytoplasm. Conversion of viral genome is needed because ribosomes were made to read single mRNA strands. Therefore out of the 7 types of viruses in baltimore's classification mentioned earlier, only one type does not require conversion. This is the single stranded positive sense RNA virus.
This type of virus do not require conversion because the positive sense RNA itself is similar to RNA meaning it is already readable by ribosomes. However for the other types of genome, they have to be converted by polymerases to positive sense single stranded RNA before ribosomes can read them.
Now, this conversion is what leads to the high mutation rates of certain viruses. For DNA viruses, this is not really a problem, because during conversion in the nucleus, they are exposed to the host cell DNA polymerases which are able to correct mistakes in transcription. RNA viruses on the other hand, perform conversion in the cytoplasm with their own RNA polymerases. RNA polymerases lack the proof reading ability of DNA polymerases meaning that large changes to the genome could go undetected. This could lead to new protein formation that could in turn increase the virulence of that virus.
In this stage, the production of all viral proteins also begins.
5. Maturation
In this stage, the whole virions are assembled. For unenveloped viruses, they are ready to be released from the cell. For enveloped ones, they will obtain their final component in the next step.
6. Release
At this stage, there are two options. Lyse the cell, or bud off the cell.
Lysing the cell kills the cell instantly. This is performed by unenveloped viruses. After they are fully assembled, they gather in the cell until the cell is either full or exhausted of nutrients before bursting out and infecting surrounding cells.
Budding off the cell does not kill the cell instantly but weakens it till a point of eventual death. This is for enveloped viruses that once fully completed, bud of the cell taking a part of the cell membrane with it to form the envelope. As the continuous taking of membrane continues, the cell soon looses its ability to repair the membrane and dies.
The infection of the cell leading up to its death can be observed via certain symptoms, termed as cytopathic effects. These include multiple nuclei in the cell, empty cells, etc. They can be observed under a microscope with sufficient magnification power depending on the size of the host cells being viewed.
Viral Replication Cycle
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