Virus-host interactions

In order for a virus to infect a host, like you and I, several processes must occur involving the virus and its target cells. They are, in the following order:

1. Attachment & Entry
2. Uncoating
3. Integration
4. Replication
5. Assembly
6. Exit

Attachment & Entry

This is when a virus binds to the surface of a cell.

In one type of attachment, called receptor-mediated endocytosis, the virus must bind to a specific protein on the surface of the cell’s membrane, using one of its own surface proteins. Once this occurs, the cell is stimulated to take in the virus by infolding its membrane until a vesicle containing the virus is pinched off inside the cell itself.



Normally, this process is used to take in certain substrates that the cell uses to grow and perform its function. However, since the virus is able to replicate the signals of these substrates, the cell takes the virus in anyway.

The other type of attachment, fusion, occurs without the need for the cell to perform endocytosis. However, this method requires the virus to be an enveloped virus.

When the envelope of the virus makes contact with the cell surface membrane, the two begin to fuse together. This is possible because of the fluid nature of the phospholipid bilayer(what membranes are made of). Once the fusion occurs, the virus is released into the cytoplasm of the cell.



Uncoating

Most viruses have a protein coat surrounding their genetic material, called the capsid. Once inside the cytoplasm of the host cell, the capsid must be broken down in order to release the genetic material and its other contents.
This is done by the host cell’s own hydrolytic enzymes. The capsid is digested, releasing the genetic material.


Integration

Integration does not happen for all types of viruses. Only DNA viruses need to have their genetic material integrated into the genome of the host cell. This is done using enzymes called restriction enzymes and integrases.

The restriction enzymes cut a section out of the cell’s genome, and the viral genome is placed in the gap using the integrases. This effectively makes the viral genome part of the cell’s genome.



Replication

This step is when the viral genomes are expressed to form viral proteins. DNA viruses will have their DNA transcripted and translated in the same way that the host genome produces proteins. RNA viruses can act as messenger RNA, bypassing the need for transcription.

The genome of the virus is also replicated, using the host cell’s machinery.

Assembly

The viral proteins and nucleic acids that are produced will then be transported together, and then assembled into new viruses. A single virus infecting a cell can produce many more viruses.

Exit

The exit of the new viruses can occur in two main ways.

The first method is known as budding. The newly assembled viruses move to the edge of the cell, and the viruses move out of the cell, taking a portion of the cell’s membrane with them. The stolen bit of cell membrane becomes the envelope of the virus, and will be used to aid infection of other similar cells in the future.
The other method involves cell lysis. One way this can happen is that the cell produces such a large number of viruses that the cell cannot contain them, and simply bursts, releasing the viruses. Or, the viral genome can cause the cell to destroy itself with its own enzymes, also releasing the virus.

When the viruses are released out of the cell, they move on to infect new cells, repeating the cycle.

Here is an overview of the viral lifecycle: