- Replication and Expression
All viruses contain one more important proteins known as the attachment proteins or docking proteins. The attachment protein is needed by the virus to attach to its target cell before it can enter that cell. Attachment proteins lie on the outer surface of the virus and contact the appropriate receptor sites on the target host cells, in other words, receptor sites are specific. Receptors on host cells may be protein, glyroprotein or glycolipid. These attachment proteins are often called spikes because they can extend away from the cell so as to better be able to contact the host receptor (think of the viral AP & the host receptor site as being the pairs of a velcro systemVELCRO SYSTEM). In addition, virus may contain small quantities of carbohydrate (glycoprotein). Cells lacking in specifics proteins are resistant to virus infection.
Penetration follows right after attachment process. There are 3 ways of penetration, namely, receptor-mediated endocytosis (enveloped virus), clathrin-mediated endocytosis (naked virus) and fusion for eukaryotic cells. Endocytosis is the process whereby cell absorbs molecules from outside the cell by engulfing them with their cell membrane. In the case of receptor-mediated endocytosis, enveloped virus contains attachment proteins which bind to the host’s receptor site, after binding, the cell membrane will engulf the virus into cell, thus successfully penetrating the cell. As for clathrin-mediated endocytosis, it can be shown from the animation below.
As for fusion, the envelope of virus fuses with the cell membrane as the virus approaches the cell membrane, thus releasing the virus nucleic acid into the cell.
For prokaryotic cells, the phage binding to the host cell receptor sites through its docking proteins at the end of the tail fibres. Once firmly attached to the cell, a viral enzyme in the tail punches a hole in the host's cell wall and the core region of the tail is thrust through and the DNA ejected into the host's cytoplasm. In the case of phage the protein coat remains on the outside, but with eukaryotic cells the entire virus may be taken into the host's cytoplasm.
3. Uncoating ( only in animal viruses)
Uncoating is the process whereby the nucleic acid is separated from its protein coat. This is done so by the digestive enzymes, lysozymes, produced in the host lysosomes. The viral enzymes produced by the virus itself, also separates the nucleic acid from the protein coat.
They are many types of uncoating,
- Pore formation: picornavirus
- Direct plasma membrane fusion: Paramyxoviridae
- Partial plasma membrane fusion: Human Immunodeficiency Virus (HIV)
- Endosomal plasma membrane fusion: influenza virus
- Endosomal plasma membrane lysis: adenovirus
4. Replication & Expression
Once inside the host cell, the viral genome takes over the metabolism of the host, converting the host cell entirely to its needs, which is the production of more viruses. Viral nucleic acid is transcripted and translated in the nucleus of the host cell, and the capsid protein’s protomers and enzymes are produced in ribosomes within the cytoplasm of the host cell. A capsid protein is a protein shell of a virus. It is made up of several oligomeric structural subunits made of proteins, known as protomers. These protomers are produced and pieced up together to enclose the genetic material of the virus. As for the nucleic acid, there are 7 different main groups to distinguish them. (Refer to the Baltimore’s classification)
The various components of the virus that is produced in the cells accumulate and begin to spontaneously assemble into new phage. However, this assembly process is an orderly one; components have to be added in a proper sequence. For example, the genes will be assemblied, followed by the capsid protein.
Different virus types have different forms of releasing their viruses. For enveloped virus, it involves taking a portion of the host’s cell membrane to envelope its capsid. This process is known as budding. As for the non- enveloped virus, the virion is released from the cell via membrane rupture.
Example of enveloped virus : Influenza virus
Example of non – enveloped virus : Adenovirus
Taken from NYP's Lecture Virus Host Interaction Part 1 (for student).ppt
Here's a video for you to understand the virus replication cycle more.
This website below will also allow you to know more about the Virus Replication Cycle.