How do viral particles multiply itself in our body?
How do viral particles multiply itself in our body? How do they infect our body? Why are they so fatal? In these troubled times, where the world is taken aback by the impact of the Covid-19 pandemic, these are a few of the most commonly asked questions (To understand what a virus generally is and its composing structure, refer to my previous posts).
A virion multiplies by hijacking the metabolism of the host’s cell to replicate and make copies of its genome. It then uses the cell’s machinery to assemble protein coats to protect the genome copies, forming new viral particles.
The process of viral replication varies for different types viruses, but there are five general stages:
1. Attachment
2. Penetration
3. Uncoating
4. Replication
5. Release
ATTACHMENT
A virus particle attaches to a specific receptor, which lies on the surface of target cells, which the virus attacks. Each type of virus is compatible only with its own specific receptor or. The property of the receptor and virus compatibility depends on the type of cell on whose surface the receptor lies.
For instance, the SARS-CoV-2 virus takes action and is capable of replication only in the cells of the lungs of the host. Therefore, it attaches to its specific receptor known as ACE-2 receptor with lies on the epithelial cells lining the lungs, heart and intestine.
When the virus attaches to the receptor, it initiates fusion between the viral envelope and the cell’s plasma membrane. Non-enveloped viruses go through endocytosis where the entire particle enters the cell.
PENETRATION
The virions enter the cell by:
A) Membrane fusion
B) Endocytosis
C) Trauma to cell wall followed by spreading through plasomdesmata
D) Direct injection of viral genome into the cellular cytoplasm
MEMBRANE FUSION: This occurs when the virus has a viral envelope and infects a human cell. The envelope and the cell membrane/plasma membrane of the cell fuse together, exposing the viral capsid to the cell’s cytoplasm.
ENDOCYTOSIS: Viruses with no outer envelope enter the cells through this way. Endocytosis is the cellular process where outside particles enter into the cells. The cell membrane engulfs the virus particle, thus allowing it to come into contact with the cell’s cytoplasm.
TRAUMA TO CELL/SPREADING THROUGH PLASMODESMATA: This occurs only in plant cells. Plant cells have thick cell walls, making it hard for the virus to enter. So, the virus particle damages the cell wall by direct trauma, exposing it to the inner cell. From there, after undergoing complete replication, the new virus particles spread cell by cell through minute pores present at the part where the cells come into contact with each other. These pores are known as plasmodesmata.
DIRECT INJECTION OF VIRAL GENOME: those viruses that only affect bacterial cells or bacteriophages only do this. Bacterial cells have thick and strong cell walls. Therefore, these viruses puncture the surface of the cell and inject its genome directly into the cell’s cytoplasm.
UNCOATING
Uncoating is the process of removing the viral capsid to expose the genome into the cell for replication. The capsid is degraded by using enzymes released by the host or the virus itself. If there are no enzymes, the capsid could also be removed by simple chemical dissociation.
After degradation of the capsid, the nucleic acid i.e the genome is exposed to the cytoplasm of the cell.
REPLICATION
Replication of the viral genome inside the cell depends on the nucleic acid it contains.
Virions with RNA multiplies its genome in the cytoplasm while those with DNA multiplies itself in the nucleus.
Replication of DNA virus: these viruses use the host’s enzymes to make additional DNA strands, and converts these strands into RNA and mRNA (messenger RNA) strands. After converting, the new RNA is used as a main blueprint to produce more genome.
Replication of RNA virus: these viruses directly use the main RNA strand as a template to create more genome.
The mRNA plays a vital role in the creation of the viral capsids for the newly created genome particles. It instructs the host to synthesize proteins to make the outer coating which surrounds the genome, thus creating new virions.
RELEASE
Virions can get released from the inside the of cell by multiple methods:
- Lysis: In this process, the virions kill the cell by filling it up to an extent that its cell membrane bursts opens, thus releasing the inner contents.
2. Lysogenic Cycle: Under the instructions of the mRNA, the cell’s chromosomes allot a part of it for the virus’ genetic material to get incorporated into. After the incorporation, as the chromosome multiplies, the genetic material multiplies along with it. The material remains dormant until proper conditions occur for it branch off and start producing virions of its own.
3. Budding: The virions takes a part of the plasma membrane to create an envelope for itself and buds off the cell to go infect other cells.
