As I mentioned in Infections of the Brain, there are numerous types of infectious agents that can infect the brain. I can hear you asking though: what exactly do these things look like and how do they work? In this page, I’ll attempt to explain briefly what each of them are and what they do. I’ll be mentioning DNA and RNA in this page, so if you aren’t very familiar with them, I suggest you check out DNA and RNA: The Basics before continuing in order to avoid confusion.
What is a virus?
A virus is an extremely small infectious agent, and it is quite similar to a computer virus which infects computer software: they often arrive via e-mails and can severely affect how the computer functions. A distinctive feature of viruses is that they cannot survive on their own so therefore they are often not classified as living organisms. In order to survive and reproduce, they must invade another organism and take over their transcription and translation process. They are capable of invading any type of organism, whether it be human cells, bacteria or plant life.
Here’s a Youtube documentary further talking about viruses: The Age of Viruses
What do viruses look like?
Despite the fact that viruses can appear very different under the microscope depending on their species, they all have the same basic components and are known as virions, as shown in the picture below:
There are up to three basic components of virions (the first two are common with every virus but the third one is optional).
- The head: which contains the viral DNA (or RNA)
- A protein coat which that protects the viral DNA or RNA
- A lipid (fat) envelope which surrounds the protein coat.
Viruses that do not contain the lipid envelope are known as naked viruses, whereas those that do contain the envelope are known as enveloped viruses. The other structures are there to help the virus attach to the host cell during the replication process.
How do viruses replicate?
Viruses replicate by the following mechanisms, as shown by the picture below:
- Attachment: happens between the tail fibres and particular receptors on the host cell. The base plate then attaches itself to the host cell, in a similar way to how magnets attach themselves to magnetic objects.
- Penetration: The viral DNA or RNA enters the cell by one of two mechanisms: either through fusion with the cell membrane or by endocytosis (the cell wall engulfing the virus).
- Uncoating: Removal of the viral capsid to release the viral nucleic acid.
- Replication: The viral DNA integrates itself into the host DNA, and the regular transcription/translation process occurs.
- Self-assembly: The viruses assemble themselves into viral particles.
- a) Lysis (bursting) of cell: The overwhelming amount of viruses cause the cell to rupture (and therefore die) and the virus begins the process again with other cells or b) they remain inside the host cell within the DNA instead of causing the cell to rupture.