Do you know the difference between viruses and bacteria? Is there even a difference at all? Outside of the scientific community, I’ve heard the two used interchangeably so many times, much to the chagrin of the scientists I know. Even though they are both too small for us to see, viruses and bacteria are two completely different beasts – in fact, they’re not even related. This month, we’ll be looking at some of the features of viruses and how these critters affect us.
Alive or Nah?
One of the biggest debates in the scientific community is the classification of viruses as living things. All living things must satisfy 7 criteria:
- They eat/feed
- They move
- They breathe/respire
- They excrete
- They grow
- They respond to stimulus
- And of course, they reproduce.
While plants, animals, bacteria, fungi and archaea all satisfy these 7 characteristics, viruses don’t. Viruses also can’t exist as free living organisms (we’ll discuss a bit more soon) which means that technically, viruses are NON-LIVING things. However, viruses do display some of the characteristics of living things, e.g. they’re able to move and replicate/reproduce, which creates some confusion around the issues. This confusion led one researcher known as Rybicki to create a description for viruses as “organisms at the edge of life”. This classification in particular is the biggest distinction between viruses and other living things like bacteria.
So what exactly is a virus?
Well, a virus is a “small infectious agent that replicates only inside the living cells of other organisms.” This means that viruses can only survive if they have a HOST (like a plant or animal) to replicate inside of. They do not have their own metabolism, so they cannot exist on their own.
A single virus particle is called a virion, and it’s made up of three parts:
- Long molecules of DNA or RNA that carry genetic information
- A protein coat called the capsid, which surrounds and protects the genetic material
- Some virions have a protective envelope made up of fat molecules that surrounds the protein coat when they are outside a cell
These virions can exists in various shapes, ranging from simple helical/icosahedral forms to more complex structures. And viruses are really really really small: the average virion is about one one-hundredth the size of the average bacterium. Viruses are so small that most virus species have virions that are too small to be seen even with an optical microscope!
A very important feature of viruses is their ability to easily mutate, and the vast number of ways that viruses are actually able to mutate. This is very important medically, as it has implications for the treatment of viruses. If they are always mutating, it’s difficult to create treatment techniques that can keep up with these mutations. For some viruses like HIV, as soon as one treatment is found to work, the virus mutates to another form that is no longer susceptible to this treatment.
Virus Life Cycle
Viruses have a specific life cycle with a number of different steps:
- Attachment – the proteins on the outside coating (capsid) of the virus are compatible with receptors on the outside of the cell of the host. These proteins are usually specific to a particular cell type (e.g. HIV virions have proteins that are specific for human white blood cells)
- Penetration – once attached, virions enter the host cell. Some viruses have evolved where they can just inject their DNA into the host through a special needle-like structure, while the rest of the virion stays outside.
- Uncoating – the viral capsid is removed and just the DNA/RNA remains
- Replication – the genome of the virus contained on the DNA/RNA begins to multiply, making multiple copies of itself.
- Assembly – these copies of the genome begin reassembling into new virus particles that can then be released to form an army and go on to infect more cells.
- Release – these new virus particles are then either (i) released from the host cell by bursting it open and killing the host cell (lysis) or (ii) the virus genome is incorporated into the DNA of the host cells (lysogeny). By doing this, every time the host cell replicates itself, it will also be replicating a copy of the virus DNA.
This life cycle underscores why viruses can be so dangerous, because it results in either the production of multiple viruses which kill a bunch of the host cells, or the viruses ingrains itself into your DNA – which means you never truly get rid of it. Remember, as well that your body is making multiple copies of your DNA every single day, so these viruses basically exploit their host by using your existing replication machinery to create copies of itself.
Hopefully now you have a better idea of what viruses are, and why they can prove to be so dangerous. Next week we’ll look more on how viruses affect us and cause diseases, and what differences we have to look at when treating them.