SARS-CoV-2 has put a lot of focus on Viruses, but often people are not actually fully aware of what they actually are, so in this post, I wanted to briefly explore what viruses actually are, and how they reproduce.

It should be noted that with many of my posts, this is a VERY introductory piece, and if you want more information you will no doubt find much more available on the internet. This is a combination of my thoughts, some rambling and some information.

What is a Virus

In its simplest form, a virus can be considered to be a very simple microorganism that infects cells and may cause disease. As viruses rely on a host for replication (discussed below), they are often not considered to be ‘alive'.

There are many different types of viruses, but in essence, a virus is genetic information (either DNA or RNA) that is surrounded by a protein coat. Viruses can have a range of different structures (shapes and sizes) which in general includes:

• Helical
• Icosahedral
• Prolate
• Enveloped
• Complex

Usually, viruses are very small, much smaller than bacteria with variable sizes. In some cases, thousands of virus particles could fit inside one bacterial cell (which gives some perspective of size).

As mentioned, viruses contain genetic information which is surrounded by a protein coat. This complete virus particle is known as a virion and the protective coat is known as a capsid. The function of this protein coat is two-fold: 1. to protect the genetic information inside, and 2. to allow for the genetic information to be inserted into a host well.

When we talk about genetic information inside the virion, there are two types of genomes. One is DNA whilst the other is RNA-based. In general RNA viruses have smaller genome sizes as there can be errors in copying this information whilst DNA-based viruses have systems in place to allow for more accurate copying and hence can have a larger genome (with the exception of single-stranded DNA viruses).

How does a virus infect a cell?

Viruses are in effect genetic information and require a host cell in order to replicate (discussed below). A virus cannot replicate by itself, and furthermore, without the assistance of a host cell, it cannot replicate. In order to reproduce the virus usually needs to get ‘ínto' the host cell.

Viruses typically enter a cell through two different methods, one is called Receptor-mediated fusion whilst the other is called the Endocytotic pathway or often membrane fusion / direct fusion. Whilst the mechanism of entry varies in effect both of these methods are simply transporting external molecules into the host cell.

From a technical perspective, receptor-mediated fusion is a form of endocytosis where macromolecules are taken inside the cell selectively from the extracellular fluid. This requires the macromolecules (in this case the virus) to bind with receptors on the host cell, the host cell then effectively ‘ingests' the macromolecule in a bubble (vesicle) and the contents are released to the cell.

Direct fusion is a mechanism where the two membranes (the virus membrane/coating) fuse and the genetic information from the virus are delivered to the cytoplasm.

There are in effect six steps to a virus life cycle:

• Attachment
• Penetration
• Uncoating
• Gene expression and replication
• Assembly
• Release

Attachment, penetration and uncoating are all essentially how the virus ‘infects' the cell, whilst gene expression and replication, assembly and release are more about the production of the virus.

How do Viruses reproduce inside cells?

Once the virus is inside the cell, it undergoes a process known as uncoating, this is effectively presenting the genetic information to the cellular machinery for replication.

Some viruses need to present the genome to the nucleus of the host cell (DNA viruses) whilst other viruses (RNA) replicate outside of the nucleus

Once the genetic information is within the host cell, the host's existing organelles read the genetic information and start to produce the components. These include the Viral proteins (structural which make up the virus particle, and nonstructural which are not found in the virus particle and are mainly for genome replication) made in the ribosomes, the host cell also makes new viral genomes

Once the cell has produced the respective viral proteins and genetic information (genomes) the virus is then assembled into a new virus particle, this can take place in the cell's nucleus, cytoplasm or plasma membrane.

Finally, the viruses are released from the cell through sudden rupture of the cell or being extruded/forced out of the cell.

It should be noted that replication is more complicated than above, but in general, it gives the idea that the virus enters the cell, the genetic information from the genome is presented to the cellular machinery of the cell which starts to make the proteins encoded in the viral genome as well as replicating the viral genome and ultimately this leads to the death of the cell (most commonly). There are many different types of viruses (Double-Stranded DNA (dsDNA), Single-Stranded DNA (ssDNA), Double-Stranded RNA (dsRNA), Single-Stranded RNA (ssRNA), Single-Stranded RNA – Positive Sense) and each of these replicates in slightly or sometimes significantly different ways.

Does the immune system know about the presence of a virus?

Immune responses are hugely complicated and will often vary based on the host cell, the immune response in a person is going to be significantly different to other species, and because viruses can affect so many different hosts (from bacteria through to people) there cannot be a one-size-fits-all response.

That said, in many cases, the immune system detects the presence of a virus and instructs the cell to self-destruct in order to prevent the replication (or slow the replication) of the virus. Sadly though, in many cases, the viruses also take steps to prevent an immune response (i.e., in HIV the virus affects the immune cells) which can mean that they replicate unabated.

Viruses also have some other interesting tricks, where the genetic material (i.e., HSV-1) hides within the cell and can often re-activate later on down the track so you never actually ‘clear' the virus.