How Viruses Take Over Our Cells In Order To Replicate – Anatomy and Physiology

Why are viruses so freaking cool?

If you ignore the reputation they have about making people sick, they are by far the coolest thing you will come across in Microbiology. The word virus means venom or poison (Lee and Bishop, 2013).
Viruses are selective as all get out, they are specific to animals, plants and bacteria, and to narrow it down even further, they usually are species specific, but some aren’t, so keep that in mind (Cough*Bird Flu*Cough).

Lets start with the characteristics of Viruses:

  • They are the smallest infectious agents known, ranging from a diameter of 20nm to 400nm. You need an electronic microscope to be able to see them.
  • They lack enzymes necessary for metabolism, and due to their lacking of important enzymes, they can’t even synthesis complex molecules. This is why they need a living host, and there they will replicate inside a host cell. It is  inside the host cell that they ‘hijack’ the metabolic machinery.

Viruses take over our cell reproduction in our DNA in order to replicate. During replication, the host cells often get damaged resulting in cell death. (Lee and Bishop, 2013).

Viruses are categorised by shape and structure. They only produce one type of nucleic acid which holds their genetic code, this can be single stranded RNA, double stranded RNA, single stranded DNA, double stranded DNA. Due to the small size of viruses they can’t not carry the equipment needed, this is where a living host cell is required.

How do Viruses take over our cells in order to replicate?

The Lytic Phase:

  • A virus attaches to a host
  • The virus either enter the cell or injects genetic instructions for the host enzymes to make a new virus.
  • A new virus is assembled
  • The virus busts out of the host cell = killing the cell
  • Moves on to Infect a new cell.

They Attach and Enter:
The capsid’s proteins can have antigenic properties, which means that they will produce a corresponding antibody to the immune system – enabling the virus to attach and enter the host cell. A bit like a lock key fit. Whether or not a virus attaches is dependent on the attachment site on the surface of the host cell.
Some viruses have an envelope surrounding them, which have a lipid bilayer, on this lipid bilayer, there are proteins and glycoproteins that happen to look like spikes. These ‘spikes’ help the virus attach. For example in the Influenza virus, the ‘spikes’ to the surface receptors on the cells of the respiratory epithelium.  These proteins on the surface of the virus bind to the receptor molecules on the surface (plasma membrane) of the host cell if they correspond.

Replication, Assembly and the Infective Process:
The Virus has genes which will code for enzymes required for replication – in order to start infection. When a host cell is infected, the virus will ensure that the cell will produce viral protein and genetic material from it’s ‘hijacked’ equipment through nucleic acid transcription. The actual replication process is dependent on whether the nucleic acid of the virus is single or double stranded, or, DNA or RNA.
The new virus will be made inside the cell and when ready to go out and infect other cells, it will burst the cell, killing it. Up until this point the host cell will still be functioning, just at a very reduced rate, this is because the virus needs to make all the parts necessary before destroying the host cell.

*The envelope can be easily damaged and some viruses don’t have one, these are called naked viruses, and are generally more resistant to environmental changes.*

Our DNA contains the ‘blue-print’ for over 100,000 different proteins AND it contains genes that are just for us! So we should really consider how cool our DNA is too. I mean, can you believe that it knows the code for so many things??

All information was taken from Lee and Bishop’s textbook ‘Microbiology and Infection Control for Health Professionals‘, 5th edition, 2013 and  Introduction to the Viruses by the University of Berkley.

For Further Info check out this youtube video : Entry of a Virus into the Host Cell

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