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Author Topic: Largest viruses - how big are they?  (Read 9821 times)

Offline DoctorBeaver

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Largest viruses - how big are they?
« on: 30/03/2007 09:07:10 »
Here's a post for young Chris to show off his expertise:-

What are the largest & smallest known viruses and are they harmful?
Are they really the pretty colours you see in pics or is that just enhancement?
Why are coldsores so named? They don't only erupt when it's cold or when the sufferer has a cold.
 
« Last Edit: 07/04/2007 22:04:52 by chris »


 

Offline chris

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Re: Largest viruses - how big are they?
« Reply #1 on: 07/04/2007 21:56:18 »
Hi Eth

The smallest viruses are about 20-30nm and include the notorious noroviruses, which are the bane of P&O's life because they cause outbreaks of diarrhoea and vomiting on cruise ships. 30nm is about 1/30,000'th of a millimetre across, meaning you could comfortably fit 600 million or so of them on the head of a pin.

The largest viruses that affect humans are the pox viruses, which include smallpox, vaccinia (cowpox), orf, molluscum contagiosum and so on. These are about 200nm across (1/5,000'th of a millimetre). By comparison, herpesviruses (such as the coldsore virus HSV-1) are about 150nm.

But the king of the viruses in terms of sheer physical size is a new family known as mimiviruses; they were first isolated in 1992 from amoebae (which they infect) in a cooling tower in Bradford. These viruses are truly massive; they encode 900 genes (in contrast herpes simplex (HSV) has about 75 genes), and are almost the same size as bacteria, making them readily visible by light microscopy. Although they are not a natural infection of humans, the French scientists who characterised them found antibodies to them in human patients, indicating that the people they had examined had in the past come into contact with the viruses.

To answer your other questions, unfortunately viruses are not the exciting colours you see in rendered images of them. Most of them are far too small to see other than by electron microscopy, which is grayscale anyway. Scientists add the colours to designate the arrangements of different proteins which link together to form the shell of the virus. This is because viruses are made from interlocking protein units, a bit like a football is made of hexagons and pentagon-shaped pieces of leather. The colours are therefore a useful way to visualise the different protein subunits.

Coldsores are caused by herpes simplex viruses (usually type 1). This virus persists indefinitely in the nervous system as a small circle of DNA, which sits next to our own chromosomes. Periodically, in response to numerous ill-defined and poorly-understood stimuli, the viral genes switch on and produce new viral particles which are shipped down the nerve fibre to the patch of skin that it supplies. Here the particles bud out of the nerve and infect the overlying skin, producing an infectious lesion so you can pass your herpes on to someone else. They probably get their name because whenever people are laid low, by a cold or flu for instance, they are more likely to suffer a viral reactivation and develop a coldsore.

I wrote an article about herpes simplex viruses and their clinical manifestations. Here's the URL:

http://www.thenakedscientists.com/HTML/articles/article/chrissmithcolumn4.htm/

Chris

Chris
« Last Edit: 07/04/2007 22:03:58 by chris »
 

Offline DoctorBeaver

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Largest viruses - how big are they?
« Reply #2 on: 07/04/2007 22:06:18 »
Thanks Chris. I read the article & that's what prompted my questions.
 

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Largest viruses - how big are they?
« Reply #2 on: 07/04/2007 22:06:18 »

 

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