so... what are viruses

[Teo yuyuan]

okay, i'm just a normal teenager, just want to find out...

can anybody out there help me explain why viruses are not so counted as living thing? viruses and bacteria seem quite similar to me, can anyone out there help me simplify it?

[ewmon]

If you describe some similarities and differences, we can help you from there.

[Teo yuyuan]

okay... wait hur, currently not at home yet, will try to:)

[Arete]

Defining life is a prickly subject when you get down to the details of it, but to grossly oversimplify - one of the defining features of being "alive" is being able to reproduce.

 

Most bacteria are capable of independent reproduction, whereas most viruses cannot reproduce without the co-option of another organism. Ergo, as a virus can't reproduce on its own, it isn't "alive" under many definitions of life.

 

Of course, there are plenty of exceptions, such as symbiotic bacteria who can't survive without a host, and encapsulated organelles like chloroplasts that grey up the boundary.

[Danijel Gorupec]

 

Of course, there are plenty of exceptions, such as symbiotic bacteria who can't survive without a host, and encapsulated organelles like chloroplasts that grey up the boundary.

 

Do you consider those symbiotic bacteria to be organisms?

 

Long time ago I accepted the following rough "definition" of life: a 'mechanism' able to maintan its complexity in a less complex environment. Now I don't know what to think about those symbiotic bacteria .

[Arete]

   

Do you consider those symbiotic bacteria to be organisms?

 

 

I think it would be tough to be a biologist without accepting that virtually any definition has grey areas and exceptions which don't fit neatly. As such I'm content with them falling somewhere in between autonomous organisms and part of their host.

[CharonY]

It is also quite a bit dependent on perspective. Scientists from different (sub-)discipline. From a physiological perspective the lack of own metabolic activities is probably the most important distinction (whereas symbionts engage in metabolite exchange, for instance).

[jp255]

If one is to say that viruses cannot independently reproduce, who is to say that humans can?

 

Viral replication is dependent on the existence of other organisms just like Human reproduction is (I think we can't live without our gut flora). Either way, most, if not all organisms, are dependent on their environment in some way, and so reproduction is dependent on the environment too. It seems rather meangingless and quite pointless to describe viruses (but not other "dependent" organisms) as non-living according to that argument, the justification is inadequate.

 

I prefer a definition of life which includes the capability to evolve.

It is also quite a bit dependent on perspective. Scientists from different (sub-)discipline. From a physiological perspective the lack of own metabolic activities is probably the most important distinction (whereas symbionts engage in metabolite exchange, for instance).

There are weaknesses with this reasoning as well, Humans are not totally metabolically independent either. So how much independence is required for an organism to be considered as living?

[CharonY]

I never mentioned metabolic independence. In fact, interdependence is quite common. The distinguishing factor is that viruses are unable to do any type of metabolic activities on their own.

[Arete]

If one is to say that viruses cannot independently reproduce, who is to say that humans can?

 

 

Mammals are not dependent on their microbiome to live - sterile mice lines are commonly used in research. E.g. http://www.bbc.co.uk/news/science-environment-12306431

 

It's a biological definition of reproduction. Viruses lack the mechanistic components required to reproduce themselves - they have to "steal" the mechanisms to reproduce themselves from a cell which has them. The organisms we typically define as being "alive" have all the mechanisms required to reproduce themselves - even if they are functionally dependent on a mutualism for another biological function.

Edited May 10, 2013 by Arete

[sam1123]

The way I think of it is that they lack all of the cellular apparatus required to sustain life on it's own. It has to use the ribosomes of the host in order to translate/code for proteins to be made. Most viruses are essentially little balls of genetic information (can be DNA or RNA) in a protein coat.

They can survive on their own but cannot replicate. They are a lot smaller than other pathogens (disease causing micro-organisms) such as bacteria.

The proteins that they code for are usually damaging, and are what make us ill, along with the immune response such as a temperature, mucous production, which are designed to help reduce the spread and/or kill the virus.

 

To summarise:

 

viruses are very simple organisms, with few cellular components, and are very small. They cannot reproduce asexually or sexually. They require certain host mechanisms to multiply. They cannot respire/convert energy. However they are classified taxonomically.

 

You might find this link helpful

 

http://www.ncbi.nlm.nih.gov/books/NBK8174/