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I read an article attempting to explain why viruses attack cells. The argument mentioned by the author hinges around the idea that a virus is genetically programmed to attack cells. We already know that. What is missing is how this viral genetic code was written in the first place. This is not a philosophical question!

The point is as follows: evolution is an ongoing biological process and different life forms are different form one another. Having said that, it is not clear how the genetic make-up of a virus can be designed in order to successfully attack cells. Consider the differences between a bacteriophage and the Human Immunodeficiency Virus (HIV). Clearly, they have different structural properties that allow them to successfully invade the corresponding targets. Why? How did it happen? What are the fundamental mechanisms underlying viruses, i.e., design and build a virus that will be able to infect the following cell? Scientific answers are required to tackle these hard questions.

Question Date: 2013-10-31
Answer 1:

This is an interesting question and in fact, the origins of viruses, as well as life in general, are something that scientists are still trying to determine. There are ideas about how certain "characteristics" are incorporated into the structure of viruses (and living organisms!), such as an iterative process that basically looks like:

1) mutation and crossing over between parents leads to higher viability of the organism
2) thus these genes get passed on to a next generation
3) repeat 1) and 2).

Through this iterative process, the "fitness landscape" of a species is explored, and ultimately, the individuals at the local maxima (peaks) of the fitness landscape, who are more "fit" to survive are the ones whose traits get passed on. It just happens that the characteristics that resulted in higher fitness in individuals (i.e. viability) might be different from one kind of organism to another because of the randomness that went into the iterations. Inherently, a mutation or a crossing over event during reproduction is random.

Answer 2:

Viruses are contained within protein sheathes when not in cells. The sheath of each virus is different. Each virus' protein sheath both enables it to recognize cells that it can infect and physically inserts the viral DNA/RNA into the cell(s) that the virus does infect. How it does this is going to depend on the cell in question and presumably by the proteins or other chemicals embedded within the membrane of the cell the virus is going to infect. Not being a microbiologist, I can't tell you the specifics of how any one virus manages to infect its host cell, but I can tell you that a virus' ability to do so is maintained by natural selection. I'm also sure that the fundamental means by which viruses can infect cells varies considerably from one virus to the next, and is an area of ongoing research (in other words, we don't have all of the answers yet).

Where viruses come from is another question where we have only partial answers. As far as I know, most viruses are thought to be derived from the organisms that they infect; they did not create other forms of life. There are bits of free-floating DNA in some cells, especially bacteria (they're known as plasmids), but they aren't encased in protein sheaths like viruses. How that happens I certainly don't know and I doubt anybody.

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