One of the things that I find most interesting about viruses is the diversity in their replication cycles. It seems that for every barrier that viruses encounter during replication it is overcome in a myriad of ways.
Imagine you are a warrior invading a castle. How many different ways can you penetrate the defenses, cross the moat, get through the walls, and then access and use all the stuff inside for your own benefit? Will you knock down the gate? Will you sneak through the windows? Catapult yourself over the wall?
A Virus must enter a host cell, take over the machinery to make lots of copies of itself, then get out and transmit to the next host. While the viral replication cycle of all viruses follows the same general patterns, the subtle differences are fascinating. Its like an evolutionary brainstorm that resulted in thousands of different ways to solve the same basic problem.
Ive asked my Biol 105 class to read Ch 19 of Campbell’s Biology and post a comment about an interesting thing about viruses that caught their attention. DId you learn something new and surprising? What is it about it that is interesting to you?
A question I often get in class is: “Are there viruses that infect other viruses?”
The answer is still “no” but a recent discovery reported in Nature does starts to blur that line. A newly identified virus, called Sputnik, tags along with another virus called Mamavirus (so called because its bigger than the previous record holder for large viruses, Mimivirus).
Interestingly, Sputnik virions can be found within Mamavirus virions, so they travel together. Since Sputnik can not replicate with in the Mamavirus virion, this is not an active infection of Mamavirus, but rather it is a passive particle traveling within the Mamavirus virion.
Importantly, Sputnik is a parasite of Mimivirus. It can only replicate in ameoba that are co-infected by Mamavirus. Parasites live off of other organisms, with a deleterious effect on the host, such as reduced nutrient uptake or growth rate. Viruses are parasites – infected cells usually halt all or most cellular activity and eventually die, the most deleterious effect of all. Sputnik is a parasite of Mamavirus it replicates at the expense of Mamavirus. Cells infected with both viruses produce less 70% less infectious Mamavirus particles than a cell not co-infected with Sputnik. A few other viruses have been identified that require co-infection with another virus (Hepatitis D virus for example) but these do not reduce the infectivity of the host virus so are not really parasites of the other virus.
Mamavirus hijacks a cell to replicate. Sputnik hijacks Mamavirus for transportation and its cell-hijacking capabilities. It is the first described virus of a new group of viruses called virophages.
An ongoing question in virology is whether viruses are to be considered living creatures. Its easy to tell that a groundhog is alive but a book is not. But what properties does a groundhog have that a book does not? We can look up basic properties of living things in a biology textbook, and yet it remains difficult to define life in a simple sentence.
I would argue that a virus is not alive. Viruses are completely dependent on host cells to replicate. That said, in absence of the host cell the virus clearly lacks most of the properties of a living thing. (Does stealing those properties from a living thing count towards being alive?) Life seems to emerge from a collection of parts where the whole is greater than the sum of the parts. This emergent property, life, is present in animals, plants, bacteria etc, but in a virus infected cell, that property remains a part of the cell, not the virus.
Alive or not, viruses are an integral part of biology. They help us understand life and they certainly have an effect on living things.