I thought this week we’d spend a little time talking about
viruses, because I’ve been spending some quality time this winter with them. I
imagine many of you out there listening have too.
Viruses are the strange fruit of the biosphere, they don’t
quite meet all of the typical criteria for being alive, yet are clearly alive
none the less, in a way that things like rocks and other inanimate objects are
not. In the technical jargon, viruses are obligate intracellular parasites, which
decoded means that viruses require access to the cells of a host organism to
complete their reproduction; the amount of time they can survive outside the
host cell varies from virus to virus, from just minutes to years. Regardless of
how long they can lie dormant in the outside world, they can’t do anything
active until they are inside a host cell.
The reason for this is largely due to their anatomy (or lack
there of). Viruses are not cells, they do not have a phospholipid cell
membrane. All other living cells do have these membranes. Additionally viruses lack
the molecular machinery needed to reproduce their genetic information. The only
things they can call their own are their viral genetic molecules (a piece of
DNA or RNA) surrounded by a protein coating called a capsid. What cells lack in
anatomy they make up for in ingenuity, they are the MacGyvers of the natural
world. They can take the machinery and structure of a living cell and use it for
their own nefarious purposes. It’s perfect really—why bother to evolve the
enzymes required to replicate DNA or the organelles needed to generate a cell
membrane, when you can just appropriate the use of some one else’s? Viruses do
just that, many of them construct an envelope out of material from a host
cell’s membrane, which functions as an invisibility cloak and grants passage
past a host’s immune cells. Viruses lack the enzymes with which to replicate
their genetic material, and have solved this problem by evolving the mechanism
by which they hijack the replication molecules of a host cell. That is why they
have to get inside a cell to do their business.
Viruses are incredibly small and incredibly diverse. Part of
this diversity stems from the fact that viruses also are incredibly specific.
Each virus is adapted to hijack a specific kind of cell, and only that kind.
Generalists are the exception not the rule the virosphere. Human evolution has
been shaped by viruses throughout our history; influenza, hepatitis, small pox,
dengue, rabies, measles, polio, now we have zika and ebola to add to the mix.
Each of these viruses has a favorite cell type, influenza goes for respiratory
epithelial cells, polio prefers motor neurons.
Hepatitis reproduces primarily in the liver, causing the inflammation we
recognize as the disease, and more than 200 different types of viruses,
including the rhinoviruses, infect the cells lining the inside of your nose and
cause the common cold. Developing vaccines to combat rapidly evolving or newly
discovered viruses informs much of our biomedical research.
All that moving in and out of our cells has left a lasting
imprint not just on human social history, but on our very genome as well. The
more we learn about the human genome, the more we realize that viruses have
been a part of our world for a very long time. Scientists estimate that as much
as 8 percent of our DNA is viral in origin. When the virus moves into a cell to
reproduce, sometimes things don’t quite go according to plan, and instead of
hijacking the cell machinery to make viral DNA, the viral DNA gets incorporated
into the cell nuclear material (the same kinds of mechanisms are used in
genetic engineering today). If that
viral DNA ends up in the genome of a germline cell (one that is destined to
become a gamete), it can be passed on to offspring, which apparently happened a
lot. There is evidence emerging that this viral DNA is actually influencing the
expression of some of our human genes. The deeper we look the more connected we
are.
Stay well out there, wash your hands, cover your cough, stay
home in bed if you are sick. Virus season will be over soon.
References:
University of California Berkley to the rescue again: http://www.ucmp.berkeley.edu/alllife/virus.html
The Journal Nature has a wonderful and well stocked
educational website: http://www.nature.com/scitable/topicpage/cell-membranes-14052567
Viral DNA is in our DNA, and it can influence the sex of
your kid: http://www.livescience.com/54247-ancient-viral-genes-may-determine-babys-sex.html
A list of human viral pathogens http://viralzone.expasy.org/all_by_species/678.html
It wouldn’t be a show about viruses without something from
Carl Zimmer: http://phenomena.nationalgeographic.com/2015/02/01/our-inner-viruses-forty-million-years-in-the-making/
