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"More than a hypothesis, it's a theory!"
Every winter as the season turns and Santa comes to visit, a less welcomed
traveler makes the rounds. It's influenza, a truly horrid little
critter. And right now, in the throes of delirium, flush with fever and
cold, I confess; I really despise influenza. I go to sleep feeling chipper
and wake up feeling beaten with baseball bats and heated by a microwave to
a toasty 103 degrees.
This damn flu is caused by a virus, of course, just one of many viral
pathogens that curse mankind. No one knows how these quasi-living,
self-replicating packets of genetic material first came to be. Perhaps
they're a vestigial remnant of a long-gone RNA-based world that thrived in
the nearly boiling sea of a planet unrecognizable as Earth, billions of
years ago. Maybe they're errant bits of code from a more traditional
bacterial microbe that began freelancing on its own.
However they arose, the viral vector is here to stay.
They all have the same grisly modus operandi. First, the dastardly buggers
break through the cell wall. Some pick the lock of an existing portal and
sneak past the molecular doorman disguised as legitimate cargo. Others are
constructed like an off-shore drilling platform. They land on jointed legs,
drop a drill bit onto the exterior of their target, and then begin boring
in for all they're worth.
Once the membrane is breached by whatever means, viruses inject a packet of
trouble into the interior, which makes for the nucleus and commandeers the
genetic machinery to make more viruses. The end result is a person who
sickens, and sometimes dies.
But there is a consolation prize. Geneticists have found that this viral
scourge is incredibly useful in shedding light on all manner of mysteries
biological.
Viruses Are Our Friends, Part 1
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Some of these little influenza bugs are retroviruses like HIV and feline
leukemia.
| retrovirus (n): genes carried on
single-stranded RNA, which co-opts the nucleus of the infected cell to
convert itself into DNA. This DNA sometimes inserts itself into the cell's
chromosomal DNA. |
These viruses are RNA, and typically "reverse-transcribe" their RNA into
DNA for integration into the host's genome. When that happens, they snip
the DNA and insert themselves surreptitiously into the host's chromosome,
where they lie dormant, sometimes for years, before activating. Later, they
activate and begin making more RNA, which in turn inserts itself into the
genome, until the entire cell basically falls apart. Then the RNA
retroviruses are off to seek new, healthy cells for more viral adventures.
But if the retrovirus doesn't reactivate, this life cycle fails. While the
infection is contained, the cell is left with inert viral sequences
scattered throughout its genome. They can cause problems. The inert
sequences may disrupt a key gene, for example, and uncontrolled replication
may ensue-the beginning of cancer. Or it could spark an autoimmune disorder
like multiple sclerosis, or exotic forms of painful, debilitating
inflammatory disease. But often the retrovirus simply sits in the
chromosome, inert and disregarded.
Now consider inheritance. Sometimes, by a stroke of fate, the infected host
is a sperm or egg cell, which becomes a child. That child has those inert
viral sequences in every single cell of his body. If those sequences happen
to lie near an allele (one member of a pair of genes) that becomes
ubiquitous in the population through natural selection, then all members of
the species will carry the same genetic "signature" in every cell of their
body. These ghosts of infections past are called endogenous retroviruses,
or more affectionately, ERVs.
An ERV found in the same DNA location in two people provides powerful
evidence, admissible in court, that they share a common ancestor. All
humans share many ERVs in identical locations, which is no surprise. We all
have common ancestors if one looks far enough back. But here's where it
gets interesting: Chimps and humans also have ERVs in common. If identical
ERVs serve as evidence of relatedness in court, then ERVs are equally
convincing evidence that chimps and humans are related!
The human and chimp genomes have been sequenced over the last decade, and
we've found a dozen separate ERVs (hundreds of repeats each) identical in
both genomes, in exactly the same locations. When these sequences are
checked for time of introduction, they indicate a common ancestor five to
seven million years ago. That is consistent with other molecular divergence
studies and the fossil record.
But wait, there's more!
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Consider a repeat sequence of base pairs like an ERV-or any other
nonfunctional DNA sequence for that matter-found in the same locations in
the genome between two species. Geneticists have looked at shared sequences
between humans and mice, and compared those to the shared sequences between
humans and chimps. They found that in humans and chimps, the same mouse
sequences have been overwritten, just like a new CD recorded over an old
one. This is exactly what we'd expect if ancestors of chimps and humans
diverged more recently than primates and rodents diverged.
Creationists hate ERVs. They usually ignore them, or recursively label them
"intelligent common design." ERVs have no known function. They code for
viral proteins used only by viruses. Even if these things did have a
function to us, common descent (common ancestors) explains how they got
where they are in both species. The odds against two species by sheer
chance being identically infected are enormous: one in ten followed by a
hundred zeros. You have a better chance of winning the lottery ten times in
a row than of being infected by a dozen distinct ERVs in the same hundreds
of locations in your genome as any other individual or creature.
Viruses Are Our Friends, Part 2
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Viruses do more than shed light on evolutionary relationships. Their study
holds great promise for novel health therapies. We may someday tailor
viruses that eat only cancer cells. And since a virus can literally perform
nano-surgery on the chromosomes of a cell without so much as breaking the
patient's skin, we might enlist them to remove genetic abnormalities and
replace the pathological sequences with healthy ones. Inherited
susceptibility toward cardiopulmonary disease, diabetes, and cancer-the
three greatest killers of humanity-might be permanently
eliminated. Afflictions like sickle-cell anemia, lupus, irritable bowel
syndrome, muscular dystrophy, Lou Gehrig's disease, and on and on, would
all become words found only in history books. What a wonderful gift of
health to bestow on our children!
Science is slowly but surely domesticating the virus. This is only
fair. We've suffered from these little beasts for way too long. A little
payback is overdue. At least that's my firm opinion as I lie here
immobilized with flu on the couch, temporarily lucid between waves of total
mental incapacitation. God, my body aches...Where the hell did I put that
aspirin?
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