Everything old is new again?
Disabled or broken retroviruses, fossils of molecular battles that raged for generations, make up about eight per cent of the human genome. They are only beginning to be understood. Michael Specter in the New Yorker explores how these and other viruses have contributed to human evolution:
in the nineteen-sixties, Howard Temin, a virologist at the University of Wisconsin, began to question the “central dogma” of molecular biology, which stated that genetic instructions moved in a single direction, from the basic blueprints contained within our DNA to RNA, which translates those blueprints and uses them to build proteins. He suggested that the process could essentially run in the other direction: an RNA tumor virus could give rise to a DNA copy, which would then insert itself into the genetic material of a cell…in 1970, he and David Baltimore…discovered reverse transcriptase, the special enzyme that can do exactly what Temin predicted: make DNA from RNA…
When the sequence of the human genome was fully mapped, in 2003, researchers also discovered something they had not anticipated: our bodies are littered with the shards of such retroviruses, fragments of the chemical code from which all genetic material is made.
It takes less than two per cent of our genome to create all the proteins necessary for us to live. Eight per cent, however, is composed of broken and disabled retroviruses, which, millions of years ago, managed to embed themselves in the DNA of our ancestors. They are called endogenous retroviruses, because once they infect the DNA of a species they become part of that species.
One by one, though, after molecular battles that raged for thousands of generations, they have been defeated by evolution. Like dinosaur bones, these viral fragments are fossils. Instead of having been buried in sand, they reside within each of us, carrying a record that goes back millions of years. Because they no longer seem to serve a purpose or cause harm, these remnants have often been referred to as “junk DNA.” Many still manage to generate proteins, but scientists have never found one that functions properly in humans or that could make us sick.
Then, last year, Thierry Heidmann brought one back to life. Combining the tools of genomics, virology, and evolutionary biology, he and his colleagues took a virus that had been extinct for hundreds of thousands of years, figured out how the broken parts were originally aligned, and then pieced them together. After resurrecting the virus, the team placed it in human cells and found that their creation did indeed insert itself into the DNA of those cells. They also mixed the virus with cells taken from hamsters and cats. It quickly infected them all, offering the first evidence that the broken parts could once again be made infectious…
researchers from the State University of New York at Stony Brook “built” a polio virus, using widely available information and DNA they bought through the mail. To test their “polio recipe,” they injected the virus into mice. The animals first became paralyzed and then died…Then, two years ago, after researchers had sequenced the genetic code of the 1918 flu virus, federal scientists reconstructed it, too.
Specter also mentions the work of Luis P. Villarreal, the director of the Center for Virus Research at the UC Irvine. Villarreal argued in a 2004 essay, “Can Viruses Make Us Human?” that viruses represent “a major creative force’’ in our evolution. Very interesting. It turns out perhaps that humans carry within them their own fossils. HT: LGF