Almost immediately, two staff scientists in his Salk laboratory found that a curious DNA sequence muddled their efforts to discover how neural stem cells produced new neurons.
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At first, these artificial rodents seemed normal enough. Yet upon close study, some of the creatures seemed dimwitted. A few had memory problems. Others had trouble learning.
Intrigued, the scientist compared the genetically engineered neurons to natural cells. The only major difference she could detect was the activity of this puzzling genetic sequence.
Her experiments had taken two years. Crestfallen, she turned her attention elsewhere.
"At the time, we could not make heads or tails of it," Gage recalled. "We would have long discussions, but I could not get anyone interested in working on this."
Scientists called the curious genetic sequence a "jumping gene."
It could move up and down the double helix of DNA to insert itself into the genetic structure, like a black snake crawling along a branch into a bird's nest.
Despite the name, the sequence was not a gene but a primitive precursor — called a long interspersed nuclear element, or LINE — that struggled for survival inside the microcosm of a cell. The LINE sequence belongs to a mysterious family of mobile genetic elements called retrotransposons.
For 600 million years, it existed solely to copy itself.
All mammals contain such LINE sequences. But as species became more intelligent, they retained fewer types. No one knew why.
Mice harbored 3,000 different kinds of LINE elements, rats 500. Humans had about 100 types that differed from one person to the next.
All told, there are as many as 850,000 copies of such junk DNA in the human genetic structure, composing almost half of every cell's heredity. Most researchers dismissed it all as the detritus of parasites, viral infections and evolution's failed experiments.
Unlike the other molecular relics littering the human genetic code, however, the particular human sequence that cropped up in the Salk laboratory, called an L1 LINE, was still on the move.
So many thousands of times had it copied itself into the human genome that it now made up one-fifth of a cell's DNA.
Most copies were stranded far from any functional gene. Many were truncated, broken off like an aria interrupted by a cough.
Every once in a while, the sequence landed close enough to a gene to disrupt its behavior or change its expression.
A single jumping retrotransposon is the reason that Great Danes, dachshunds, border collies and certain other domestic dogs have patchy black-and-tan coats, researchers at Texas A&M University recently reported. It landed in a gene that affects the color of dog hair.
But no one had ever heard of these DNA strands reweaving the genetic fabric of individual brain cells.