The researchers found that individual genes interact with one another in more complex ways than previously suspected. They also found that large stretches of DNA once called "junk DNA" because they had no known purpose may actually play a significant role in regulating biological processes.
The findings point to the need for much more research, the researchers say, and could influence the way scientists search for the genetic causes of diseases.
"This is a landmark in our understanding of human biology," Francis Collins, director of the National Human Genome Research Institute, said at a news conference Wednesday.
The institute was one of more than 80 research institutions from 11 countries that participated in the Encyclopedia of DNA Elements (ENCODE) project. The researchers published their findings Thursday in a series of articles in the journals Nature and Genome Research.
The human genome consists of a string of more than 3 billion DNA letters. However, only about 3 percent of those DNA letters are part of the approximately 20,000 genes that contain instructions for making proteins, which regulate the body's cellular processes. The protein-making is a two-step process. First, the DNA in the gene is transcribed into RNA, and then that RNA serves as a template to produce a protein.
Until now, most genome research has focused on investigating genes -- often the genes thought to cause or contribute to heritable diseases -- and has ignored the vast stretches of so-called "junk DNA."
The new study, in contrast, picked 44 sites on the genome to comb through in detail. Some were chosen because they already contained genes of interest, but others were randomly chosen to include large stretches of DNA with no known purpose.
The researchers found, to their surprise, that much of the supposedly purposeless DNA was transcribed into RNA -- leading them to believe that the DNA serves some purpose, although they don't yet know what that purpose might be.
"There now appear to be thousands of places in the genome that were long thought to be useless or meaningless, but which we now see to have a functional role," ENCODE researcher Thomas Tullius of Boston University told the Boston Globe. "But we don't really understand what that role is."
One role, the researchers say, may be to act as control regions that help regulate individual genes and turn them on and off. Researchers already knew that some sections of DNA performed this function, but the new research suggests that it might be more widespread than previously thought.
Other segments of DNA may simply be "clutter in the attic," according to Collins. However, even this clutter could serve a purpose, providing basic building blocks for evolution to call into use when needed.
"Most of the time, the human genome is operating on the 'first and second floor,' with 5 percent of the genome doing what needs to be done on a daily basis," Collins said at the news conference. "But over evolutionary time, a much larger part of the genome, the stuff in the attic, becomes important. It's waiting for natural selection to call for it."