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| Science News - updated 7:43 PM ET Feb 12 |
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| Reuters | SPACE.com | AP | ||
Junk DNA May Not Be Such Junk, Genome Studies Find
By Maggie Fox, Health and Science Correspondent WASHINGTON (Reuters) - The first in-depth look into the human genome shows it is much more complicated than the clear blueprint of how to make a human that scientists had hoped for. Instead of having DNA packed with tens of thousands of new genes that make people different from mice, fruit flies and worms, it seems we have relatively few genes -- just 30,000 or 40,000, researchers will announce later Monday. Earlier estimates had ranged from 60,000 to 100,000. The two separate teams of scientists, who say they were shocked and awed by their findings, say this means that genes may not be the be-all and end-all of what makes an organism. They know that each gene ``expresses'' or controls a protein. And they now know that the proteins must mix and match in ways more important than previously thought. But they also know they are going to have to go back and dig through the trash can of the genome -- the so-called ``junk DNA'' that many had believed played no important role at all. ``I call it the alleged junk,'' Eric Lander, head of genome sequencing at the Whitehead Institute in Cambridge, Massachusetts, said in a telephone interview. ``The junk is amazing.'' Looking Long And Hard At The Junk Lander, whose institute, part of the Massachusetts Institute of Technology (news - web sites), played a large role in the publicly funded Human Genome Project (news - web sites), said researchers will be taking a long hard look at the junk. When the two efforts, public and private, announced the first step, the sequence of the human genome last June, they knew little more than that there were 3.1 billion base pairs of DNA in the human genome. This amounted basically to a read-out of the A's, C's, T's and G's -- the nucleotides that form the rungs in the twisted double helix of DNA. If the right combinations of letters are together, say an A, and T and a G plus an A, G and C, they make an amino acid. There are 20 different amino acids, and these can join up in a variety of ways to make 250,000 different proteins. There is no set number of amino acids needed to make a protein, thus the variety. Each of the body's 100 trillion cells, except for red blood cells, has a full copy of this complement of DNA. But each cell does not express all of them. Brain cells need to express certain proteins, muscle cells and immune cells need to express others. Genes sometimes control what is expressed by other genes, but it could be the ''junk'' DNA plays a role as well, said the scientists who publish their findings in the journals Science and Nature this week. Their surprising finding is that the relatively few genes found in the 3.1 billion base pairs are clumped up. In between are vast spaces of ``desert,'' repeats of nucleotides that look like meaningless stutters. However, Lander said some of these, which often repeat the same sequence over and over again, look like guideposts to evolutionary history. ``By taking all the repeat elements in genome, we can put them together into a family tree,'' Lander said. Genome Becomes A Fossil Record ``The genome now becomes a fossil record.'' It had been known that viruses known as retroviruses could make their DNA a permanent part of ours -- and also of all the other mammals -- but the scientists found evidence that bacteria did the same thing. Lander said his team can already tell that, way back before humans became humans, our ancestors stopped getting so many new genes from viruses and bacteria and stopped moving genes around inside the genome, a process known as transposition. ``The rate of transposition, the rate of hopping, has plummeted in recent times, in the past 30 million to 40 million years,'' Lander said. ``We don't know why. This hasn't happened in the mouse. Entire classes of junk DNA have gone extinct.'' But other junk DNA thought to have been useless, hints at being very important. One example is a piece of repetitive DNA called an AL sequence. ``It turns out the genome cares a lot about getting the Alums to be near genes,'' he said. The Alums seem to have come into the genome fairly recently, and into gene-poor areas. But the transposition process moves them closer to actual working genes. ``If they are selected for, they have a function,'' he said. One possibility is dealing with stress. ``Suppose you need to regulate proteins under stress -- do you want to use a protein? No,'' he said. Any regulatory protein would also get stressed. ``You'd want something that was extremely abundant and near genes. Maybe it turns out AL is our friend. We have been calling it junk for all these years.'' So perhaps humans have learned to make so with so few genes by using other DNA elements to help them out.
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