Humans Need Fewer Genes Than Thought to Survive
WASHINGTON — Scientists conducting the first thorough survey of human DNA have made a remarkable discovery: To create the complex organism known as a human takes only twice the genes of a fruit fly or a roundworm.
At the same time--though scientists once described the overwhelming majority of the human genetic code as “junk†with little apparent purpose--it is actually brimming with remnants of long-dead genes and bits of DNA that “live†amid the genes like parasites, reshuffling and reshaping them over time. This suggests that the “junk†actually plays a role in the process of evolution and deserves intensive study.
The researchers also concluded that men, far more than women, produce the genetic mutations that both bring disease into the human family and allow evolution to move forward.
These and other findings will be reported Monday by the two teams of researchers that raced each other to map the chemical composition of human DNA, the inherited material within most cells that controls basic cellular operations and plays a role in most disease.
That race ended in a tie in June, giving both teams a claim to one of the most important scientific achievements in history. Since then the teams--one a private U.S. company, the other an international group funded largely by the U.S. government and a British charity--have been scouring their data in order to conduct the first broad analysis of what lies within DNA.
The teams agreed to release their findings at about the same time; they will be published this week in the journals Science (by the private company Celera Genomics Corp.) and Nature (by the international consortium). The results were to be released Monday, but a British newspaper broke the embargoed story in its edition today.
The two teams’ conclusions, which largely agree with each other, say little directly about potential cures for disease, though that is ultimately the major goal of the research. Still, by revealing more about how genes work, as well as where in the DNA they are clustered, the work over time will help researchers studying a large variety of illnesses.
Working independently, the two teams have concluded that humans have somewhere from 30,000 to 40,000 genes. One team, led by Celera, said the number could be as low as 26,000.
Most experts until recently believed it took 100,000 genes or more to build and operate a human. But the new reports say evolution has produced the human’s magnificent complexity using only a few more than needed by a roundworm, which has a mere 959 cells, or a mustard weed, a plant with 25,000 genes.
“That’s a ‘knock you over with a feather’ kind of result,†said Dr. Francis S. Collins, leader of the international team and director of the National Human Genome Research Institute.
“On one level, it’s a blow to the pride of our species: How can we hold our heads up if we have only a few more genes than a worm?†Collins said. “But what it tells you is that our complexity arises from some other source, and we will have to start looking for it.â€
The reports offer evidence of how the body accomplishes so much with such a small gene set. Where simpler organisms rely on each gene to produce a single protein, human cells are able to skip over parts of genes at times, allowing each gene to produce on average of three versions of a protein, and sometimes as many as five.
This suggests that to understand the root causes of disease, researchers will have to intensively study how proteins, as well as genes, interact and how they go awry. Proteins are the workhorses of the body, handling such basic tasks as turning food into energy, signaling among cells and growing from an embryo into a child.
Moreover, said Collins, the studies suggest that each human protein can handle more functions than those in a simpler organism. “If a worm protein is made to clip another protein . . . then it’s like a cutting knife that does one simple thing,†he said. But the analogous human protein “would be like a Cuisinart: It would have lots of settings and dice and slice.â€
The teams also found evidence of how DNA changes over time, causing organisms to evolve.
In one surprising conclusion, the international team found that about 220 genes did not evolve in a straight line from all animals that came before humans. Instead, human ancestors adopted these genes millions of years ago directly from bacteria. In essence, this shows evolution making use of whatever material it found at hand, Collins said.
Intriguingly, at least one of the genes in question plays a role in depression. Still, an independent scientist questioned the finding. “I think it’s more likely that the gene transfer went the other way, from vertebrates to bacteria,†said Philip Green of the University of Washington in Seattle.
The two teams also found hundreds of thousands of copies of mysterious DNA bits that act like parasites, detaching themselves from the genome and then reinserting themselves in a new location. Their existence had been known, but there are far more of them than scientists had thought.
Moreover, one type of these parasites has the ability to move other pieces of DNA with them. The international team found that they could move more DNA than previously known, suggesting that these elements play a role in reshuffling genes to form new ones.
Another type was found to exist only where genes are plentiful, and it may help the body respond to stress, the international team said.
All this, as well as evidence of genes that became inactive long ago, exists in the 98% of DNA that does not produce proteins. Once, scientists considered this vast material to be junk. Now, they are finding that much of the material has, or had, some kind of biological function.
In addition to studying how genes work, the teams determined that genes are not scattered uniformly throughout DNA. Instead, some areas of the genome, as the sum total of human DNA is called, are packed densely with genes, like a busy, urban area, while others are like “deserts,†barren of genes.
Researchers are not sure why this clustering occurs. But they noted that genes responsible for one of the most complicated functions--the laying out of an embryo’s anatomy--are very densely clustered and contain no junk at all. They surmised that this lowers the chance for the junk to interfere with the operation of those genes.
