Dan Rockwell
Joined: 10 Dec 2001
Posts: 1988
Location: Stamford, CT, USA |
Human-chimp DNA difference trebled
 Thu Sep 26, 2002 10:32 pm
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Human-chimp DNA difference trebled
22:00 23 September 02
NewScientist.com news service
We are more unique than previously thought, according to new comparisons of human and chimpanzee DNA.
It has long been held that we share 98.5 per cent of our genetic material with our closest relatives. That now appears to be wrong. In fact, we share less than 95 per cent of our genetic material, a three-fold increase in the variation between us and chimps.
The new value came to light when Roy Britten of the California Institute of Technology became suspicious about the 98.5 per cent figure.
Ironically, that number was originally derived from a technique that Britten himself developed decades ago at Caltech with colleague Dave Kohne.
By measuring the temperature at which matching DNA of two species comes apart, you can work out how different they are.
But the technique only picks up a particular type of variation, called a single base substitution. These occur whenever a single "letter" differs in corresponding strands of DNA from the two species.
But there are two other major types of variation that the previous analyses ignored. "Insertions" occur whenever a whole section of DNA appears in one species but not in the corresponding strand of the other. Likewise, "deletions" mean that a piece of DNA is missing from one species.
Littered with indels
Together, they are termed "indels", and Britten seized his chance to evaluate the true variation between the two species when stretches of chimp DNA were recently published on the internet by teams from the Baylor College of Medicine in Houston, Texas, and from the University of Oklahoma.
When Britten compared five stretches of chimp DNA with the corresponding pieces of human DNA, he found that single base substitutions accounted for a difference of 1.4 per cent, very close to the expected figure.
But he also found that the DNA of both species was littered with indels. His comparisons revealed that they add around another 4.0 per cent to the genetic differences.
Junk and genes
"We're not any more different than we were," says Britten. "But we see a bit more divergence than before because insertions and deletions are taken into account. It almost triples the difference."
The result is only based on about one million DNA bases out of the three billion which make up the human and chimp genomes, says Britten. "It's just a glance," he says. But the differences were equally split between "junk" regions that do not have any genes, and gene-rich parts of the genome, suggesting they may be evenly distributed.
Britten thinks it will be some time before we know what it is about our genes that makes us so different from chimps. He thinks the real secrets could lie in "regulatory" regions of DNA that control whole networks of genes.
"It'll be a while before we understand them," he says.Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.172510699)
Andy Coghlan
This story is from NewScientist.com's news service - for more exclusive news and expert analysis every week subscribe to New Scientist print edition.
http://www.newscientist.com/news/news.jsp?id=ns99992833
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Scientists sort the chimps from the men
19:00 11 April 02
NewScientist.com news service
A team of molecular biologists have taken a step towards defining what makes us human. It is not so much our differing gene sequences that distinguish us from our primate cousins, but how active those genes are, the team has discovered.
Chimp and human genomes vary by only 1.3 per cent and only a tiny fraction of this actually affects genes. The new research shows how variation in the amount of product of a gene may be as significant to our recent evolution as structural changes.
The greatest changes in gene expression have been in the brain, say the researchers, perhaps explaining why human mental capabilities have evolved so rapidly.
"This is the first open door to understanding how humans became humans and chimps became chimps," says Wolfgang Enard, of the Max Planck Institute for Evolutionary Anthropology in Leipzig, the study's first author.
Hidden changes
"Expression patterns are certainly the key to the differences between chimps and humans," says Chris Stringer of the Natural History Museum in London. "And these changes have been hidden up to now."
But Walter Messier, director of Evolutionary Genomics, Denver, Colorado told New Scientist: "It's sometimes difficult to devise an adequate control when you do expression pattern work. Let's face it, expression patterns can change after one has had lunch, for example. But it is a very valuable approach."
Evolutionary Genomics is trying to identify differences between chimp and human gene sequences, for example in relation to HIV susceptibility.
Five-fold changes
The new analysis compares gene expression patterns in blood cells, liver and brain tissue from chimps and humans, and also orangutans and monkeys for reference.
The researchers used a molecular tool called a DNA chip to detect the products of up to 12,000 active genes.The greatest variations in gene expression patterns were found in the brain.
The researchers estimate that there have been more than five times the number of changes in human brains than chimp brains since their evolutionary lines diverged.This may explain how humans have developed such unique mental capabilities as language, culture and planning, the researchers say.
But changes in gene expression must in the end originate from DNA changes and the researchers do not yet know which groups of genes are responsible
Disease genes
So far only two genetic differences have been biochemically characterised, one affecting cell surface structure and one that may affect hair structure.Identifying which genes are responsible could help us to learn more about the genetic basis of diseases, says Enard, if we can correlate gene expression patterns with differences in susceptibility to cancer, Alzheimer's or AIDS.
Finally, Enard cautions: "The question of what makes us human can be answered in so many different ways. Never will molecular biology tell us everything."
Journal reference: Science (vol 296, p 340)
Helen Phillips
This story is from NewScientist.com's news service - for more exclusive news and expert analysis every week subscribe to New Scientist print edition.
http://www.newscientist.com/news/news.jsp?id=ns99992160
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