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Ärende: Tiny genetic changes have
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Human, chimp DNA changes mapped
Tiny genetic changes have huge impact on protein-coding


By By Maggie Fox
Health and Science Correspondent

WASHINGTON - Tiny genetic changes add up to huge differences when human
DNA is compared to that of chimpanzees, researchers said Wednesday, in a
report that explains how people and apes can be so close, yet so far
apart.

Genetically, chimpanzees are 98.5 percent identical to humans. But the
differences between the species are clearly profound, and geneticists
have been laboring to find out how such subtle variations in DNA can be
so important.

"Clearly, the genomic differences between humans and chimps are much
more complicated than conventional wisdom has portrayed," Asao Fujiyama
of the RIKEN Genomic Sciences Center in Yokohama, Japan, and colleagues
in Japan, Taiwan and China wrote in their report, published in
Thursday's issue of the journal Nature.

The comparison will help understand disease and also help in comparing
one person's genetic sequence with another's by helping to set a "base"
genetic sequence that can be used to determine the individual human
variations in DNA.

Chromosome counterparts compared
Fujiyama's team compared chromosome 22 on three different chimpanzees to
its counterpart in humans, chromosome 21.

They looked for differences that would help separate the human sequence
from the chimp sequence.

Fujiyama's team found that just 1.44 percent of the DNA was different at
the level of single letters of genetic code.

These letters, A, C, T and G, stand for the nucleotides that make up the
DNA of all living creatures - adenine, cytosine, thymine and guanine.
The nucleotides match up to make amino acids, which in turn string
together into genes that control the proteins made by cells.
There are vast stretches of DNA that do not make up genes, and
scientists are struggling to understand their importance.

Big protein differences

Fujiyama's team found differences that may be more important than the
single-letter changes.
"There is also an impressive number (68,000) of small to large stretches
of DNA that have been either gained or lost (these are called
'insertions or deletions', 'indels' for short) in one species or the
other," the researchers wrote.

"These differences are sufficient to generate changes in most of the
proteins: Indeed, 83 percent of the 231 coding sequences, including
functionally important genes, show differences at the amino-acid
sequence level," they added.
"Our data suggest that indels within coding regions (genes) represent
one of the major mechanisms generating protein diversity and shaping
higher primate species."
In other words, while the genes and other DNA may look the same in
chimpanzees and humans, the proteins they eventually code for can be
very different. This supports what genetic researchers have been saying
lately - that subtle changes in the genetic code that reach far beyond
the genes themselves may be extremely important to biology.

The power of proteins

While there may be no more than about 30,000 to 40,000 human genes,
there are more than 250,000 different proteins.

The researchers tried to calculate what the genetic code of the original
ancestor of both looked like, 6 million to 7 million years ago.
It looked to them as if the original ancestor of human chimps had a
larger genome, and each species pared it down differently as they
evolved.

Some of the genetic differences they found may have direct implications
for disease. They found differences between chimp and human immune
system genes, for instance, and molecules involved in early brain
development.

Copyright 2004 Reuters 

"It's uncertain whether intelligence has any long term survival value.
Bacteria do quite well without it."
 Stephen Hawking
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