Chimp chromosome creates puzzles First sequence is
unexpectedly different from human equivalent. 27 May 2004
LAURA NELSON
What is the difference between a chimp and a human? There
could be a lot more to the answer than scientists thought,
according to the first accurate DNA sequence of a chimp
chromosome.
We already knew that around 98.5% of the base pairs that
make up our DNA are the same as those in chimps. So the
old idea was that all the things that differentiate us
from apes, such as highly developed cognitive functions,
walking upright and the use of complex language, should
come from the other
1.5%.
Scientists had hoped this would mean the key genetic changes
that enabled such traits to evolve would be easy to find.
But the latest evidence suggests that the journey from ape
to human was much more complex.
Todd Taylor at the Riken Genomic Sciences Center in
Yokohama, Japan, and his colleagues have read the DNA
sequence of chimpanzee chromosome 22, and compared it to its
human counterpart, chromosome 21. Although a draft sequence
of the chimp genome has been available since August 2003,
this is the first sequence of a whole chimp chromosome that
is accurate enough for researchers to be sure that any
differences between the two species are real, and not just
data errors.
Not so human
The sequences of chimp chromosome 22 and human chromosome 21
are roughly equivalent. Out of the bits that line up, 1.44%
of the individual base pairs were different, settling a
debate based on previous, less accurate studies.
However, the researchers were in for a surprise. Because
chimps and humans appear broadly similar, some have assumed
that most of the differences would occur in the large
regions of DNA that do not appear to have any obvious
function. But that was not the case. The researchers report
in Nature1 that many of the differences were within genes,
the regions of DNA that code for proteins. 83% of the 231
genes compared had differences that affected the amino acid
sequence of the protein they encoded. And 20% showed
"significant structural changes".
In addition, there were nearly 68,000 regions that were
either extra or missing between the two sequences,
accounting for around 5% of the chromosome. "We already knew
that at the DNA level we are similar to chimpanzees," says
Taylor. "But we have seen a much higher percentage of change
than people speculated."
The researchers also carried out some experiments to look
at when and how strongly the genes are switched on. 20% of
the genes showed significant differences in their pattern
of activity.
Chromosome 22 makes up only 1% of the genome, so in total
there could be thousands of genes that significantly differ
between humans and chimps, says Jean Weissenbach from
France's National Sequencing Centre in Evry. This could make
it much harder than scientists had hoped to find the key
changes that made us human.
Nonetheless, Taylor and his team plan to use their chimp
sequence to home in on important differences between the two
species. "We have to work out which proteins have a
functional impact," says Taylor. The researchers have
already identified two genes called NCAM2 and GRIK1, the
human versions of which contain large sections that are
missing in the chimp. Both genes are known to be involved in
neural function.
Genome feast
The sequence of chromosome 22 has also whetted researchers'
appetites for the full chimp genome sequence, due to be
published in Nature later this year. "The small section of
the genome is very valuable," says Ajit Varki, a molecular
biologist at the University of California, San Diego. "But
the whole genome will be a wonderful tool."
Weissenbach is looking even further ahead. He points out
that the best way to narrow down which of the differences
between the two genomes are really key to making us human is
to compare the sequences with those of other great apes. "Do
we now need the gorilla genome sequence?" he says.
References Fujiyama, A. et al. Nature, 429, 382,
doi:10.1038/nature02564 (2004).
Quoted message said:From Nature
Posted by Robert Karl Stonjek.