Chimps are not like humans
Whole-chromosome comparison reveals much greater genetic differences than
expected
By Cathy Holding

The vigorous debate on how different chimpanzees are from humans is fuelled
by new data in this week's Nature, as the International Chimpanzee
Chromosome 22 Consortium reports that 83% of chimpanzee chromosome 22
proteins are different from their human counterparts.

The difference is "much more complicated that we initially imagined or
speculated," Yoshiyuki Sakaki, who headed the consortium, told The
Scientist.

"Our group completed the human chromosome 21 sequence about 3 years ago,"
Sakaki, director of the RIKEN Genomic Sciences Center, Yokohama City, Japan,
told The Scientist. Chimpanzee chromosome 22 is the ortholog of human
chromosome 21. Despite being very small, human chromosome 21 contains a
number of interesting features, including functional genes, centromere and
telomere chromosomal structures, and insertions and deletions including Alu
and other repetitive elements, Sakaki said.

"So if we analyze this [chimpanzee] chromosome, we can estimate the overall
events that have taken place," Sakaki said.

The team sequenced bacterial artificial chromosome genomic DNA libraries
from male chimpanzees (Pan troglodytes) and aligned chimpanzee chromosome 22
(PTR22) clones with the orthologous human chromosome 21 HSA21q data using
NCBI BLAST2 to make a whole-chromosome comparison.

Sakaki said their analysis found about 68,000 insertions or deletions. "That
is almost one insertion/deletion every 470 bases," he said. In addition, a
small proportion of genes showed a relatively higher rate of evolution than
most other genes. "We haven't known what proportion of the genes shows
adaptive evolution. This study shows it to be about 2 to 3%," he said.

Early molecular comparisons between humans and chimpanzees suggested that
the species are very similar to each other at the nucleotide sequence
level-a difference of between 1.23% and 5%, Sakaki said. The results
reported this week showed that "83% of the genes have changed between the
human and the chimpanzee-only 17% are identical-so that means that the
impression that comes from the 1.2% [sequence] difference is [misleading].
In the case of protein structures, it has a big effect," Sakaki said.

Read the rest at The Scientist.com
http://www.biomedcentral.com/news/20040527/01

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