556
Chimpanzee Genome
an electric feld. Additional resolution can
be achieved by a preceding separation oF
the proteins according to their isoelectric
point – the pH where the net charge is 0
(2D electrophoresis). Variants oF a partic-
ular protein that diFFer in their net charge
because oF sequence diFFerences involv-
ing charged amino acids can be identifed
by their diFFerent mobility on the matrix.
The application oF protein electrophore-
s
i
sinphy
logene
t
i
cana
ly
se
si
sba
sedon
the Fact that the degree oF diFFerence
in the mobility pattern between related
proteins approximately reflects their ex-
tent oF amino acid sequence diFFerence.
Thus, similar to immunological compar-
isons, protein electrophoresis provides an
estimate oF the genetic distance between
species that Forms the basis oF phylogenetic
conclusions.
The most extensive study to reconstruct
the phylogeny oF hominoids by means oF
protein electrophoresis was based on a
2D electrophoresis oF 383 fbroblast pro-
teins. Concordant to conclusions From
immunological distances oF the homi-
noids, it supports an early separation oF the
gibbons Followed by the lineage leading
to orangutans. Notably, the study sug-
gested a smaller genetic distance between
humans and chimpanzees than between
either species and gorillas. This lends sup-
port to the model that humans are the
closest relatives to chimpanzees and bono-
bos (±ig. 2d).
However, similar to the immunologi-
cal comparison oF proteins, electrophoretic
analyses can detect only a certain subset oF
amino acid diFFerences between proteins.
Only about 30% oF all amino acid sub-
stitutions alter the charge and, thereFore,
the mobility pattern oF a protein during
electrophoresis. ±or closely related species,
this poses the same resolution problem as
already discussed For the immunological
comparison oF proteins.
1.1.3
Amino Acid Sequence Comparison
The direct comparison oF their amino acid
sequences represents the most accurate
way to assess the degree oF diFFerence
between two proteins. Even though ex-
perimentally elaborative, it provides the
opportun
itytoa
lsotracethosechangesin
the protein sequence that neither aFFect the
immunological properties nor the charge
oF a protein.
Despite this better resolution, the phy-
logenetic insights From comparative se-
quence analyses oF hominoid proteins are
limited. A data set oF 1271 amino acid
positions From 9 diFFerent polypeptides
revealed an average similarity between hu-
man and chimpanzee proteins oF 99.6%.
A similar analysis with a smaller data set
comparing humans, chimpanzees, and go-
rillas determined the average amino acid
sequence similarity between humans and
gorillas and between chimpanzees and go-
rillas to be slightly lower (99.3%). Even
though this implies a closer relationship
between humans and chimpanzees to the
exclusion oF gorillas, the number oF inFor-
mative positions was Far too low to allow
decisive conclusions.
Hindered by the disproportionate exper-
imental eFFort, this data set has never been
substantially enlarged. Rather, the Focus
oF evolutionary biologists shiFted From the
comparison oF proteins to the comparison
oF DNA sequences between species.
1.2
Phylogenetic Analyses Based on DNA
Sequence Comparisons
At least two reasons suggest that compar-
ative analyses oF DNA sequences are more
suitable For reconstructing the phylogeny
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