564
Chimpanzee Genome
1.5%. Many of these estimates were based
on the comparison of short stretches
of DNA from only one or few regions
of the genome. However, different re-
gions in the human genome differ in
aspects such as gene content, transcrip-
tion frequency, base composition, and
recombination frequency all of which are
suspected to influence the evolutionary
rate of DNA sequences. Thus, sampling
of DNA sequences from a multitude of
loci distributed throughout the genome
is required to obtain a relevant estimate
of the average amount of substitutional
DNA sequence differences between two
species. On the basis of genome-wide
comparisons of DNA sequences between
humans and chimpanzees, their aver-
age amount of DNA sequence difference
was recently determined to be 1.2%. The
results furthermore reveal that indeed ge-
nomic regions vary substantially in their
divergence among species (Fig. 5).
In absolute numbers, a mean DNA se-
quence difference of 1.2% corresponds to
a total of about 40 million substitutional
DNA sequence differences between the
human and chimpanzee genomes. How-
ever, only a tiny proportion of the human
genome – about 3% – is believed to be rel-
evant for gene function. This suggests that
less than (only) 1.2 million substitutional
differences account for the distinct pheno-
types of humans and chimpanzees.
2.4
Insertions and Deletions
Processes that are capable of removing
nucleotides from DNA or adding nu-
cleotides to DNA (deletions and insertions
respectively) comprise a further source of
genetic difference between species. Inser-
tions and deletions mediated by trans-
posable elements or by duplications of
chromosomal segments have already been
2.5
2.3
2.1
1.9
1.7
1.5
1.3
1.1
0.9
0.7
12345678
Human chromosome
9101112131415161718192021
X
22
Y
Differences
[%]
Fig. 5
Mean DNA sequence differences between humans and chimpanzees by
human chromosome. Bars indicate 95% conFdence intervals. [Reprinted with
permission from Ebersberger, I., Metzler, D., Schwarz, C., Paabo, S.
(2002) Genomewide comparison of DNA sequences between humans and
chimpanzees,
Am
.J
.Hum
.Genet.
70
, 1490–1497.]
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