562
Chimpanzee Genome
HS2
HS17
PT12
PT13
(a)
(b)
PT19
Fig. 4
Examples of cytogenetic
differences between humans and
chimpanzees. Chromosomes and
chromosome bands are not drawn to
scale. (a) Human chromosome 2 (HS2)
is present as two separate
chromosomes in chimpanzees (PT12,
PT13). (b) Human chromosome 17
(HS17) and chimpanzee chromosome
19 (PT19) differ by a pericentric
inversion. Arrows mark the inversion
breakpoints.
of the centromere (pericentric inversion)
(Fig. 4(b)). To date, the inversion break-
points of a single human chromosome,
human chromosome 17, have been ana-
lyzed on the DNA sequence level, and the
breakpoints in three additional chromo-
somes, human chromosome 4, 9, and 12,
have been mapped to intervals of 1 to 2 Mb.
So far, no gene was found to be structurally
modi±ed by these inversions.
Finally, a number of chromosomes dif-
fer between chimpanzees and humans
in the amount of constitutive heterochro-
matin, chromosomal regions that remain
condensed throughout the cell cycle and
generally do not contain genes.
Cytogenetic differences are discussed to
comprise evolutionary important changes
that contributed to the initial separation
of chimpanzees and humans. Somewhat
in contrast to this view, currently no ev-
idence exists that the function of a gene
has been affected by any of the observed
chromosomal rearrangements. However,
pericentric inversions can establish re-
productive isolation between populations
without the necessity of an altered gene
function. In a heterozygous state, an indi-
vidual carries one normal and one inverted
copy of the chromosome, pericentric inver-
sions can substantially interfere with the
fertility of the individual. Crossing-over
during meiosis in the inverted region
generates recombination products that are
generally incompatible with life. Thus, it
can be speculated that the pericentric in-
versions differentiating the chromosomes
of contemporary chimpanzees and hu-
mans comprised early reproductive bar-
riers that facilitated the separation of the
two species.
2.2
Subchromosomal Rearrangements and
Repeat Content
In contrast to the well-de±ned extent of
cytogenetic differences between humans
and chimpanzees, the amount of genomic
differences whose size is below the res-
olution of cytogenetic methods is still
unclear. Subtelomeric regions, those ge-
nomic segments that are located directly
adjacent to the chromosome ends, appear
to be dynamic structures that undergo
rapid changes over short evolutionary dis-
tances. For example, a subtelomeric region
formed by a 32-bp repeat unit present at
many chromosome ends of chimpanzees
and almost all chromosome ends of go-
rillas is not found in humans. Similarly,
agenom
iclocustha
tex
is
tson
lyasas
in
-
gle copy in the subtelomeric region of
chimpanzee chromosome 17 is found in
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