Anthology of Human Repetitive DNA
259
defned as classical satellites I, II, III,
and IV. Classical satellite I was Found
to be dominated by a simple component
called
satellite 1
. Satellite 1 contains a 42-
bp basic unit, originally described as a
17-mer (A) and a 25-mer (B), arranged in
an alternating pattern: A-B-A-B (±ig. 1b).
The major simple components oF classical
satellites II and III are named
satellites 2
and 3
, both containing the GAATG unit
(sometimes described by the complemen-
tary ATTCC). Repetitive sequences From
the satellite DNA Fraction IV are nearly
identical to those in Fraction III.
Among other well-defned and exten-
sively studied components oF classical
satellites II and III are the so-called
al-
pha satellites
, frst discovered in AFrican
green monkeys and subsequently in other
primates. Alpha satellite DNA is composed
oF a 171-bp unit (±ig. 1c). Interchromoso-
mal sequence identities suggest that all
human alpha satellites arose From two or
three diFFerent ancestral monomers, which
in turn diverged into 12 distinct types oF
alphoid monomers. They can diFFer by as
much as 20 to 40% From one another by
sequence identity.
Human chromosomes contain alpha
satellites
organized
hierarchically
into
higher-order repeat arrays (±ig. 1c), which
can diFFer between diFFerent chromosomal
locations. On the basis oF this organi-
zation, the alpha satellites From the hu-
man genome have been divided into fve
suprachromosomal Families: the frst Fam-
ily consists oF alpha satellites located on
chromosomes 1, 3, 5, 6, 7, 10, 12, 16, and
19; the second comprises chromosomes
2, 4, 8, 9, 13, 14, 15, 18, 20, 21, and 22;
the third is located on chromosomes 1, 11,
17, and X; the Fourth Family is Found on
chromosomes 13, 14, 15, 21, 22, and Y;
and, fnally, the fFth Family was detected
on chromosomes 5, 6, 13, 14, 19, and 21.
Members oF each suprachromosomal
Family share a common alpha satellite
repeat unit. The size, conservation, and
organization oF these higher-order struc-
tures depend on the chromosome and
on the physical distance From the cen-
tromere. Apart From the fve classical
suprachromosomal Families, other higher-
order structures have also been described.
±or example, the central region oF the chro-
mosome X centromere contains a specifc
alpha satellite array arranged into 2-kb long
highly identical blocks.
The evolution oF alpha satellites is
complex, and is driven by intra- and in-
terchromosomal recombination and gene
conversion. The recombination occurs
during chromosomal pairing and leads to
an unequal crossing-over between chro-
mosomes. As a result, stretches oF alpha
satellite DNA expand and contract at a very
rapid rate. In addition, the alpha satellite
regions oF chromosomes within the same
suprachromosomal Families may undergo
exchanges between nonhomologous chro-
mosomes, thus increasing the mode and
tempo at which these sequences evolve.
However, the intrachromosomal recombi-
nation within alpha satellites appears to
be the predominant evolutionary driving
Force. In general, the average intraFam-
ily identity oF alpha satellite monomers
(
90%) is much higher than the interchro-
mosomal identity (
75%). In addition,
the average intraFamily identity within a
species (99% in human and gorilla) is gen-
erally greater than that between species
(90% between the human and gorilla
satellite monomers). Humans and goril-
las share the basic monomer as well as
a higher-order repeat unit For suprachro-
mosomal Family 3, suggesting that this
alpha satellite structure predates the hu-
man/great ape divergence. Thus, the alpha
satellites within each species have evolved
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