Mechanism
of
DNA
Amplification -
The
origin
of
repeated
sequences
with
features
as
different
as
those
of
tandem
arrays
and
interspersed
repeats
would
seem
likely
to
involve
various
sorts
of
sequence
amplification
events.
And,
indeed,
several
different
types
have
been
observed
experimentally.
Tandem
arrays,
especially
those
composed
of
rather
simple
sequences
such
as
the
satellite
DNAs,
may
originate
through
a
process
of
unequal
crossover
in
which
DNA
exchange
occurs
between
out
of
register
chromatids
during
mitosis
and
meiosis.
Within
an
existing
array,
an
unequal
crossover
event
will
cause
one
of
the
resultant
sister
chromatids
to
contain
more,
and
the
other
sister
chromatid
to
correspondingly
contain
fewer
copies
of
the
repeating
unit.
Thus
unequal
crossover
can
lead
to
both
increase
and
decrease
in
copy
number.
Both
increase
and
decrease
have
been
observed
to
occur
in
the
evolution
of
several
well
studied
tandem
arrays,
such
as
those
containing
the
genes
for
ribosomal
RNA,
and
it
may
be
reliably
inferred
that
unequal
crossover
is
an
important
source
of
evolutionary
variation
in
most,
if
not
all,
tandemly
repeated
sequences.
