  An oscillation with a period of around 500 kb in guanine and cytosine content
(GC%) is observed in the DNA sequence of human chromosome 21. This oscillation
is localized in the rightmost one-eighth region of the chromosome, from 43.5 Mb
to 46.5 Mb. Five cycles of oscillation are observed in this region with six
GC-rich peaks and five GC-poor valleys. The GC-poor valleys comprise regions
with low density of CpG islands and, alternating between the two DNA strands,
low gene density regions. Consequently, the long-range oscillation of GC%
result in spacing patterns of both CpG island density, and to a lesser extent,
gene densities.
