  Spatial fluctuations of guanine and cytosine base content (GC%) are studied
by spectral analysis for the complete set of human genomic DNA sequences. We
find that (i) the 1/f^alpha decay is universally observed in the power spectra
of all twenty-four chromosomes, and that (ii) the exponent alpha \approx 1
extends to about 10^7 bases, one order of magnitude longer than what has
previously been observed. We further find that (iii) almost all human
chromosomes exhibit a cross-over from alpha_1 \approx 1 (1/f^alpha_1) at lower
frequency to alpha_2 < 1 (1/f^alpha_2) at higher frequency, typically occurring
at around 30,000--100,000 bases, while (iv) the cross-over in this frequency
range is virtually absent in human chromosome 22. In addition to the universal
1/f^alpha noise in power spectra, we find (v) several lines of evidence for
chromosome-specific correlation structures, including a 500,000 bases long
oscillation in human chromosome 21. The universal 1/f^alpha spectrum in human
genome is further substantiated by a resistance to variance reduction in
guanine and cytosine content when the window size is increased.
