Several banding techniques, especially G-banding, suggest
that there are both qualitative and quantitative variations
in the interaction of DNA and proteins along the length of
metaphase chromosomes. The chromatin of active genes is
generally considered to be more accessible to nuclease
attack than is inactive chromatin. Consistent with this
nucleases preferentially digest R-bands and T-bands of
intact mitotic chromosomes, with G-bands and C-bands
refractive to digestion. The extent of chromatin packaging
in the interphase nucleus also differs between chromosome
bands. C-band positive heterochromatin remains visibly
condensed through interphase. FISH has shown that over
the 150 kb to 1Mb size range G-band chromatin is more
tightly packaged than that of R-bands.
The N-terminal tails of core histones H3 and H4 can be
modified by acetylation of lysine residues. A consequence
of this acetylation may be to facilitate access of proteins
such as transcription factors to the DNA. Histones can be
deacetylated or acetylated throughout the cell cycle by
nuclear acetyltransferase and deacetylase enzymes. Immunofluorescence
of mammalian metaphase chromosomes
with antibodies raised against each of the
acetylated forms of H4 has shown that the differently
modified forms are found preferentially in different regions
of the chromosome. Mono-acetylated (Lys16) H4 is found
throughout euchromatin, whereas acetylation at Lys8 and
Lys12 occurs mainly in R-bands. Acetylation at Lys5 is
found in the most highly modified (tri-and tetraacetylated)
forms of histone H4. Antibodies to this H4
isoform produce a good banding pattern on metaphase
chromosomes, especially from cells that have been briefly
exposed to histone deacetylase inhibitors. Bright immuno-
fluorescence is seen over T-/R-band regions of the
karyotype and only faint staining is seen in G-bands
(Jeppesen and Turner, 1993). Hence the distribution of
histone acetylation on mammalian metaphase chromosomes
mirrors that of genes