Patterns in chromatin structureSeve

Patterns in chromatin structure
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 metaphasechromosomes.Thechromatinofactivegenesis 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 theinterphasenucleus alsodiffersbetween chromosome bands. C-band positive heterochromatin remains visibly condensed through interphase. FISH has shown that over the 150kb 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 modifiedformsarefoundpreferentiallyindifferentregions 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 immunofluorescence 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.