DNA exposure and condensation in the X and 21 chromosomes.

Fluorescence in situ hybridization (FISH) whole chromosome painting probe studies have been carried out with X and 21 chromosomes on normal human fibroblasts grown in tissue culture. The majority of the cells were in G1 phase (including G0). The X chromosome, which exhibits differential inactivation, displays an active form which is most commonly in the nuclear periphery, is diffused over a large area with dark regions interspersed with bright regions, and exhibits punctate bright spots at its edges. The inactive X, which contains a small fraction of active genes, is also most often at the nuclear periphery, is highly condensed and also exhibits punctate labeling around its outer edge. Occasional nuclei exhibit X chromosomal material adjacent to a nucleolus. These observations fit the pattern proposed by the genome exposure theory in which inactive gene regions are sequestered by chromosome condensation, and become exposed by decondensation into a condition invisible by the video-imaging technique employed. Such exposed genes can then be activated by appropriate molecular messengers. In accordance with this theory, the total fluorescence observed from the active X is appreciably less than that of the inactive. The FISH pattern from chromosome 21 is very different, displaying two fluorescent bodies usually connected with the nucleoli. Both bodies contain condensed and decondensed regions, and both are much more similar in their degree of decondensation than was the case with the X chromosomes, although a small difference cannot be ruled out. Use of DNase I treatment of nuclei reveals the existence of exposed DNA. The use of FISH as demonstrated here can indicate sequestered DNA. Together the two techniques promise elucidation of gene regions of various chromosomes which are active and inactive in particular tissues and in normal and pathologic conditions.