Abstract :
Our main research interests have focused on cation–DNA and cation–protein interactions in terms of their roles in higher order
chromosomal structure. Our previous study directly analyzed the cation composition of cryo-preserved mammalian interphase
and mitotic cells, as well as fractionated untreated and cation-depleted chromosomes at a resolution of 50 nm using the
University of Chicago high resolution scanning ion microprobe (UC-SIM).
Quantitative direct UC-SIMS signals and subsequent imaging of cryo-preserved cells demonstrated that Naþ and Kþ were
associated with chromatin throughout the cell cycle, whereas in contrast Ca2þ and Mg2þ exhibited a localization change during
interphase and mitosis. Interphase Ca2þ and Mg2þ were found mainly throughout the cytoplasm, but at mitosis associated with
chromatin. Our findings of chromatin association of Naþ and Kþ support a role of these cations in both interphase and mitotic
chromatin compaction, whereas Ca2þ and Mg2þ binding points to an essential function in mitotic chromatin compaction.
In our present investigation using SIMS images obtained from both human and Indian muntjac metaphase chromosomes, we
reveal specific binding of Mg2þ to chromosomal ‘‘p’’ and ‘‘q’’ arm heterochromatic regions correlating with conventional Giemsa
(G-) bands. In addition, detailed Mg2þ image density peaks along the chromosomes indicate that Mg2þ peaks correspond to the
location of G-bands.Our results support a direct role forMg2þ in promoting andmaintaining the higher order chromatin structure of
heterochromatic regions on chromosome arms represented by G-bands, possibly due to both Mg2þ–DNA and Mg2þ–protein
interactions.
# 2004 Elsevier B.V. All rights reserved.
