CENTROMERIC LOCATIONS IN KARYOTYPES - A RULE DERIVED FROM THE THEORY OF BRANCHED POLYMERS

It is believed that chromosomes occupy non-overlapping domains in the interphase nucleus, and that the nuclear volume can be divided into th e interchromosomal space and the chromosome domains. Concentrations of various components (e.g., small ions) are different in these compartm ents. Since nuclear volume is twice as large in G(2) as in G(1) phase, V-2/V-1 (the G(2)/G(1) ratio of total chromosomal volumes) must be tw o in order to keep the interchromosomal concentrations unchanged. The aim of this study is to test the 'V-2/V-1=2' hypothesis. It has been s hown that G(1)-chromosomes behave as real flexible polymers. If a Gr-c hromosome behaves as a four-arm star-type branched polymer, then, acco rding to polymer theory, its chromosome volume should depend on its ce ntromere position. We calculated V-2/V-1 values for 40 karyotypes, fro m yeast to human, and 19 of them have V-2/V-1=2+/-10%. There are two t ypes of exceptions from the 'V-2/V-1=2' rule: karyotypes with a large number of telocentric chromosomes (V-2/V-1>2), and karyotypes with a l arge number of metacentric chromosomes (V-2/V-1<2). It has been observ ed in the literature that for all-telocentric karyotypes of mouse and Chinese muntjac, their chromosomes form branch-like structures by asso ciation of centromeres in clusters in G(2) phase. When calculated for these temporary structures, V-2/V-1 decreases to two if the number of associated chromosomes per cluster is greater than or equal to five. T his corresponds to a number of centromere clusters per nucleus less th an or equal to 8-9 for mouse and Chinese muntjac, which is consistent with observation. For rye, all-metacentric karyotype, the calculated V -2/V-1 value increases to nearly two if B-chromosomes are taken into a ccount. (C) 1996 Academic Press Limited.