Centromeric Locations in Karyotypes: a Rule Derived from the Theory of Brahnched Polymers

It is believed that chromosomes occupy non-overlapping domains in the interphase nucleus, and that the nuclear volume can be divided into the interchromosomal spce and the chromosome domains. Concentrations of various components (e.g., small ions) are different in these compartments. Since nuclear volume is twice as large in G2as in G1phase,V2/V1(the G2/G1ratio of total chromosomal volumes) must be two in order to keep the interchromosomal concentrations unchanged. The aim of the study is to test the ‘V2/V1’ hypothesis. It has been shown that G1-chromosomes behave as real flexible polymers. If a G2-chromosome behaves as a four-arm star-type branched polymer, then, according to polymer theory, its chromosome volume should depend on its centromere position. We calculatedV2/V1values for 40 karyotype, from yeast to human, and 19 of them haveV2/V1= 2 ± 10 %. There are two types of exceptions from the ‘V2/V1= 2’ rule: karyotypes with a large number of telocentric chromosomes (V2/V1> 2), and karyotypes with a large number of metacentric chromosomes (V2/V1< 2). It has been observed in the literature that for all-telocentric karyotypes of mouse and Chines muntjac, their chromosomes form branch-like structures, by association of centromeres in clusters in G2phase. When calculated for these temporary structures,V2/V1decreases to two if the number of associated chromosomes per cluster is greater than or equal to five. This corresponds to a number of centromere clusters per nucleus less than or equal to 8–9 for mouse and Chinese muntjac, which is consistent with observation. For rye, all-metacentric karyotype, the calculatedV2/V1value increases to nearly two if B-chromosomes are taken into account.