An alternative model for ring chromosome perpetuation in a human subjecgt

TLDR: It is suggested that an alternative cytological mechanism exists for ring perpetuation that may not follow strictly McClintock’s principle of ‘ breakage-fusion-bridge cycle ’.

Abstract: The cytological mechanisms for the formation and perpetuation of varying-sized ring chromosomes have been explored extensively by McClintock (1938) in Zea mays. According to McClintock, a somatic crossover between the two sister chromatids of a monocentric ring during the duplication period gives rise to a dicentric double-sized ring at the following anaphase stage. Because of the anaphase movement, the dicentric ring might break at unequal or equal points. Thus, two unequal or equal-sized rings would be produced in the daughter nuclei. In the following cell division, these daughter rings would repeat the earlier cytological steps so that a ‘ breakage-fusion-bridge cycle ’ is maintained in the subsequent cell generations. In the present communication a comparative metaphase behaviour of two unequal rings in a human subject has been studied in leucocytes grown in vitro. Our observations suggest that an alternative cytological mechanism exists for ring perpetuation that may not follow strictly McClintock’s principle of ‘ breakage-fusion-bridge cycle ’.