Premature chromosome condensation and cell cycle analysis

The application of the phenomenon of premature chromosome condensation for cell cycle analysis in HeLa and CHO cells has been examined. Random populations of HeLa and CHO cells pulse labelled with H3-TdR were separately fused with mitotic HeLa cells using U.V. inactivated Sendai virus. The resulting prematurely condensed chromosomes (PCC) were scored and classified into G1, S and G2-PCC on the basis of both morphological and autoradiographic data, The results of this study indicated that the G1, S and G2 phase cells are equally susceptible to virus-induced fusion with mitotic cells and subsequent induction into PCC. Hence the PCC method for cell cycle analysis is both practical and accurate. This study also revealed that the process of chromosome decondensation initiated during the telophase of mitosis continues throughout the G1 period reaching an ultimate state of decondensation by the end of G1, at which point the fusion of such cells with those in mitosis yield PCC with the most diffused morphology instead of the discrete single stranded structures characteristic of early G1-PCC. Thus, the decondensation of chromatin during G1 appears to be a prerequisite for the subsequent initiation of DNA synthesis.