DNA SYNTHESIS AND CHROMOSOMAL ASYNCHRONY

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Abstract
Peripheral leucocytes of normal females were cultured in vitro and treated for a period of time with aminopterin causing a blocking of DNA synthesis. At the end of this period, thymidine-H3 was used in a 15-minute pulse both to bypass the blockage of DNA synthesis and to label the newly synthesized DNA. Counts of metaphases showed that aminopterin treatment had induced a degree of parasynchrony of DNA replication within cultures, doubling the number of metaphases in treated cultures as compared with untreated control cultures. Autoradiography of DNA synthesized at the beginning of the S phase revealed a characteristic pattern of early replication, with labeled DNA in every chromosome except one known to be in the 6[long dash]12+ X group. This late-starting chromosome is presumed to be the same X chromosome reported by earlier workers to be replicating late in the S phase.