Abstract
SUMMARYValues of mitotic cycle parameters were determined in Allium cepa root-tip cells after incorporation of tritiated thymidine. Roots were grown from bulbs cultivated in darkness (21°C). The means of the values from two independent experiments were: G1 = 3 h 20 min.; S: 12 h; G2 = 3 h 40 min; mitosis = 4 h; mitotic cycle = 23 h.The pattern of DNA replication in Allium chromosomes was also investigated. Counts of silver grains over euchromatic and heterochromatic regions of metaphase chromosomes revealed an asynchronous DNA replication in heterochromatin. In metaphases from cells treated at the end of S, heterochromatic segments (i.e. juxta-centromeric and distal segments of chromosomes) were selectively labelled. On the contrary, the labelling was distributed at random in chromosomes from cells which were at the beginning of S when the precursor was given. It was concluded that DNA synthesis lasts longer in heterochromatin than in euchromatin.
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