DNA Synthesis and Cell Division in Germinating Onion II. Mitotic Cycle and DNA Content

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Abstract
This study of the duration of the mitotic cycle in the germinating onion shows a mitotic cycle at stability of 12.8 hours, with G1, S, G2, and M-periods of 1.5, 6.5, 2.4, and 2.3 hours, respectively. Phase durations are 0.8, 0.2, 0.1, and 1.2 hours for prophase, metaphase, anaphase, and telophase, in that order. When examined as a function of developmental stage, no significant variation is seen in the lengths of the above intervals. Microspectrophotometric analysis of various developmental stages reveals that the total nuclear complement of the quiescent embryo is in G1, (essentially all 2C), and that the initiation of DNA synthesis occurs at approximately the 1.5–2.0 mm. point in root development, in agreement with the value determined earlier.