Abstract
The duration of the mitotic cycle with its component phases in Nicotiana plumbaginifolia (pbg, 2n = 20) and a hybrid derivative of N. tabacum (tbc, 2n = 48 and N. plumbaginifolia was determined from the root tips. A 30 minute pulse label of H3-thymidine was employed for the autoradiographic detection of the labeled prophases.The time intervals, for the total generation time (T), postsynthetic phase (G2), prophase, synthetic phase (S), and the combined presynthetic phase, metaphase, anaphase, telophase (G1 + M + A + T) in N. plumbaginifolia were estimated as 11.0, 3.4, 1.3, 5.7 and 0 6 hours, respectively. In the hybrid derivative the corresponding estimates were 9.0, 1.4, 0.8, 3.8 and 3.0 hours.The G1, S and G2periods of N. plumbaginifolia were found significantly different from those of the hybrid derivative, but the duration of the mitotic cycle did not show a significant difference. On the basis of the distribution of heterochromatin and its late replication in N. plumbaginifolia chromosomes, possible causes of instability of alien pbg chromosomes in N. tabacum nuclei are discussed.
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