ABSTRACT
Sister chromatid exchanges (SCEs) were first inferred by B. McClintock on the basis of the behavior of mitotic ring chromosomes of maize. The phenomenon of SCEs can be analyzed much easier since S.A. Latt reported that incorporation of the halogenated pyrimidine analog, 5-bromo-2'-deoxyuridine (BrdU) instead of thymidine (Thd) into chromosomal DNA also allows for the differentiation between sister chromatids after staining with the fluorochrome bisbenzimide. Cultivation of cells for two consecutive cell cycles in the presence of BrdU results in metaphase chromosomes that consist of one unifilarly (BrdU incorporation in only one DNA strand, TB) and one bifilarly (BrdU incorporation in both DNA strands, BB) substituted chromatid. Replication banding is widely used for chromosome identification in animals, but has only rarely been applied for plant chromosomes. Replication banding in V. faba showed that the late-replicating chromosome regions detected after FPG staining correspond to dark-stained regions of constitutive heterochromatin detected by C-banding.
