The Indian deer, Muntiacus muntjak, with its low number of individually identifiable chromosomes (♂, 2n = 7), is a suitable material for studies on the spatial interrelationships among metaphase chromosomes on the equatorial plate. Cultured monolayers not subjected to colchicine or hypotonic treatments were analyzed in situ. The emphasis mainly was placed on determining the locations of the centromeres of the six large chromosomes, which usually occupied peripheral positions at the equator. The frequencies found of certain patterns of chromosome arrangement deviated significantly from what would be expected if these patterns were random. Cells in which homologs were next to each other were more frequent than expected. In configurations in which no homologous chromosomes occupied adjacent positions, symmetrical radial arrangements, based on homology or proximity in size, were favored. The position of the Y chromosome usually was close to the centromeric region of the X-autosome, which might be a consequence of a heterochromatic association or partial homology between the X and Y. Factors relevant to the chromosome arrangement on the equatorial plate may be somatic pairing, symmetrical spatial interrelationships between chromosomes based on homology or size, and heterochromatic association.
High-resolution Giemsa-trypsin bands can be induced in the chromosomes of human skin fíbroblast cultures exposed to actinomycin D (final concentration, 2 µg/ml) before harvest.
In this paper methodology is described which yields three-way Giemsa differentiation (light-medium-dark) in human metaphase chromosomes exposed to 5-bromodeoxyuridine (BrdU) for 3 DNA synthetic periods (or exposed for 2 DNA synthetic periods and removed from exposure for the third) by means of which all of the sister chromatid exchanges (SCEs) occurring during (or shortly after) S1, S2 and S3 can be accurately counted and distinguished from one another. Using these methods it has been demonstrated that approximately twice as many SCEs occur during the first S-period in the presence of the drug (labeling=B1T0XT0B1)1 as occur during the second S-period (labeling=B2B1XT0B2)1. The three-way differentiation pattern is thought to result from a stepwise decrease in the amount of BrdU incorporated during the first, second and third DNA synthetic periods. These methods can also be used to differentiate between unlabeled (T2T0) and unifilarly labeled (B1T2) sister chromatids and are potentially useful in the detection of sub-chromatid exchanges (none were detected).
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