Sex and the Single Chromosome
In the late 1800s, microscopists began noting condensed chromatin pieces embedded in a generally diffused chromatin matrix of interphase nuclei. These condensed chromatin bodies have been known as the chromocenters. In the late 1920s and early 1930s, Emil Heitz made a series of careful cytological observations on a number of organisms and concluded that there are two major types of chromatin: euchromatin, which condenses during cell division and decondenses (becoming diffused) during interphase, and heterochromatin, which remains condensed all the time. The chromocenters are equivalent to the heterochromatin. However, it was difficult to identify the chromosomal locations of heterochromatin because at metaphase, when the chromosome morphology is the best, both euchromatin and heterochromatin are fully condensed. In interphase, when differentiation is best, recognition of individual chromosomes is not feasible. Heitz determined the locations of heterochromatin by using prometaphasic chromosomes. Here, the chromosomes are morphologically identifiable yet the euchromatin is not fully condensed. In Drosophila melanogaster, heterochromatin is located in the proximal third of the X chromosome, the entire Y chromosome and the centromeric areas of the autosomes.
KeywordsSingle Chromosome Homing Pigeon Facultative Heterochromatin Chromatin Body Prometaphasic Chromosome
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