The features of the isochromosome suggest that the interaction of chromatids during interphase is essentially caused by specific DNA sequences. Two events account for this high degree of condensation:(1) the homologues are particularly associated due to somatic pairing and (2) the arms are further tightened as a result of pericentric folding. The interphase density of chromosome 4 can exceed that of metaphase by a factor of up to 8. At this stage, a new category of extreme DNA packaging was detected. The large amount of DNA engaged in heteropycnosis and the isochromosome nature allow the identification of chromosome 4 during interphase. The sex chromosomes and chromosome 3 condense by shortening, while chromosomes 2 and 4 preferentially reduce their diameter. Further differences were observed in the longitudinal and lateral compaction of the chromosomes in males and females. This would explain the metaphase of the female where the X chromosomes shorten less than the autosomes, and why each of the X chromosomes is 15% shorter than the X chromosome in the metaphase of the male. The X chromosomes of the female probably have enhanced shortening during prophase. In contrast, chromosomes of the female display asynchrony as monitored by area and length determinations. During the metaphase of the male, chromosomes condense with similar kinetics. In the male, the order is X, 2, 3, Y, and 4. In the metaphase of the female, Feulgen density increases from the X chromosome, via 3 and 2, to chromosome 4.
The mean condensation is not equal for all chromosomes. The process of chromosome condensation is not restricted to mitotic prophase but continues into the metaphase. The results of a comparative microphotometric study on neuroblast metaphases in both sexes revealed the following picture. nasutoides, even sex specific differences can be observed.
This paper reports that chromosomes of the same complement display different degrees and kinetics of condensation. Earlier studies of heterochromatin attributed little attention to the fact that “condensed” chromosomes often vary in condensation. This element mainly consists of constitutive heterochromatin which does not polytenize. Drosophila nasutoides has an extraordinary genome since 62% of its DNA resides in chromosome 4.
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