title: Polytene chromosomes of Oxytricha: biochemical and morphological changes during macronuclear development in a ciliated protozoan.
abstract: After conjugation in the ciliated protozoan, Oxytricha, polytene chromosomes are formed during the development of a macronucleus from a micronucleus. Here we report a microscopic study of these chromosomes and an analysis of their DNA. The polytene chromosomes of Oxytricha bear a strong morphological resemblance to the polytene chromosomes of the Dipteran salivary gland. The nucleus of a developing macronuclear anlage contains 120 +/- 2 polytene chromosomes and each chromosome has an average of 81 hands; a total of about 10,000 bands per nucleus. At a later stage in development, the number of bands per chromosome is reduced by a factor of four, presumably due to fusion of adjacent bands. The polytene chromosomes then break up into their constituent bands, each of which is encased in a vesicle. There are about 2,700 vesicles per nucleus.--During the growth of polytene chromosomes, there is a change in the relative proportion of sequences in the DNA. The DNA from polytene nuclei has a buoyant density of 1.695 g/cc, significantly lighter than the density of the original micronuclear DNA (1.698 G/cc to 1.702 g/cc). We interpret this buoyant density change to be the result of differential replication of DNA sequences during polytene chromosome growth. A second change in DNA composition occurs after the polytene stage of development, shown by a shift in buoyant density to 1.701 g/cc in the DNA of the mature macronucleus. During this second process, the molecular weight of the DNA is reduced from greater than 50 x 10(6) daltons to about 2 x 10(6) daltons.
mesh_terms: D000818:Animals; D002499:Centrifugation, Density Gradient; D002875:Chromosomes / Q000032:analysis / Q000648:ultrastructure; D002906:Chromosomes, Human, 6-12 and X; D016798:Ciliophora / Q000166:cytology* / Q000254:growth & development; D003227:Conjugation, Genetic; D004247:DNA / Q000032:analysis; D004261:DNA Replication; D004276:DNA, Satellite / Q000032:analysis; D006801:Humans
chemical_list: D004276:DNA, Satellite; D004247:DNA