Harold G. Callan

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We have examined the distribution of snRNPs in the germinal vesicle (GV) of frogs and salamanders by immunofluorescent staining and in situ nucleic acid hybridization. The major snRNAs involved in pre-mRNA splicing (U1, U2, U4, U5, and U6) occur together in nearly all loops of the lampbrush chromosomes, and in hundreds to thousands of small granules (1-4(More)
Amongst the axolotl's haploid complement of fourteen mitotic chromosomes, one of the four largest, with a greater arm asymmetry than the other three, shows a nucleolar constriction subterminally in its shorter arm. Low-temperature treatment causes further secondary constrictions to appear; these constrictions enable most of the mitotic chromosomes to be(More)
We show by immunofluorescence microscopy of amphibian oocyte nuclei that small nuclear ribonucleoproteins (snRNPs) occur in lampbrush chromosome loops, in a few dozen extrachromosomal organelles previously described as "spheres," and in thousands of smaller granules. Spheres are variable in size (up to approximately 20 microns in diameter in the newt(More)
Details are given of a technique for making permanent preparations of the lampbrush chromosomes of Xenopus laevis. Stained preparations allow all 18 bivalent chromosomes to be identified, and a working map showing the major features has been constructed. Fifteen of the Xenopus chromosomes have one telomere conspicuously larger than the other; the two(More)
In male Triturus vtdgaris at 16 °C the pre-meiotic synthesis of DNA (5-phase) takes 0-10 days. The 5-phase starts 1-2 days after completion of the ultimate spermatogonial mitosis, and extends into leptotene. Heterochromatic regions, most of which lie close to the centromeres, start and finish replicating about 1 day later than other parts of the(More)