ISSN:
1432-0886
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Medicine
Notes:
Abstract Nuclei from three different developmental stages of Strongylocentrotus purpuratus were examined for stage-specific differences in transcription. After dispersal in hypotonic solution and preparation for electron microscopic (EM) observation, nuclei from cleavage, gastrula, and prism stages all displayed beaded chromatin with similar numbers of nucleosomes per μm chromatin. The DNA/chromatin packing ratios computed from these nucleosome frequencies are 1.9–2.0 for both transcribed and nontranscribed chromatin. Several stage-specific differences were also observed. (1) Prism stage nuclei exhibited a smooth, nonbeaded chromatin configuration within the active ribosomal RNA genes (packing ratio=1.1 μm DNA/1.0 μm chromatin). (2) The rapid cell cycle of cleavage stage embryos produced some nuclei with metaphase chromosomes and some with DNA replication intermediates. (3) The average lengths of the nascent nonribosomal transcripts were comparable in gastrula and prism nuclei but shorter in cleavage nuclei. (4) And, the cleavage stage chromatin had a class of short, repeated ribonucleoprotein (RNP) fibril arrays that were not observed at any other developmental stage. This special class consisted of long stretches of chromatin (50–160 μm) covered with clusters of very short RNP fibrils (length=0.04±0.02 μm). These repeating clusters ranged in length from 0.1–0.2 μm chromatin. The characteristics of these arrays suggest they are active histone genes. If so, then this is the first report of the EM morphology of transcribing histone genes. These putative histone genes contributed significantly to the amount of transcription observed in cleavage nuclei while the number and distribution of all other RNP fibrils were similar on cleavage and gastrula chromatin. These data are discussed in relation to reported changes in the RNA synthetic rates of sea urchin embryos.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00328475
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