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Showing papers on "Ribosomal DNA published in 1982"


Journal ArticleDOI
TL;DR: The observation that regions of rDNA and histone repeats have become homogeneous for variants that can discriminate between the two most closely related and recently diverged species suggests that additional processes might be required to drive variants to complete fixation.

284 citations


Journal ArticleDOI
TL;DR: The wheat rDNA clone pTA250 was examined in detail to provide a restriction enzyme map and the nucleotide sequence of two of the eleven, 130 bp repeating units found within the spacer region showed some sequence heterogeneity, but polymorphism was not so great as to prevent the use of spacer sequence variation as a probe for evolutionary relationships.
Abstract: The wheat rDNA clone pTA250 was examined in detail to provide a restriction enzyme map and the nucleotide sequence of two of the eleven, 130 bp repeating units found within the spacer region. The 130 bp units showed some sequence heterogeneity. The sequence difference between the two 130 bp units analysed (130.6 and 130.8) was at 7 positions and could be detected as a 4 °C shift in Tm when heterologous and homologous hybrids were compared. This corresponded to a 1.2% change in nucleotide sequence per ΔTm of 1 °C. The sensitivity of the Tm analysis using cloned sequences facilitated the analysis of small sequence variations in the spacer region of different Triticum aestivum cultivars and natural populations of T. turgidum ssp. dicoccoides (referred to as T. dicoccoides). In addition spacer length variation was assayed by restriction enzyme digestion and hybridization with spacer sequence probes.

249 citations


Journal ArticleDOI
TL;DR: It is suggested that the Aegilops umbellulata nucleolus organisers are dominant over those of wheat because they compete more effectively for some limiting factor.
Abstract: Two pairs of chromosomes (1U and 5U) in Aegilops umbellulata possess ribosomal RNA genes. This has been proven by studying wheat plants into which 1U and 5U chromosomes have been introduced separately. These plants have more ribosomal RNA genes than the recipient wheat plants and additional clusters of rDNA when examined by in situ hybridisation. The repeating rDNA unit in Aegilops umbellulata is longer than most of the units in the wheat variety Chinese Spring, the additional DNA probably being in the non-transcribed spacer. This was determined from restriction endonuclease maps of rDNA. In Chinese Spring plants possessing 1U or 5U chromosomes, the largest nucleoli are formed on 1U or 5U chromosomes and the wheat nucleolus organisers form micronucleoli. This is not because the nucleolus organisers on chromosomes 1U and 5U have many more rRNA genes than wheat nucleolus organisers. It is suggested that the Aegilops umbellulata nucleolus organisers are dominant over those of wheat because they compete more effectively for some limiting factor. — The partial inactivation of the wheat nucleolus organisers by chromosomes 1U or 5U does not result in a reduced total nucleolus volume in root tip or pollen mother cells, because of the compensation by the nucleolus organisers of chromosomes 1U or 5U. The amount of RNA in seedlings is not markedly affected by the partial inactivation of the wheat nucleolus organisers.

87 citations


Journal ArticleDOI
TL;DR: From a comparison of the sequences and transcription template-effectiveness of various rDNA subclones, it is inferred that a major promoter of RNA polymerase I activity lies between -150 and -30 in the rDNA nontranscribed spacer.
Abstract: Tandem repeats of ribosomal RNA transcription units in Drosophila melanogaster are separated by a nontranscribed spacer that is comprised in part of serial repeats of a 0.24 kb sequence. DNA sequence analysis shows that such repeats are imperfect copies of a region that includes the site of in vivo rRNA transcription initiation (ca. -240 to +30). Subclones of the rDNA spacer that are copies of the sequence extending from -34 through the initiation site support detectable in vitro transcription in a mixture involving a Drosophila cell-free extract, but accurate in vitro transcription is considerably enhanced when a nontranscribed spacer template includes a copy of the sequence extending upstream of -34. From a comparison of the sequences and transcription template-effectiveness of various rDNA subclones, we infer that a major promoter of RNA polymerase I activity lies between -150 and -30 in the rDNA nontranscribed spacer. The nontranscribed spacer copies of the initiation region are less effective templates for transcription than is the region of in vivo initiation, and there are differences between spacer repeats and the authentic sequence downstream of -240 that may account for this.

79 citations


Journal ArticleDOI
TL;DR: The DNA sequence of the intragenic region of the rat 45S ribosomal RNA precursor was determined in this paper, where the internal transcribed spacers were found to be 1066 and 765 nucleotides long and had little homology with those of Xenopus and yeast.
Abstract: The DNA sequence of the intragenic region of the rat 45S ribosomal RNA precursor was determined. This sequence contains 2282 nucleotides and extends from the conserved EcoR I site near the 3' terminus of 18S rRNA to 69 nucleotides downstream of the 5' terminus of 28S rRNA. The sequences corresponding to 18S and 5.8S rRNA were identified by comparison with previously published data. The 5' terminus of rat 28S rRNA was identified by S1 nuclease protection and reverse transcriptase elongation assays. The internal transcribed spacers were found to be 1066 and 765 nucleotides long and had little homology with those of Xenopus and yeast. Regions of sequence homology between rat and Xenopus were found at the junctions of the internal transcribed spacers with 18S, 5.8S and 28S rRNA. These homologies suggest that these sequences may function as recognition sites for the processing of the ribosomal precursor RNA.

79 citations


Journal ArticleDOI
TL;DR: The qualitative analysis of the Ag-stainability of the NORs was in very good agreement with the results obtained for mammals: Ag-stained NORs are detectable during the entire meiotic prophase up to the pachytene stage, completely absent in the meiotic metaphases I and II, and again demonstrable in early spermatid nuclei.
Abstract: The patterns of activity of the nucleolus organizer regions (NORs) in the spermatogeneses of ten species of all non-mammalian classes of vertebrates and one species of the cephalochordates were investigated with the silver (Ag)-staining technique. The Ag-stainability of the NORs is a measure of the transcriptional activity of the ribosomal RNA genes. In all species, there is a very similar pattern of NOR-activity in the various stages of Spermatogenesis. The qualitative analysis of the Ag-stainability of the NORs was in very good agreement with the results obtained for mammals: Ag-stained NORs are detectable during the entire meiotic prophase up to the pachytene stage, completely absent in the meiotic metaphases I and II, and again demonstrable in early spermatid nuclei. The results confirm the occurrence of postmeiotic reactivation of the RNA genes. The preferential inhibition of rRNA synthesis by low doses of actinomycin D induced a rapid decline of the Ag-stainability of the postmeiotically reactivated NORs. The significance of the evolutionary conservation of the postmeiotic NOR-reactivation is discussed.

78 citations


Journal ArticleDOI
TL;DR: The results show that five of the length variant classes can be divided into three discrete linkage groups, and one particular VrDNA linkage group is localized to chromosome 12, suggesting that these and other restriction fragment polymorphisms can be used in the construction of detailed mouse linkage maps.
Abstract: The ribosomal genes (rDNA) in mouse inbred strains have a multichromosomal distribution. Using a structural feature of rDNA [variable length rDNA segment (VrDNA)] that shows length polymorphism within and among inbred strains, we studied the chromosomal distribution of the variant ribosomal gene type through genetic analysis. Our results show that five of the length variant classes can be divided into three discrete linkage groups. The variants present on a particular chromosome pair appear to be unique to that pair and absent from nonhomologous chromosomes. The chromosomal location of particular variants appears to be the same in two unrelated inbred strains suggesting that the observed linkage patterns predate the origin of inbred mice. The nonrandom chromosomal distribution of these rDNA classes suggests that only a limited degree of genetic exchange occurs among nucleolus organizer regions on nonhomologous chromosomes. We have localized one particular VrDNA linkage group to chromosome 12. These and other restriction fragment polymorphisms can be used in the construction of detailed mouse linkage maps.

75 citations


Journal ArticleDOI
TL;DR: The sequence of 1,100 nucleotides surrounding the transcription initiation site of a cloned rat ribosomal RNA gene (rDNA) has been determined and regions of sequence homology were found when the sequences of the initiation regions of rat and mouse rDNA were compared.
Abstract: The sequence of 1,100 nucleotides surrounding the transcription initiation site of a cloned rat ribosomal RNA gene (rDNA) has been determined. The location of the 5' terminus of 45S pre-rRNA was determined by S1 nuclease mapping, reverse transcriptase elongation and confirmed by in vitro capping of 45S rRNA and in vitro transcription. Two different plasmid subclones, from two separate genomic clones of rat rDNA, contained the identical sequence surrounding the transcription initiation site: -10GGAGATATAT 1GCTGACACGC TGTCCTTTTG+20. Relatively long, greater than 15 base pairs, regions of sequence homology were found when the sequences of the initiation regions of rat and mouse rDNA (Urano, Y., Kominami, R., Mishima, Y., and Muramatsu, M., Nucleic Acids Res. 8, 6043-6058, 1980) were compared. When both the rat and mouse sequences were compared to that of human rDNA (G. Wilson, personal communication) a sequence of 15 nucleotides immediately following the initiation sites were found to be preserved.

62 citations


Journal ArticleDOI
TL;DR: Drosophila rDNA units that have an interruption in the 28S rRNA coding region are not transcribed in vivo, but restriction digests of a recombinant phage DNA that contains such a unit are active as template for in vitro rDNA transcription.
Abstract: An extract of Drosophila melanogaster Kc cells is shown to give specific and accurate transcription of truncated segments of cloned D. melanogaster ribosomal DNA (rDNA). When clones are digested with restriction enzymes so that the initiation site is flanked by 0.3 kilobase (kb) of nontranscribed spacer and greater than 0.4 kb of external transcribed spacer, RNA polymerase I activity in the extract parallels in vivo rRNA synthesis in selection of the coding strand of template and the site of transcription initiation. When greater than 0.3 kb of the nontranscribed spacer is contiguous with transcribed spacer, in vitro initiations evidently also occur in repeated sequences adjacent to the site of in vivo initiation; when less than or equal to 0.4 kb of the external transcribed spacer is present in a segment, expected transcripts are heterogeneous in length or not detectable. Transcription in the cell-free system requires the specific addition of D. melanogaster rDNA: neither D. virilis rDNA, vector plasmid, nor clones of D. melanogaster genes that are transcribed in vivo by RNA polymerases II and III serve as templates in the system. Drosophila rDNA units that have an interruption in the 28S rRNA coding region are not transcribed in vivo, but restriction digests of a recombinant phage DNA that contains such a unit are active as template for in vitro rDNA transcription.

60 citations


Journal ArticleDOI
01 Mar 1982-Cell
TL;DR: It is concluded that in T. thermophila heterozygotes, selective amplification of the rdnA1 allele is not caused by the production of only one type of free, single rRNA gene during amplification, and the single, free, nonpalindromic rRNA genes, which are synthesized during rDNA amplification, are derived from micronuclear gene copies from both chromosomal homologs.

55 citations


Journal ArticleDOI
TL;DR: It is demonstrated faithful transcriptional initiation of cloned Xenopus rRNA genes upon injection into Xenopus oocytes is demonstrated, made possible by the use of an S1 nuclease assay that is both sensitive and quantitative.
Abstract: We have demonstrated faithful transcriptional initiation of cloned Xenopus rRNA genes upon injection into Xenopus oocytes. This observation has been made possible by the use of an S1 nuclease assay that is both sensitive and quantitative. In order to detect rRNA synthesis from the injected template above the large background of rRNA endogenously present in oocytes, the divergence of ribosomal DNA sequences between two Xenopus species was utilized. Cloned X. laevis ribosomal DNA was injected into the nuclei of X. borealis oocytes. Total oocyte RNA was then isolated and hybridized to a radioactive DNA probe that over laps the 5' end of X. laevis rRNA; endogenous rRNA of the X. borealis oocytes does not hybridize to the probe. RNA/DNA hybrids were treated with S1 nuclease and protected fragments were sized by polyacrylamide gel electrophoresis. RNA made from the injected rDNA protects the same region of probe as does authentic X. laevis precursor rRNA. Thus, transcription appears to initiate on the cloned, microinjected X. laevis rDNA at the same site as is used in vivo. This synthesis is not impaired by coinjection of an amount of alpha-amanitin sufficient to inhibit RNA polymerase II and III; therefore the reaction is mediated by RNA polymerase I. The amount of transcription may be reproducibly quantitated and we have varied a number of parameters in order to maximize transcriptional expression of the injected rDNA. Eight independently isolated X. laevis rDNA clones as well as several subcloned initiation regions of these genes are all accurately transcribed at approximately equal efficiency. This assay should facilitate analysis of several aspects of rRNA transcription, including deleniation of the Xenopus RNA polymerase I promoter location.

Journal ArticleDOI
TL;DR: In the archaebacterium Thermoplasma acidophilum, each of the structural genes for 5S, 16S and 23S rRNA occur once per genome, and in contrast to those of eubacteria and eukaryotes, they appear unlinked.
Abstract: In the archaebacterium Thermoplasma acidophilum, each of the structural genes for 5S, 16S and 23S rRNA occur once per genome. In contrast to those of eubacteria and eukaryotes, they appear unlinked. The distance between the 16S and the 23S rDNA is at least 7.5 Kb, that between 23S and 5S rDNA at least 6 Kb and that between 16S and 5S rDNA at least 1.5 Kb. No linkage between those genes has been found by the analysis of recombinant plasmids carrying Bam HI and Hind III rDNA fragments as by hybridizing those plasmids to fragments of Thermoplasma DNA generated by 6 individual restriction endonucleases, recognizing hexanucleotide sequences.

Journal ArticleDOI
TL;DR: It is demonstrated that the rRNA coding region contributes to X / Y pairing, however, no single region of Xh is required for fidelity of male meiotic pairing of the sex chromosomes.
Abstract: The proximal breakpoints of the inversion chromosomes In ( 1 )ω m 4 and In ( 1 ) m 51 b were shown, by in situ hybridization, to define the boundaries of the ribosomal DNA region located within the X chromosome heterochromatin ( Xh ). We estimate that at least 95% of the rDNA is located between the In ( 1 )ω m 4 and In ( 1 )ω m 51 b proximal breakpoints. In contrast only 60–70% of the Type I intervening sequences located in Xh are located between these breakpoints. The Type I intervening sequences in the rDNA region occur as inserts in the 28S rRNA sequences while the remainder of the sequences are distal to the In ( 1 )ω m 4 breakpoint and not associated with rRNA genes. The regions of Xh which contain rDNA and Type I intervening sequences were related to regions shown by Cooper (1964) to contribute to meiotic pairing between the X and Y chromosomes in male Drosophila. We demonstrate that the rRNA coding region contributes to X / Y pairing. However, no single region of Xh is required for fidelity of male meiotic pairing of the sex chromosomes.

Journal ArticleDOI
01 Dec 1982-Gene
TL;DR: The ribosomal DNA repeat unit of Aspergillus nidulans has been cloned in pBR322 and a restriction map constructed and the 5S rRNA is not present in the repeat unit.

Journal ArticleDOI
TL;DR: The entire intervening sequence of Tetrahymena thermophila ribosomal DNA has been determined and has the same splice junctions as those in T. pigmentosa.
Abstract: The entire intervening sequence of Tetrahymena thermophila ribosomal DNA has been determined. It is 413 nucleotides long and has the same splice junctions as those in T. pigmentosa. There is 93% homology between the intervening sequences in the two species, and 100% homology between their adjacent 26S RNA coding regions.

Journal ArticleDOI
TL;DR: A model to explain the evolutionary origin of the several palindromic axes in the Physarum rDNA spacer is proposed and some of the CpG sequences in the spacer resist cleavage by certain restriction endonucleases and thus appear to be methylated.

Journal Article
TL;DR: A group of human ribosomal DNA (rDNA) recombinants that include the probable site for initiation of transcription have been examined for sequence polymorphism and a detailed restriction map of one rDNA insert was constructed using plasmid subclones and end-labeled segments.
Abstract: A group of human ribosomal DNA (rDNA) recombinants that include the probable site for initiation of transcription have been examined for sequence polymorphism. A detailed restriction map of one rDNA insert was constructed using plasmid subclones and end-labeled segments. Comparison of 16 similar rDNA inserts by restriction and heteroduplex analysis demonstrated striking conservation of the external transcribed spacer and 18S gene regions, but defined a region where restriction sites for the enzymes Sma I, Hpa II, and Hha I become frequent or variable. This region extends for about 400--800 base pairs (bp) at the left end of the rDNA insert and is postulated to contain nontranscribed spacer sequences. The use of cloned rDNA segments as probes for the restriction analysis of genomic rDNA has demonstrated certain fixed sites in the nontranscribed spacer that do not vary significantly among different individuals or tumor cell lines. In contrast, restriction with the enzyme Sal I reveals several variable fragments, one of which has been found only in a retinoblastoma cell line.

Journal ArticleDOI
01 Jan 1982-Gene
TL;DR: Four EcoRI fragments, which contain the transcribed portion of the rat rDNA repeat, have been isolated from a rat genome library cloned in lambda Charon 4A vector by cleavage of the fragments with various restriction endonucleases and Southern hybridization with 18S, 5.8S, and 28S rRNA.

Journal ArticleDOI
TL;DR: It is shown that as embryonic cleavage continues, the amount of amplified rDNA decreases until it is no longer detectable in the early gastrula embryo, and it is concluded that the precursor rRNA sequences synthesized in the oocyte are neither processed nor degraded during early development.

Journal ArticleDOI
TL;DR: The number of rRNA cistrons decreases gradually in the successive vegetative progeny, approximating the parental haploid value by the eleventh vegetative transfer.

Journal ArticleDOI
TL;DR: Since it is clear from cytological investigations that Ch.
Abstract: The ribosomal DNAs from Ch. thummi piger and Ch. th. thummi were cloned and analysed by a variety of restriction endonucleases. Comparison of rDNA clones from the two subspecies revealed a considerable length difference: the length of the analysed rDNA cistrons is approximately 9.0 kb for Ch. th. piger and approximately 14.5 kb for Ch. th. thummi. The nearly 5 kb additional DNA in Ch. th. thummi is clearly located within the non-transcribed spacer region, and consists of AT-rich, reptitive DNA elements. These elements with a basic repeat length of approximately 120 bp, are arranged tandemly in stretches of up to about 50 identical copies, which are characterized by a cleavage site for ClaI restriction endonuclease. They are found only in the Ch. th. thummi rDNA clones and not in the Ch. th. piger clones. Southern hybridizations between cloned ribosomal DNA and “centromeric” highly repetitive DNA have shown that the ribosomal repetitive Cla-elements are closely related to a highly repetitive DNA sequence family, which is present in various chromosomal sites particularly the centromeres. Sequence analysis has revealed more than 90% homology between the ribosomal Cla-elements and the “centromeric” Cla-elements. — Since it is clear from cytological investigations that Ch. th. piger with the small rDNA repeating unit is the phylogenetically older subspecies, we postulate a transposition of Cla-elements into the nucleolar DNA during the evolution of Ch. th. thummi.

Journal ArticleDOI
TL;DR: The DNA content of the hemoflagellate Leishmania brasiliensis, strain Y, has been determined by colorimetric reactions and found to be nearly 0.226 pg/cell.
Abstract: . The DNA content of the hemoflagellate Leishmania brasiliensis. strain Y, has been determined by colorimetric reactions and found to be nearly 0.226 pg/cell. When this DNA is bound to filters and hybridized with labeled rRNA from the same organism, saturation is reached at 0.47% of the DNA, corresponding to an estimated 160 ribosomal gene copies. When the DNA is sheared and centrifuged to equilibrium in CsCl gradients, two major satellites of the main band (p = 1.712 g/cm3) are observed: a heavy one (1.720 g/cm3), which hybridizes with labeled rRNA, and a light one (1.699 g/cm3) with the electron microscopic characteristics of the kinetoplast DNA network.

Journal ArticleDOI
TL;DR: It is concluded that the autosomal rDNA sequences are inactive in salivary glands and supply additional evidence in favour of a transcriptional inactivity of ribosomal genes with an IVS in Drosophila.
Abstract: The locations of nucleolus organizer regions in the genomes of D. neohydei, D. eohydei and D. repleta were studied by in situ hybridization. All three species carry one nucleolus organizer in the X heterochromatin. The Y chromosomes of D. neohydei and D. eohydei carry two nucleolus organizers in terminal positions comparable to those of D. hydei. The Y chromosome of D. repleta appears also to carry two nucleolus organizer regions, but this is difficult to establish because of the small size of this chromosome. In situ hybridization of ribosomal RNA with salivary gland chromosomes revealed the presence of additional ribosomal DNA sequence clusters in autosomes 2, 3 and 4 of D. neohydei. They form no nucleolus in salivary glands nor are they associated with the nucleolus originating from the sex chromosomal nucleolus organizers. After in vitro pulse-labelling salivary glands with radioactive RNA precursors, no autoradiographic label is associated with the autosomal ribosomal RNA genes. Transcript in situ hybridization gives no label in these autosomal loci. We conclude therefore that the autosomal rDNA sequences are inactive in salivary glands. In situ hybridization with clones of a major part of the 28S intervening sequence (IVS) of D. hydei to polytene chromosomes of D. neohydei results in intense hybridization of the 28S IVS fragment to the autosomal ribosomal DNA sequences. It is therefore suspected that most if not all of the rDNA copies in the autosomes contain an IVS. The inactivity of these ribosomal genes might be the consequence of the presence of intervening sequences in the 28S gene. These observation supply additional evidence in favour of a transcriptional inactivity of ribosomal genes with an IVS in Drosophila. Additional loci with IVS sequences are found in different positions of the genome which do not hybridize with rDNA sequences without IVS.

Book ChapterDOI
01 Jan 1982
TL;DR: Genetic expression during the early stages of embryogenesis is largely under the control of the maternal genome rather than the embryo genome and appears to be a manifestation of genetic activity realized during oogenesis.
Abstract: Genetic expression during the early stages of embryogenesis is largely under the control of the maternal genome rather than the embryo genome and appears to be a manifestation of genetic activity realized during oogenesis (reviewed by Davidson, 1976). In many organisms, ribosomes assembled during oogenesis provide the protein synthetic machinery of early embryogenesis. In such organisms, large amounts of ribosomal RNA are synthesized and accumulated during oogenesis (Harris and Forrest, 1967). In order to accomplish this, the oocytes may amplify the genes coding for ribosomal RNA (see Gall, 1969, and Tobler, 1975, for reviews).

Journal ArticleDOI
01 Jan 1982
TL;DR: The rDNA methylation level might be correlated directly with the number of rDNA genes and there are discrete hypomethylation sites at similar sites for both Hpa II and Hha I and show a tissue-specific pattern.
Abstract: Ribosomal DNA (rDNA) methylation was studied in various strains of mice. We used restriction enzymes that are sensitive to methylation and a cloned probe containing the transcribed spacer and part of the 18S and 28S gene. Strains C3H/He3, C57/B6-3, and AKR/J were found to have less than 9% of the rDNA methylated. In sharp contrast, Balb/c mice showed 30-50% of the Hpa II and Hha I sites to be methylated. Further study of the Balb/c DNA showed that there are three groups of rDNA sequences. In the first group, all the Hpa II and Hha I sites are almost completely unmethylated; in the second group these sites are all methylated (greater than 30 sites for each enzyme); in the third group most sites are methylated, but there are discrete hypomethylation sites. These hypomethylation positions are at similar sites for both Hpa II and Hha I and show a tissue-specific pattern. Comparison of AKR/J with Balb/c copy level showed that AKR/J had about 60% fewer rDNA genes. The rDNA methylation level might thus be correlated directly with the number of rDNA genes. Finally, analysis of F1 mice from a cross between Balb/c and AKR/J showed both low copy number and low methylation levels.

Journal ArticleDOI
TL;DR: The rDNA of Ch.
Abstract: The rDNA of Ch. tepperi is homogeneous in structure with a repeat size of 8.4 kb. This size seems to be typical for the basic repeat unit in Chironomus species. Ch. th. piger rDNA cistrons are slightly increased in length (9.0 kb). In the non-transcribed spacer (NTS) an appr. 0.18 kb segment is additionally present in about 50% of the repeats. Ch. th. thumni DNA contains largely heterogeneous rDNA repeats, mainly between 10 and 16 kb. The heterogeneity is due to varying numbers of 120 bp elements present in the NTS. The different spacer size classes are not randomly distributed. The short repetitive 120 bp elements (Cla I elements) hybridize in situ with the nucleolus and with centromere regions. The Cla I elements are regularly present in the thummi NTS, but are absent in the piger NTS. Only very few piger rDNA cistrons may contain Cla I elements.

Journal ArticleDOI
TL;DR: In most eukaryotic cells, the genes for the ribosomal RNAs (rRNAs) occur as tandemly repeated units arranged in head-to-tail arrays within the chromosome; however, those of the yeasts Saccharomyces cerevisiae and Torulopsis utilis are located within the Ribosomal DNA (rDNA) repeating unit.

Journal Article
TL;DR: The localization of DNA and RNA synthesis at the various stages of meiosis is discussed in relation to current concepts of chromosome pairing, crossing-over, ribosomal DNA amplification and cycles of RNA metabolism.
Abstract: In an electron microscopic autoradiographic study of DNA and RNA synthesis during meiosis isolated Lilium microsporocytes were supplied with [3H]thymidine and [3H]uridine. DNA synthesis occurred in the nucleus during the zygotene and pachytene intervals of meiotic prophase. Most of the activity was associated with the chromatin, but some synthesis early in zygotene was located at the nucleolus. RNA synthesis occurred throughout prophase until diplotene, when all activity ceased until after division. The newly synthesized RNA was found mostly in association with the chromosomal peripheries or in the space between chromosomes. There was also a peak of [3H]uridine incorporation at the nucleolus, which followed shortly after the synthesis of DNA at that site. The localization of DNA and RNA synthesis at the various stages of meiosis is discussed in relation to current concepts of chromosome pairing, crossing-over, ribosomal DNA amplification and cycles of RNA metabolism.


Journal ArticleDOI
TL;DR: Findings indicate that 18S coding sequences in X. laevis are largely homogeneous and the previously established sequence is the predominant one, thus providing a reliable basis for studies on 18S rRNA.
Abstract: We have used two approaches to search for sequence variants in the 18S coding region of amplified ribosomal DNA (rDNA) from Xenopus laevis oocytes. First, using clones derived from amplified rDNA, we compared the equivalent of a complete 18S coding region from two clones and short regions from two other clones with the 18S sequence previously determined from a "reference" clone. The respective sequences in all the clones were identical. Secondly, we examined greater than 60% of the 18S sequence in "pooled 18S genes" in uncloned amplified rDNA. The predominant sequence corresponded to that in the reference clone and no heterogeneities were apparent. Since many chromosomal rDNA units contribute to rDNA amplification the findings indicate that 18S coding sequences in X. laevis are largely homogeneous. The previously established sequence is the predominant one, thus providing a reliable basis for studies on 18S rRNA. Sequencing gels on uncloned amplified rDNA confirmed the absence of methylated cytosine in this DNA. The 18S sequence lacks major open reading frames.