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  • 1
    Electronic Resource
    Electronic Resource
    Rate of Fixation of Nucleotide Substitutions in Evolution (1969)
    [s.l.] : Nature Publishing Group
    Nature 224 (1969), S. 149-154 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Nucleotide sequences of rodents and of artiodactyls have been compared by DNA/DNA duplex formation. The rate of evolution in terms of fixation of nucleotide substitutions inferred from non-repeated artiodactyl DNAs is three times greater than that inferred from comparative amino-acid sequence of ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/224149a0
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  • 2
    Electronic Resource
    Electronic Resource
    A new regulatory pathway for fragile X syndrome? (2002)
    [s.l.] : Nature Publishing Group
    Nature medicine 8 (2002), S. 1204-1205 
    Details
    ISSN: 1546-170X
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Medicine
    Notes: [Auszug] Research on human fragile X syndrome continues to elicit exciting but humbling discoveries. Once studied primarily because of its major effect on cognition—an IQ score reduction in males by an average of 50 points—and its highly unusual genetics, fragile X syndrome has informed us of ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/nm1102-1204
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  • 3
    Electronic Resource
    Electronic Resource
    Three euchromatic DNA sequences under-replicated in polytene chromosomes of Drosophila are localized in constrictions and ectopic fibers (1987)
    Springer
    Chromosoma 95 (1987), S. 227-235 
    Details
    ISSN: 1432-0886
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract We examined three regions of under-represented euchromatic DNA sequences (histone, Ubx, and 11 A), for their possible correlation with euchromatic constrictions in polytene chromosomes of Drosophila melanogaster. Cloned sequences were hybridized to filters and to chromosomes prepared for light microscopy. Under-represented sequences hybridized to DNA within constrictions and in ectopic fibers. In contrast, adjacent sequences that were fully endoreplicated in the Ubx and 11A regions in polytene cells hybridized to sites just adjacent to their respective constrictions. For one region (Ubx), sequences under-represented in salivary gland cells were fully endoreplicated in fat body cells. For this particular region, the morphology of the polytene chromosomes differs between these two cell types in that the specific constriction is absent at this region in fat body polytene chromosomes, thus strengthening the correlation between under-representation and chromosome constrictions. Although all three sequences are in regions that have been classified by others as “intercalary heterochromatin,” we detect no common functional or sequence organizational feature for these examples of under-represented DNA. We suggest that the lower efficiencies of the replication origins, or special regions of termination at these sites, are the primary cause of the under-replication, and that this under-replication is sufficient to confer the properties of intercalary heterochromatin.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00294779
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  • 4
    Electronic Resource
    Electronic Resource
    Chromatid structure: relationship between DNA content and nucleotide sequence diversity (1971)
    Springer
    Chromosoma 32 (1971), S. 378-406 
    Details
    ISSN: 1432-0886
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract Models of chromatid structure are based on inferences made from genetic, cytological, and cytochemical observations. An alternative approach can provide limits as to the number of identical subunits present in chromatids. This method is based on the demonstration that nucleotide sequence diversity may be estimated from the kinetics of renaturation of denatured DNA. Measurements of DNA content and renaturation rate constants are given for several eukaryotic DNAs. Control experiments involved measurements of renaturation kinetics of DNAs from bacteria and bacteriophage. These estimates show that most of the nucleotide sequences in mouse, Drosophila, and Ciona DNA are present only once per sperm. Since the reduction of DNA content during meiosis indicates that mouse sperm contain a haploid set of chromatids, it follows that a set of mouse meiotic chromatids contains a single copy of most sequences. Models of chromatid structure which postulate multiple subunits with identical nucleotide sequences are therefore not tenable for mouse meiotic chromatids. This method of analyzing nucleotide sequence diversity may be of general use in designing models of chromatid structure in other organisms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00285251
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  • 5
    Electronic Resource
    Electronic Resource
    Relationship between relative dry mass and average band width in regions of polytene chromosomes of Drosophila (1980)
    Springer
    Chromosoma 76 (1980), S. 175-189 
    Details
    ISSN: 1432-0886
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract Whole-mounted polytene chromosomes from Drosophila melanogaster were prepared for high-voltage electron microscopy. Relative dry mass of chromosome regions was estimated by densitometry of electron microscopic negatives. Comparison of dry mass of regions of the male X chromosome with that of regions of associated autosomes established that dry mass values are proportional to DNA content. Relative dry mass values of regions of polytene chromosomes from salivary glands, fat body, and malpighian tubules were correlated with the average diameter of bands in these regions: as mass doubled, band width increased by a factor of approximately √2. To provide a standard for estimating absolute levels of polyteny, band widths were measured for chromosomes representing one major polytene class, 256n. These chromosomes were observed to have an average band width of 0.9 μm — These observations provide limits to models of chromatin organization in bands. For each chromatid, this area can accommodate up to five chromatin fibers of 250 Å diameter. This value may represent the extent of folding of a chromatin fiber in an average band. Alternatively, a chromatin fiber of higher-order structure could have a maximum diameter of 560 Å in an average band.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00293416
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  • 6
    Electronic Resource
    Electronic Resource
    Sequence analysis of long FMR1 arrays in the Japanese population: insights into the generation of long (CGG) n tracts (1997)
    Springer
    Human genetics 〈Berlin〉 101 (1997), S. 214-218 
    Details
    ISSN: 1432-1203
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract The human fragile-X syndrome is associated with expansions of a (CGG) n triplet repeat within the FMR1 gene. Whilst normal FMR1 arrays consist of variable numbers of (CGG)7–13 blocks punctuated with single AGG triplets, unstable arrays contain longer blocks of uninterrupted (CGG) n . The degree of instability, and subsequent risk of expansion to the fragile-X mutation, is dependent upon the length of this uninterrupted repeat. Detailed analyses of normal FMR1 array structures suggest that longer uninterrupted blocks of repeat could arise either through a process of gradual slippage or a more dramatic loss of an intervening AGG triplet. Up to 15% of Japanese and Chinese individuals have FMR1 triplet arrays centred on 36 repeats in length, a modal group not found in Caucasians. As longer FMR1 arrays have been associated with high-risk fragile-X haplotypes in some populations, we investigated the nature of these larger arrays. Sequence analysis revealed that the unusual length is due to the presence of a novel (CGG)6 block within the array. Several haplotypically related arrays contain blocks of (CGG)16 or (CGG)15, consistent with the fusion of adjacent (CGG)9 and (CGG)6 blocks after loss of the intervening AGG triplet. This is compatible with inferences from the Caucasian population that AGG loss is a mechanism by which long blocks of identical repeats are generated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004390050618
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  • 7
    Electronic Resource
    Electronic Resource
    Estimating the stability of the proposed imprinted state of the fragile-X mutation when transmitted by females (1992)
    Springer
    Human genetics 〈Berlin〉 88 (1992), S. 335-343 
    Details
    ISSN: 1432-1203
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary Fragile-X syndrome is a major cause of mental retardation in humans. The X-inactivation imprinting model accounts for the unusual pattern of inheritance and expression of this syndrome. According to this model, the fragile-X mutation creates a local block to the attempted reactivation of the mutant X chromosome prior to oogenesis. This local block results in an “imprinted” fragile-X chromosome that is deleterious in males and in females for whom this chromosome is predominantly the active X chromosome. The imprinted state of the fragile-X mutation is inferred to be stable when transmitted by an imprinted female because the penetrance of the syndrome in sons of affected females is estimated to be 1.0. To provide a more precise estimate of the stability of the proposed fragile-X imprint, we have analyzed published pedigrees that include restriction fragment length polymorphism and cytogenetic data from sibships with mothers who are interpreted as having an imprinted fragile-X allele. We conclude that the fragile-X imprint was stable in 46 out of 48 female meioses. This analysis leads to a preliminary estimate of about 96% for the stability of the imprint through female meiosis. Two imprinted females had progeny who appeared to be carriers of a nonimprinted fragile-X allele. If this interpretation is correct, then reversion from the imprinted to the nonimprinted state, or “erasure,” can occasionally occur when the mutant fragile-X allele is transmitted by an imprinted female. We discuss the genetic and epigenetic significance of possible female erasure. We request DNA and cytogenetic information from unpublished pedigrees to quantify further the stability, during female meiosis, of the proposed imprinted state of the mutant fragile-X allele.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00197270
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  • 8
    Electronic Resource
    Electronic Resource
    Analysis of chromatin-associated fiber arrays (1976)
    Springer
    Chromosoma 58 (1976), S. 169-190 
    Details
    ISSN: 1432-0886
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract Electron microscopic examination of chromatin from embryonic nuclei ofOncopeltus fasciatus andDrosophila melanogaster reveals arrays of chromatin associated fibers. The lengths and spacings of these fibers were analyzed to provide a basis for defining and interpreting regions of transcriptionally active chromatin. The results of the analysis are consistent with the interpretation of some fibers as nascent RNA with associated protein (RNP). The chromatin segments underlying these fiber arrays were classified as ribosomal or non-ribosomal transcription units according to definitions and criteria described by Foe et al. (1976). — Nascent fibers on active ribosomal transcription units were analyzed and compared forDrosophila melanogaster, Triturus viridescens, andOncopeltus fasciatus. A common feature of the fiber patterns on ribosomal TUs is that origin-distal fibers exhibit greater length variability and a lower slope relative to proximal fibers. The region of increased variability in fiber lengths is correlated with the expected location of 28S ribosomal RNA sequences in the distal half of each ribosomal transcription unit. Because 28S ribosomal RNA appears to contain more extensive regions of base sequence complementarity, we suggest that the length of ribosomal RNP fibers is influenced under our spreading conditions by the secondary structure of the nascent RNA. — In order to calculate the RNA content of RNP fibers, chromatin morphology was used to estimate lengths of transcribed DNA. The packing ratio of DNA in chromatin, which we express as the length of B-structure DNA ÷ length of chromatin, is 1.1.–1.2. and 1.6 for the DNA in active ribosomal and non-ribosomal chromatins, respectively. These DNA packing ratios are used to determine the extent to which nascent RNP fibers are shorter than the transcribed DNA (expressed as DNA/RNP length ratio). For non-ribosomal transcription units and for proximal fibers of ribosomal transcription units, DNA/RNP length ratios are relatively constant within each array. However, considerable variability in this ratio (4–23) is observed for different arrays of fibers. Possible sources of this variability are considered by comparing ratios derived from the presumably identical ribosomal transcription units. — Further analysis of the morphology of nascent fibers may elucidate the contributions of proteins and successive RNA sequences to RNP structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00701357
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  • 9
    Electronic Resource
    Electronic Resource
    Morphology of transcription units inDrosophila melanogaster (1976)
    Springer
    Chromosoma 58 (1976), S. 193-218 
    Details
    ISSN: 1432-0886
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract We have used an electron microscopic analysis to define and to characterize active transcription units ofDrosophila melanogaster. The lengths and spacings of nascent ribonuclear protein (RNP) fibers were determined on embryonic chromatin that was spread using techniques introduced by Miller and Beatty (1969). The data are consistent with the occurrence of specific sites of transcription initiation and termination. We apply the termtranscription unit (TU) to a chromatin region bounded by these control sites. Two classes of TUs are active inDrosophila melanogaster embryonic cells—those synthesizing ribosomal RNA and those synthesizing non-ribosomal RNA. The classes can usually be distinguished on the basis of TU size, chromatin morphology and inferred DNA packing ratio, frequency of RNP fibers (number of fibers per μm of chromatin), and the solitary vs. tandem repeat occurrence of fiber arrays. The results indicate that nonribosomal transcription units have lengths in accord with the expectation that DNA of each chromomere is transcribed as a unit.—Some nascent fiber arrays inD. melanogaster have more complex patterns of RNP fiber lengths. We suggest that these are a consequence of cleavage of RNP fibers at specific sites during transcription. — These sites of transcriptional control and the amounts of DNA between them provide a basis for further relating units of transcription to units of gene function.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00701359
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  • 10
    Electronic Resource
    Electronic Resource
    Dispersity of repeat DNA sequences in Oncopeltus fasciatus, an organism with diffuse centromeres (1973)
    Springer
    Chromosoma 43 (1973), S. 349-373 
    Details
    ISSN: 1432-0886
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract DNA of Oncopeltus fasciatus, an organism with diffuse centromeres, has been characterized by determination of its base composition, buoyant density, thermal stability, and reassociation kinetics; renatured DNA was characterized similarly. We conclude that repeated sequences are primarily short and scattered throughout the genome. This is in contrast to the extensive tandem repeats which are found in DNAs of organisms with discrete centromeres.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00406743
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