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The chromosomes of three species of salamander belonging to the genus Plethodon have been studied with regard to the absolute length of the haploid set of lampbrush chromosomes and the number and distribution of chromomeres per haploid set of lampbrush chromosomes. Each aspect has been considered in relation to the substantial difference in genome size between P. cinereus (C=20 pg), P. vehiculum (C=36.8pg) and P.dunni (C=38.8pg). - Karyotype analyses carried out on unfixed preparations of lampbrush chromosomes showed that the absolute length of a haploid complement of lampbrush chromosomes from mediumsized yolky oocytes is much greater for P. vehiculum and P. dunni than for P. cinereus. Nonetheless, the relative dimensions of chromosomes are nearly identical in all three species. - Chromomeres were counted along the whole length of the 14th (shortest) bivalent, and the total number of chromomeres in the haploid set of lampbrush chromosomes was determined by extrapolation on the basis of the known relative length of the 14th bivalent in the respective species. Chromomeres were also counted in measured segments of undamaged lampbrush preparations in which all chromosomes could be clearly identified. The average distance between chromomeres (chromomere distribution) was determined and the total chromomere number was estimated on the basis of measured total length of lampbrush chromosomes in a preparation. Chromomere distribution is the same for lampbrush chromosomes from all 3 species, and since P. vehiculum and P. dunni have longer chromosomes than P. cinereus, it is clear that the former 2 species have many more chromomeres (60-70%) per haploid set of lampbrush chromosomes. The term chromomere was used here in the descriptive sense to define a discrete granule of deoxyribonucleoprotein on the axis of a lampbrush chromosome. - These findings are discussed in relation to what is known of the molecular organization of eukaryotic genomes, and in relation to ideas on gene action and transcription in lampbrush chromosomes. [ABSTRACT FROM AUTHOR]

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DNA extracted from Chironomus thummi larvae was studied by isopycnic centrifugation in CsCl, thermal denaturation and DNA-DNA reassociation techniques. The mean G + C content of the C. thummi DNA is 28-29 % as indicated both by centrifugation in CsCl and thermal denaturation. According to optical reassociation analysis of total DNA and of isolated DNA fractions the C. thumini genome is composed of at least four components. About 80 % of the DNA is classified as unique with a kinetic complexity of nearly 7 x 1010 daltons. 6-8 % inter- mediate DNA exhibits a kinetic complexity slightly above 108 daltons with a mean repetition frequency of 35. 11-13 % fast-reassociating DNA has a kinetic complexity slightly above 106 daltons with a mean repetition frequency of 6000. 3-5% of the DNA cannot be properly studied by the optical reassociation technique and probably contains inverted repeats. The thermal denaturation behaviour of isolated DNA fractions indicates that most of the repetitive sequences in the C. thummi genome are tightly interspersed. [ABSTRACT FROM AUTHOR]

DNAs from different chicken tissues were purified. After denaturation, the reassociation reaction of two different molecular weight DNAs was measured by the hydroxyapatite method. The results of these experiments indicated that, under the generally accepted standard conditions, the chicken genome is comprised of 20% repetitive DNA. Furthermore, it was found that a significant amount of the repetitive DNA was interdispersed among the unique DNA. It can be concluded from the kinetic data that there was not a significant difference either in the amount or in the distribution of the unique and repetitive sequences of the DNA samples tested. Isolation and further characterization of the repetitive sequences were carried out.

Cytogenetic analysis indicates that Balbiani Ring 2 (BR 2) in the two sibling species Chironomus tentans and Chironomus pallidivittatus arises from identifically banded segments in the salivary gland polytene chromosomes, although chromosomal rearrangements have occurred. In situ hybridization of BR 2 RNA to the polytene chromosomes of each individual species, as well as of their F1 hybrids, reveals that the repetitious BR 2 DNA in the two species has, within the limits of the technique, retained identity of nucleotide sequences and degree of repetition. The DNA of the naturally expressed BR 1 and BR 3 in both species and that of the galactose induced BR 6 in C. pallidivittatus did not hybridize with BR 2 RNA, indicating that these BR's are different from BR 2 with regard to sequence content. [ABSTRACT FROM AUTHOR]

Human meiotic prophase spermatocyte nuclei were studied by electron microscope autoradiography after a 3 hours H-uridine labeling pulse, followed by postincubation in non-radioactive medium. In autosomes, H-uridine nucleolar labeling reaches a peak during early-middle zygotene prior to the peak labeling of chromosomal RNA species at middle pachytene. Transcription activities of sex chromosomes are inconspicuous when compared with that of autosomes. An increasing condensation of nucleolar-associated chromatin in acrocentric bivalents contributes to the formation of basal knobs in human pachytene spermatocytes. Upon completion of knob formation, nucleolar components segregate and the uptake of H-uridine decreases. These findings suggest that the template capability of ribosomal DNA cistrons, located next to the basal knob region, is largely associated with a dispersed state of chromatin whereas increased chromatin condensation is correlated with a restriction of ribosomal RNA transcription. [ABSTRACT FROM AUTHOR]

Whole mount metaphase chromosomes, from cultured L cells, have been centrifuged onto grids and examined by electron microscopy. Compact and dispersed chromosome forms provide extensive ultrastructural information. Condensed chromosome arms appear as packed fibers with centromeric heterochromatin identifiable because it stains more intensely than the rest of the chromosome. Kinetochores are readily visible in these preparations. Under appropriate isolation conditions, it is possible to obtain mitotic spindles in which bundles of microtubules remain attached to kinetochores, suggesting that the kinetochores retain basic structural integrity throughout the isolation procedure. Dispersal of metaphase chromosomes by treatment with formalin and distilled water shows that these chromosomes are composed of a basic fiber that is normally highly condensed. This fiber is made up of regularly repeating 70-90 Å diameter nucleoprotein granules separated from neighboring granules by a 20-40 Å diameter fiber whose continuity is maintained by DNA. This structural arrangement is totally analagous to that reported for interphase chromatin from a variety of sources. [ABSTRACT FROM AUTHOR]

The method of spreading transcription complexes has been applied to amphibian oocytes of Pleurodeles genus. Complexes of nucleolar origin show a regular and homogeneous organization similar to that described in other materials. The observations add to the interpretation as an amplification of nucleolar DNA and a redundancy of ribosomal cistrons in the two species studied. -On the other hand, complexes of chromosomal origin display a great diversity. Two main characteristics can be drawn: the existence of several transcription units in a chromosomal organization unit and the possibility to point out a special architecture at the RNP fibril level. Applying a shadowing technique used for isolated molecules is an improvement compared with earlier methods based on PTA coloration. [ABSTRACT FROM AUTHOR]

Drosophila melanogaster DNA, together with λ and E. coli DNAs as controls, was digested with three different restriction endonucleases: EcoR, Hind, and Hae. The size distributions of the segments were characterized by gel electrophoresis. More than 85% of the D. melanogaster DNA was found in a broad distribution of segment lengths consistent with random location of restriction sites. However, some DNA was spared and recovered in very long (≥20500bp) segments. These segments proved to be mostly simple sequence DNA. No complex spared segments could be found in Hind and Hae digests, while 50% of the spared EcoR segments had a complexity exceeding that of the E. coli DNA spared by this enzyme. These data do not support the hypothesis that chromomeres contain long regions of purely tandemly repeating sequences. [ABSTRACT FROM AUTHOR]

The article discusses the significance of somatic cell genetic contributions to human gene mapping. It includes outlining of the methodological aspects of the subject and at the same time provides an up-to-date account of the human gene map, and pertinent genetic information based on family studies.

Twenty-two independently occurring spontaneous reciprocal translocations were isolated from monoploid X diploid crosses in maize and their breakpoints were determined. As 12 of the translocations involved the same two chromosomes and had breakpoints at approximately the same positions (6L. 2-3, 7L. 2-3) and two other translocations appeared to be identical with breakpoints at 2L. 9, 6L. 4, 14 of the 22 translocations probably arose by crossing over within duplicate segments of nonhomologous chromosomes. Thus, at least part of the bivalents seen at diakinesis and chromatid bridges seen at anaphase I in monoploid plants appear to be generated by recombination between redundant chromosome segments. The other eight translocations each occurred once. Because our evidence indicates that recombination between nonhomologous illegitimately synapsed chromosome segments does not occur in maize, these were probably also produced by recombination between redundant segments. If one assumes that their breakpoints also mark regions of interchromosomal redundancy, other conclusions can be reached: A) corn does not contain detectable homoeologous chromosomes, thus it is precently a true diploid, and B) as exchanges giving rise to translocations did not occur in the centromeres or proximal heterochromatin, these regions either do not possess redundancy or are rarely involved in chiasma formation. Furthermore, the duplicated segments in the genome giving rise to translocations in haploid microsporocytes probably have the same serial order with respect to the centromere. [ABSTRACT FROM AUTHOR]