Scanning electron microscopy of giemsa-stained chromosomes and surface-spread chromosomes

Giemsa-stained chromosomes as prepared for light microscopy, and including G-banded, C-banded, and FPG-stained chromosomes, were examined by scanning electron microscopy. Although suitable for light microscopy, these chromosomes were too flat for a close examination of their fine structure by scanning electron microscopy. The surface of Giemsa-positive regions was rough and bright, whereas that of unstained or poorly stained regions was smoother and less bright. Giemsa-staining, therefore, seems to produce the bulkiness of the chromosomes. On topographical examination by scanning electron microscopy, the transparent chromosomes as observed with the light microscope proved to be footprints. Stereographical examinations of surface-spread chromosomes showed that minimally stretched chromosomes were composed of a mass of nodular and twisted looping fibers with an average diameter of about 300 Å. The substructure of these chromosome fibers was not determined. The kinetochore region was discernible as a constriction in the mass of the chromosome fibers, and was distinguishable from gaps by the presence of several chromosome fibers parallel to the axis of the chromatid. The organization of the chromosome fibers, however, was disordered rather than regular.     

Structure of chromosomes

The number of gyres, mode, and direction of coils, centromere, secondary constriction, and telomere were studied in the human somatic chromosomes in terms of spiral structure, employing the newly developed technique involving a hypotonic treatment. Results obtained are: 1. the number of gyres is most probably constant in analogous chromosome from different cells; 2. the direction of coils is random; 3. each ehromatid is accepted as a unit of chromonema coiling; 4. the centromere seems to be a regionally elongated portion of the chromonema, without showing any differentiated structure; 5. chromonemata at secondary constrictions usually appear as uncoiled or less tightly coiled entities, and 6. two different types of the telomere structure were observed, one being a pointed-end type, while the other is a rounded-end type (Summary see p. 424). Supported by a grant from the National Institute of General Medical Sciences, U.S.P.H.S. (GM 14185).     

Dynamic chromosomes

A report on the sixth biannual FASEB conference on Yeast Chromosome Structure, Replication and Segregation held in Snowmass, Colorado, 19-24 August, 2000.     

Isoacentric and isocentric chromosomes originating after deletions of human chromosomes

Summary The analysis of a sample of 100 isoacentric (IA) and isocentric (IC) chromosomes, which had originated from spontaneous or radiation-induced deletions in human lymphocytes, is reported. IC and also IA have a strong tendency to be formed after breakage in juxtacentromeric heterochromatin. When euchromatic regions are involved, the breaks are not distributed at random since they frequently occur at places where juxtacentromeric heterochromatin exists in other primate species. It is assumed that intercalary structures conserving some of the properties of heterochromatin exists in human chromosomes in intercalary positions.     

Isolated chromosomes

Nucleoprotein threads isolated from mammalian tissues are interphase chromosomes rather than random fragments of stretched nuclei. Structures similar to the isolated threads are visible in intact nuclei. The isolated threads are often clearly microscopically double, consist of more or less tightly coiled chromonemata and show individuality in longitudinal differentiation. A nucleolus organizing chromosome with trabant and secondary constriction carrying the nucleolus can be recognized and is of identical structure in preparations from beef liver and pancreas.     

Protein-depleted chromosomes

Protein-depleted isolated Chinese hamster chromosomes have been obtained by different protein extraction procedures and examined by electron microscopy and SDS-polyacrylamide gel electrophoresis. Salt-resistant centromeric and telomeric structures are visible in protein-depleted chromosomes and the protein-depleted chromosomes appear to have a regular, longitudinal pattern in critical point dried preparations. The scaffold-like structure of protein-depleted chromosomes is highly affected by the ionic strength and composition of the extraction medium and by the spreading conditions. Nucleosomal histones of isolated chromosomes proved to be more sensitive to the sodium chloride treatment than histones of isolated chromatin. A small, but constant quantity of core histones was detected in 2 M salt extracted chromosomes and H3 and H4 histones of isolated chromosomes appeared to be resistant to the sodium deoxycholate treatment.     

Fungal chromosomes

There are now four well-established methods to examine the chromosomes of filamentous fungi: mapping genes to linkage groups by recombination analyses, light-microscopic observation of chromosomes in meiotic divisions, electron-microscopic observation of the synaptonemal complexes between homologous chromosomes in prophase of meiosis, and separation of chromosomes as individual bands by pulsed field gel electrophoresis. These techniques and their contributions are described in brief with special reference toNeurospora. A fifth technique will be used more and more in characterizing chromosomes at the molecular level as DNA sequencing is completed for a limited number of the fungi. However, only the molecular studies of chromosome structures as they relate to centromeres, telomeres or nucleolus organizer regions are discussed, as is the potential usefulness of DNA sequencing to identify the junctions of chromosome rearrangements.     

Cytogenetic relationship ofAegilops longissima chromosomes with common wheat chromosomes

The relationships of three wheat-Aegilops longissima chromosome addition lines A, C, and D with homoeologous wheat chromosomes were studied in PMC meiosis. Substitutions of alien chromosome A for wheat chromosome 6 B, chromosome C for 1 B and chromosome D for 4 B were obtained. The production of 4 BS/C and 7 BS/D chromosome translocations indicated cytogenetic relationships of C partially to homoeologous wheat chromosomes of group 1 and 4, and D partially to homoeologous wheat chromosomes of group 4 and 7.     

Association of centrioles and chromosomes observed in preparations of whole-mounted human chromosomes

The replication of extrachromosomal rDNA molecules from the macronucleus of Tetrahymena was studied by electron microscopy. Replication begins in the center of the palindromic molecule and proceeds by means of bidirectional fork movement toward the free ends of the molecule.