The chromosomes of lower eukaryotes have short telomeric 3' extensions. Using a primer-extension/nick-translation technique and nondenaturing hybridization, we find long 3' G-rich tails at human chromosome ends in mortal primary fibroblasts, umbilical vein endothelial cells, and leukocytes, as well as in immortalized fibroblasts. For all cells tested, >80% of the telomeres have long G-rich overhangs, averaging 130-210 bases in length, in disagreement with the conventional model for incomplete lagging-strand replication, which predicts overhangs on 50% of the chromosome ends. The observed G tails must exist during most of the cell cycle and probably result from degradation of both chromosome ends. The average lengths of the G tails are quantitatively consistent with the observed rates of human chromosome shortening.

Rat liver interphase chromosomes have telomeres 20-100 kb in length. Micrococcal nuclease digestion of nuclei cleaves telomeres with a uniform 157 bp periodicity, producing soluble particles that sediment in sucrose gradients exactly like oligonucleosomes. The monomeric telomere particles comigrate with nucleosome core particles on nucleoprotein and DNA gels but do not bind H1. DNAase I cleaves telomere nucleoprotein into a series of bands spaced by about 10.4 bp and with the same intensity distribution as bands from bulk nucleosomes. Removal of H1 from chromatin alters the sedimentation properties of telomeres in parallel with bulk chromatin. Thus, telomeres of mammals are constructed of closely spaced nucleosomes, in contrast with the telomeres of lower eukaryotes, which show no evidence of nucleosomal structure.

Eukaryotic chromosomes terminate with telomeres, nucleoprotein structures that are essential for chromosome stability. Vertebrate nucleosomes. Gel electrophoresis of nucleoproteins indicated that telomere core particles did not bind histone Hi, yet sedimentation analysis showed that the mononucleosomes and oligonucleosomes of telomere and bulk chromatin cosediment at low ionic strength and are sensitive to removal of Hi. Several of these experiments have been replicated with human and mouse cell lines, giving the same results (13).Studies of the origin and nature of these telomere-specific nucleosomes might give insight into the general process of nucleosome assembly and into the roles of telomeres in chromosome stability and cellular senescence. In this paper we address the question of telomere DNA and nucleoprotein structure in organisms representing the vertebrate classes Mammalia, Reptilia, Aves, Amphibia, and Pisces, as well as the invertebrate class Echinodea. The results support the hypothesis that animal cells have highly conserved telomere DNA sequences of (TTAGGG), organized largely into short nucleosomes of variable length, usually "40 bp less than nucleosomes of bulk chromatin. In addition, the distinctness of the nucleosomal ladder appears to be correlated with the length of the telomere tracts, suggesting that short telomeres might be less homogeneous than long telomeres.Telomeres are functionally and structurally distinct structures at the ends of eukaryotic chromosomes that are essential for chromosome stability and also seem important for the expression of adjacent genes, spatial arrangement of chromosomes in nuclei, and initiation of chromosome pairing during meiosis (1, 2). In protozoa and fungi the telomere DNA tracts are very short (18-600 bp), contain a 3' G-rich single-stranded tail, and are bound to nonhistone proteins, in contrast to the rest of the genome, in which the DNA and histone proteins are organized into nucleosome arrays (3). The telomeres of animals and plants are substantially longer (2-100 kb) and less well characterized (4-7). The length of telomeres from human somatic cells is directly related to the mitotic history of the cells, with an average shortening of '100 bp per division (8, 9). As telomeres reach a critical length, chromosomes seem to become unstable (10). Only immortal cells from lower eukaryotes and germline and tumor cells from higher eukaryotes have telomeres of constant length, apparently stabilized by the enzyme telomerase, which is able to add telomere sequences to the 3' termini (11).We recently characterized the nucleoprotein structure of rat telomeres by nuclease and sedimentation analyses (12). Micrococcal nuclease (MNase) studies revealed very regular arrays of nucleosomes spaced by 157 bp on the telomeres, representing the shortest nucleosomes found in animals and plants. DNase I digestion patterns and electrophoretic mobilities of the telomere nucleosomes were identical to those of bulk chromatin, suggesting that the protein composition of...

The chromatin structure in solution has been studied by the flow linear dichroism method (LD) in a wide range of ionic strengths. It is found that increasing the ionic strength from 0.25 mM NaZEDTA, pH 7.0 to 100mM NaCl leads to a strong reduction of the LD amplitude of chromatin and inversion of the LD sign from negative to positive at 2 mM NaCl. Chromatin exhibits a positive LD maximum value at 10-20 mM NaC1. These data enable us to conclude that in very low ionic strength (0.25 mM Na2EDTA) the nucleosome discs are oriented with their flat faces more or less parallel to the chromatin filament axis. Increasing ionic strength up to 20mM NaCl leads to reorientation of the nucleosome discs and to formation of chromatin structures with nucleosome flat faces inclined to the fibril axis. A conformational transition of that kind is not revealed in H1 -depleted chromatin. The condensation of the chromatin filaments with increasing concentration of NaCl from 20 mM to 100 mM slightly influences the orientation of the nucleosomes.Chromatin structure and the nature of conformational transitions induced by variable parameters of the environment have recently been the subject of vigorous study in connection with the function of the cellular genetic apparatus of eukaryotes. Considerable progress in solving these problems by now is closely related to the application of modern physical methods, e.g. electron microscopy [l -61, X-ray diffraction [7-lo], neutron [ l l , 121 and light [13,14] scattering, flow linear dichroism [15-171, electric birefringence [18,19] and electric dichroism [18 -271. The latter two methods have been extensively used to study the structure of both isolated nucleosomes and high-molecular-weight chromatin fragments. These methods, however, suffer from two serious disadvantages : (a) limited application to high-ionic-strength solutions, due to the thermal and electrochemical processes in the Kerr's cell, and (b) the electric field used for orientation of the molecules probably induces structural distortions in the chromatin fibrils [15,27]. These limitations can be overcome by flow orientation of the chromatin fibrils [15,16]. This paper reports the results of flow linear dichroism measurements of calf thymus chromatin preparations in a wide range of NaCl concentrations. It is shown that at very low ionic strength (0.25 mM Na2EDTA) the nucleosome discs are oriented more or less parallel to the filament axis. Increasing the concentration of NaCl up to 20 mM caused a reorientation of the nucleosomes, resulting in a chromatin structure with nucleosomal discs inclined to the filament axis. This transition was not observed with histone HI-depleted chromatin. Further increase of the ionic strength up to 100 mM led to negligible changes in the orientation of the nucleosomes.

The sequencing of the human genome has led to the availability of an extensive mapped clone resource that is ideal for the construction of DNA microarrays. These genomic clone microarrays have largely been used for comparative genomic hybridisation studies of tumours to enable accurate measurement of copy number changes (array-CGH) at increased resolution. We have utilised these microarrays as the target for chromosome painting and reverse chromosome painting to provide a similar improvement in analysis resolution for these studies in a process we have termed array painting. In array painting, chromosomes are flow sorted, fluorescently labelled and hybridised to the microarray. The complete composition and the breakpoints of aberrant chromosomes can be analysed at high resolution in this way with a considerable reduction in time, effort and cytogenetic expertise required for conventional analysis using fluorescence in situ hybridisation. In a similar way, the resolution of cross-species chromosome painting can be improved and we present preliminary observations of the organisation of homologous DNA blocks between the white cheeked gibbon chromosome 14 and human chromosomes 2 and 17.