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.

Theoretical and experimental studies indicate that, with a high-resolution scanning electron microscope, it is now possible to obtain pictures of a single heavy atom resting on a thin carbon substrate.

Four classes of models have been proposed for the internal structure of eukaryotic chromosome fibers--the solenoid, twisted-ribbon, crossed-linker, and superbead models. We have collected electron image and x-ray scattering data from nuclei, and isolated chromatin fibers of seven different tissues to distinguish between these models. The fiber diameters are related to the linker lengths by the equation: D(N) = 19.3 + 0.23 N, where D(N) is the external diameter (nm) and N is the linker length (base pairs). The number of nucleosomes per unit length of the fibers is also related to linker length. Detailed studies were done on the highly regular chromatin from erythrocytes of Necturus (mud puppy) and sperm of Thyone (sea cucumber). Necturus chromatin fibers (N = 48 bp) have diameters of 31 nm and have 7.5 +/- 1 nucleosomes per 10 nm along the axis. Thyone chromatin fibers (N = 87 bp) have diameters of 39 nm and have 12 +/- 2 nucleosomes per 10 nm along the axis. Fourier transforms of electron micrographs of Necturus fibers showed left-handed helical symmetry with a pitch of 25.8 +/- 0.8 nm and pitch angle of 32 +/- 3 degrees, consistent with a double helix. Comparable conclusions were drawn from the Thyone data. The data do not support the solenoid, twisted-ribbon, or supranucleosomal particle models. The data do support two crossed-linker models having left-handed double-helical symmetry and conserved nucleosome interactions.

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...