Isolated mammalian metaphase chromosomes. II. Fractionated chromosomes of mouse and Chinese hamster cells

Principles and functions of pericentromeric satellite DNA clustering into chromocenters

Simple non-coding tandem repeats known as satellite DNA are observed widely across eukaryotes. These repeats occupy vast regions at the centromere and pericentromere of chromosomes but their contribution to cellular function has remained incompletely understood. Here, we review the literature on pericentromeric satellite DNA and discuss its organization and functions across eukaryotic species. We specifically focus on chromocenters, DNA-dense nuclear foci that contain clustered pericentromeric satellite DNA repeats from multiple chromosomes. We first discuss chromocenter formation and the roles that epigenetic modifications, satellite DNA transcripts and sequence-specific satellite DNA-binding play in this process. We then review the newly emerging functions of chromocenters in genome encapsulation, the maintenance of cell fate and speciation. We specifically highlight how the rapid divergence of satellite DNA repeats impacts reproductive isolation between closely related species. Together, we underline the importance of this so-called 'junk DNA' in fundamental biological processes.

Measurement of transcribed human X-chromosomal DNA sequences transferred to rodent cells by chromosome-mediated gene transfer

Transfer of genetic information can be effected by incubation of cultured eucaryotic cells with isolated metaphase chromosomes. In most cases, a resulting transformed cell contains only a fragment of a donor chromosome. The amount of transferred donor DNA has been quantified in 11 independent mouse A9 transformants by nucleic acid hybridization analysis. Each transformant had been selected for hprt (hypoxanthine phosphoribosyltransferase; EC 2.4.2.8) transfer and contained part of the human X chromosome. A labeled probe of transcribed human X-chromosomal DNA was prepared by hybridization of nick-translated unique-sequence human DNA with whole cellular RNA from a human-mouse hybrid cell line, A9/HRBC2-A, containing a single human chromosome., X. The amount of human X-chromosomal DNA in the transformants was quantitated by comparing the hybridization of this probe with transformant and A9/HRBC2-A DNAs. Two unstable transformants which had a microscopically detectable donor chromosome fragment contained 15% of the human X-chromosomal single-copy DNA. Four other unstable transformants contained 4 to 7% of human X-chromosomal DNA sequences. The transferred DNA was below the level of detection in three other unstable and in all three stable transformants. We conclude that the initial transfer event can introduce a substantial amount of genetic information but only smaller amounts of DNA are stably incorporated by integration.

Preferential occurrence of sister chromatid exchanges at heterochromatin-euchromatin junctions in the wallaby and hamster chromosomes

Chromosomes of two mammalian species, the white-throated wallaby and the rat-like hamster, possessed large amounts of constitutive heterochromatin which is detectable as C bands. By making use of this character the frequency of sister chromatid exchanges (SCEs) was determined for the C band and the euchromatic regions of the chromosome. In both species, the distribution of SCEs in the euchromatin of chromosomes was found to be proportional to its metaphase length, while the number of SCEs localized in the C band regions was clearly fewer than expected on the basis of the relative length of those regions at metaphase. Many SCE's were, however, detected at the junctions between the euchromatin and the C band heterochromatin. All of these findings were consistent with previous observations on the Indian muntjac and the kangaroo rat chromosomes.

Genetic analysis by chromosome-mediated gene transfer

A general method is presented for stable transfer of genetic information to eukaryotic cells, utilizing metaphase chromosomes as the vehicle. Recent progress, current problems and large areas of uncertainty in this field are reviewed; particular consideration is given to frequency of transfer, size of the transgenome, evidence of cotransfer of linked genes and serial chromosome transfer. A reasonable model for chromosome transfer is considered with respect to the available information, and various descrepancies are noted. The utility of this method for fine structural mapping, cloning small regions of the eukaryotic genome and other potential applications are discussed.

Chromosome-mediated gene transfer between closely realted strains of cultured mouse cells

Gene transfer between two closely related mouse cell lines has been carried out, using as the vector a cell-free preparation of metaphase chromosomes and nuclei. Distinction between gene transferents and revertants of the recipient mutant phenotype was achieved by the use of a donor strain carrying a mutationally altered (8-azaguanine-resistant) hypoxanthine-guanine phosphoribosyltransferase (HPRTase; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8). The transferred HPRTase gene is initially unstable; in nonselective medium, it is lost at a rate of about 0.1 per cell per generation. Stabilization occurs as a rare event, with a frequency on the order of 1 X 10(-5) per cell per generation. The unstable state can be maintained for at least 200 generations through serial passages of the transferent in selective medium. Under the conditions of cultivation used in these experiments, the unstable HPRTase-positive cells are eventually replaced by the stable HPRTase-positive cells in the population.

Localization of mouse satellite DNA on chromosomes of experimentally induced glioblastomas; non-centromeric lable in one glioblastoma producing C-type particles

The chromosomal localization of satellite DNA in two tissue culture lines derived -rom malignant mouse CNS tumors was investigated by in situ hybridization of 3H single-stranded satellite DNA purified by isopynic centrifugation in alkaline CSC1. Both tumors were glioblastomas originally induced by a methylcholanthrene implantation into the cerebrum of C3H mice; both displayed aneuploid chromosomal constitutions. One of these glioblastomas (TC 541) revealed labelling only of centromeric portions of the chromosomes even in cells containing greater than 200 chromosomes and thus it had a pattern of satellite distribution comparable to that of normal cells. The other glioblastoma (TC 509), that produced C-type particles and had a decrease in satellite DNA, displayed interstitial and telomeric label in some chromosomes in addition to labelling of the centromeres. "Hoechst 33258" fluorescence showed some interstitial and telomeric bright bands as well as centromeric bright regions, though to be consistent with in situ studies. The localization of satellite DNA to the chromosome arms and its possible relation to C-type virus is discussed.

Factors influencing the yield of satellite DNA in extractions from Drosophila virilis and Drosophila melanogaster adults and embryos

The application of different DNA extraction methods to identical batches of Drosophila virilis and Drosophila melanogaster flies or embryos has revealed that the ionic strength of a homogenization medium is of critical importance if chloroform extractions are performed. The low yield of satellite DNA after homogenization in low salt buffers is less severe if EDTA is included in the buffer. Phenol extraction procedures result in no such differential behavior of satellite and main band DNA, but under certain circumstances a particular satellite fraction of Drosophila virilis DNA may be lost.

Percent satellite DNA as a function of tissue and age of mice

A selective loss of satellite DNA was found to occur to different extents as a function of tissue and age of mice using several common DNA extraction and purification procedures. This result emphasizes a serious problem that may be encountered in comparative studies of DNA structure and composition if selective loss of specific DNA sequences occurs. We have developed a DNA extraction and purification procedure that is simple and reliable and gives a high percent DNA yield, which substantially reduces the selective loss of heterochromatin DNA sequences. The method features a centrifugation step of a proteolytic digest of chromatin in 2.4 M CsCl. Percent DNA yield of 82-98% are routinely obtained with no apparent loss of satellite DNA sequences from different tissues or ages of mice. Utilizing this method, percent satellite DNA was found to remain essentially constant at 11 +/- 1% for spleen, kidney, and brain tissues obtained from mice of 10-780 days of age. However, for liver, percent satellite DNA remained at about 7-8% from 10 to 300 days of age and then increased to about 12-13% from 300 to 600 days of age. During this latter time interval (300-600 days), an increase of DNA per nucleus of about 3-fold occurred, due to the formation of tetra- and octaploid cell types. A steady loss in the total number of nuclei per gram of liver as a function of age was also found. These two opposing effects resulted in a nearly constant amount of DNA per gram and per organ for liver throughout the lifespan of the mouse.

Relatedness of mouse satellite deoxyribonucleic acid to deoxyribonucleic acid of various Mus species

Mouse (Mus musculus musculus) satellite DNA is able to reassociate with repeated DNA sequences of Mus caroli and Mus cervicolor, but low thermal stability of the products indicates significant differences between satellite and related DNAs of these two Mus species. There appear to be several satellite-related populations in M. caroli DNA, each of which forms hybrids of low thermal stability with repeated sequences of M. cervicolor DNA. DNAs from the subspecies Mus musculus molossinus and Mus musculus castaneus reassociate with mouse satellite to form hybrids of very high thermal stability, but the satellite content of M. m. musculus DNA is only about 60% that of M. m. musculus DNA. Reassociation of M. m. musculus nonrepeated DNA with M. m. molossinus DNA reveals no detectable differences between them; reassociation with M. caroli (or M. cervicolor) DNA yields a product whose melting temperature depression relative to homologous DNA is about 5 degrees .

Satellite DNA

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Transfer of genetic information by purified metaphase chromosomes

Transfer of genetic information from isolated mammalian chromosomes to recipient cells has been demonstrated. Metaphase chromosomes isolated from Chinese hamster fibroblasts were incubated with mouse A(9) cells containing a mutation at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus. Cells were plated in a selective medium, resulting in death of all unaltered parental A(9) cells. However, colonies of cells containing hypoxanthine phosphoribosyl transferase (EC 2.4.2.8) appeared with a variable frequency of about 10(-6) to 10(-7). The enzyme from these cells was indistinguishable from that from Chinese hamster cells, as shown by DEAE-cellulose chromatography and gel electrophoresis, and differed clearly from the mouse enzyme. The colonies, thus, did not result from reversion of A(9) parental cells to wild type, but appeared to represent progeny of individual cells that had ingested chromosomes, replicated, and expressed the hprt gene. These colonies differed from each other in stability of expression of the transferred gene.

Isolation and chromosomal localization of highly repeated DNA sequences in Drosophila melanogaster

The nuclear DNA of D. melanogaster contains DNA sequences that are repeated between ten and a hundred times more often than the next class of redundant DNA. This DNA, as a renatured duplex isolated on the basis of its renaturation kinetics, has a buoyant density of 1.691 (g/ml). In its native state it bands within the unique nuclear DNA peak (rho = 1.701). These sequences have been localized by "in situ" hybridization in the chromocenter of the chromosomes of the salivary gland. The properties of centromeres are discussed in terms of the occurrence of repeated sequences at this locus.