Heterochromatin, Satellite DNA, and Cell Function

Abstract

With the assumption that a portion that comprises some 10 percent of the genomes in higher organisms cannot be without a raison d'être, an extensive review led us to conclude that a certain amount of constitutive heterochromatin is essential in multicellular organisms at two levels of organization, chromosomal and nuclear. At the chromosomal level, constitutive heterochromatin is present around vital areas within the chromosomes. Around the centromeres, for example, heterochromatin is believed to confer protection and strength to the centromeric chromatin. Around secondary constrictions, heterochromatic blocks may ensure against evolutionary change of ribosomal cistrons by decreasing the frequency of crossing-over in these cistrons in meiosis and absorbing the effects of mutagenic agents. During meiosis heterochromatin may aid in the initial alignment of chromosomes prior to synapsis and may facilitate speciation by allowing chromosomal rearrangement and providing, through the species specificity of its DNA, barriers against cross-fertilization. At the nuclear level of organization, constitutive heterochromatin may help maintain the proper spatial relationships necessary for the efficient operation of the cell through the stages of mitosis and meiosis. In the unicellular procaryotes, the presence of a small amount of genetic information in one chromosome obviates the need for constitutive heterochromatin and a nuclear membrane. At higher levels of organization, with an increase in the size of the genome and with evolution of cellular and sexual differentiation, the need for compartmentalization and structural components in the nucleus became imminent. The portion of the genome that was concerned with synthesis of ribosomal RNA was enlarged and localized in specific chromosomes, and the centromere became part of each chromosome when the mitotic spindle was developed in evolution. Concomitant with these changes in the genome, repetitive sequences in the form of constitutive heterochromatin appeared, probably as a result of large-scale duplication. The repetitive DNA's were kept through natural selection because of their importance in preserving these vital regions and in maintaining the structural and functional integrity of the nucleus. The association of satellite (or highly repetitive) DNA with constitutive heterochromatin is understandable, since it stresses the importance of the structural rather than transcriptional roles of these entities. Nuclear satellite DNA's have one property in common despite their species specificity, namely heterochromatization. In this sense the apparent species specificity of satellite DNA may be the result of natural selection for duplicated short polynucleotide segments that are nontranscriptional and can be utilized in specific structural roles.

Comparative Analysis (Hippotragini versus Caprini, Bovidae) of X-Chromosome's Constitutive Heterochromatin by in situ Restriction Endonuclease Digestion: X-Chromosome Constitutive Heterochromatin Evolution

“…This observation leads to the assumption that the CH heterogeneity found between the Hippotragini and Caprini X-chromosomes is ancient, or it is a consequence of a rapid turnover of repetitive DNA families, that deserve further investigations at the molecular level. Nevertheless, and for both suppositions, it seems reasonable that the presence of CH facilitates the occurrence of chromosome rearrangements, as it is in accordance with several authors (Yunis & Yasmineh, 1971;Peacock, Dennis & Gerlach, 1982;John, 1988).…”

Cytogenetic analysis of the neotropical spider Nephilengys cruentata (Araneomorphae, tetragnathidae): standard staining, NORs, C-bands and base-specific fluorochromes

“…In N. cruentata, the NORs in the pair 1 seem to be intercalated with constitutive heterochromatin, which could have the function of protecting the NORs against mutations and changes, avoiding alterations in the cistrons of rDNA, as proposed by Yunis & Yasmineh (1971), Hsu (1975. NORs adjacent to C bands have also been observed in some insects (Palomeque et al, 1988;Rodríguez-Iñigo et al, 1992;Lorite et al, 1997;Souza et al, 1998).…”

Chromosomal heterochromatin differentiation in North American cyprinid fishes.

“…The function(s) of constitutive heterochromatin are essentially unknown although several hypotheses have been forwarded, some of which are supported by empirical data (John and Miklos 1979). Because many if not most species appear to differ trenchantly in C-band amount and/or chromosomal distribution, one long held hypothesis is that heterochromatin may function as a chromosomal reproductive isolation mechanism and hence in part underlie new species formation (Yunis and Yasmineh 1971, Hatch et al 1976, Fry and Salser 1977.…”