Chromosome in eukaryotes

Eukaryotic cells are more complex than prokaryotic cells and usually contain more DNA, which is partitioned between several chromosomes. In both prokaryotes and eukaryotes, almost all cells of the same organism contain the same number of chromosomes. In eukaryotes most of the chromosomes are localized in the nucleus. Thus the DNA is isolated from the main body of the cytoplasm—a unique feature of eukaryotes and the primary distinction between prokaryotes and eukaryotes. Some organelles, notably the mitochondria and the chloroplasts, contain a single circular chromosome. 

From the complementary duplex structure of DNA described in chapter 25, it is a short intuitive hop to a model for replication that satisfies the requirement for one round of DNA duplication for every cell division. In chapter 26, DNA Replication, Repair, and Recombination, key experiments demonstrating the semiconservative mode of replication in vivo are presented. This is followed by a detailed examination of the enzymology of replication, first for how it occurs in bacteria and then for how it occurs in animal cells. Also included in this chapter are select aspects of the metabolism of DNA repair and recombination. The novel process of DNA synthesis using RNA-directed DNA polymerases is also considered. First discovered as part of the mechanisms for the replication of nucleic acids in certain RNA viruses, this mode of DNA synthesis is now recognized as occurring in the cell for certain movable genetic segments and as the means whereby the ends of linear chromosomes in eukaryotes are synthesized. 

Telomerase is an enzyme that has a specific role in replicating and maintaining the integrity of the ends of linear chromosomes in eukaryotes. Such ends are absent in DNA from E. coli. 

The complete gene complement of any organism, contained in a set of chromosomes in eukaryotes, a single chromosome in bacteria, or a DNA or RNA molecule in viruses the complete set of genes inside the cell or... 

Tubulins arose very early during the course of evolution of unicellular eukaryotes and provide the machinery for the equipartitioning of chromosomes in mitosis, cell locomotion, and the maintenance of cell shape. The primordial genes that coded for tubulins likely were few in number. As metazoan evolution progressed, natural selection processes conserved multiple and mutant tubulin genes in response to the requirements for differentiated cell types (Sullivan, 1988). 

No results indicating genotoxicity were observed in in vitro studies that examined six organophosphate ester hydraulic fluids for gene mutation, deoxyribonucleic acid (DNA) damage, or chromosomal aberrations in eukaryotes (see summarized data in Table 2-11). 

The structure and sequence of the catalytic moiety have been determined (06, V6, W6). The enzyme consists of 362 amino acids and 40,638 daltons of the protein predicted by the cDNA sequence. The ADA gene spans 32 kb and consists of 12 exons. The apparent promoter region of the gene lacks the TATA and CAAT sequences often found in eukaryotic promoters and is extremely G/C rich. The location of the ADA gene is on chromosome 20ql2-ql3.11 (Jl). 

Abstract. In eukaryotic cells, replicated DNA molecules remain physically connected from their synthesis in S phase until they are separated during anaphase. This phenomenon, called sister chromatid cohesion, is essential for the temporal separation of DNA replication and mitosis and for the equal separation of the duplicated genome. Recent work has identified a number of chromosomal proteins required for cohesion. In this review we discuss how these proteins may connect sister chromatids and how they are removed from chromosomes to allow sister chromatid separation at the onset of anaphase. 

Yoshimura SH, Kim J, Takeyasu K (2003) On-substrate lysis treatment combined with scanning probe microscopy revealed chromosome structures in eukaryotes and prokaryotes. J Electron Microsc (Tokyo) 52 415-423... 

Telomeres are r etitive sequences at the ends of linear DNA molecules in eukaryotic chromosomes. With each round of replication in most normal cells, the telomeres are shortened because DNA polymerase cannot complete synthesis of the 5 end of each strand. This contributes to the aging of cells, because eventually the telomeres become so short that the chromosomes cannot function properly and the cells die. 

Hydroquinone induces alterations of the DNA in eukaryotic cells (micronuclei, chromosomal aberrations and disintegrations) but is nonmutagenic in Salmonella tester strains with or without metabolic activation/... 

Electron microscopy shows that DNA consists of either linear or circular structures. The chromosomal DNA in bacteria is a closed circle, a result of covalent joining of the two ends of the double helix (Figure 10.11). Note the presence of supercoils, branch points, intersections, and the generally thin and open structure. The chromosomal DNA in eukaryotic cells, like ours, is believed to be linear. 

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