Chromosomes eukaryotes

The native form of chromatin in cells assumes a higher order stmcture called the 30-nm filament, which adopts a solenoidal stmcture where the 10-nm filament is arranged in a left-handed cod (Fig. 5). The negative supercoiling of the DNA is manifested by writhing the hehcal axis around the nucleosomes. Chromatin stmcture is an example of toroidal winding whereas eukaryotic chromosomes are linear, the chromatin stmctures, attached to a nuclear matrix, define separate closed-circular topological domains. 

Bacterial as well as eukaryotic chromosomes contain too much DNA to fit easily into a cell. Therefore, the DNA must be condensed (compacted) to fit into the cell or nucleus. This is accomplished by supercoiling the DNA into a highly condensed form. When relaxed circular DNA is twisted in the direction that the helix turns, the DNA becomes positively supercoiled, if it is twisted in the opposite direction, it is called negatively supercoiled. Bacterial DNA is normally found in a negatively supercoiled state. Supercoiling reactions are catalyzed by topoisomerases. 

Kornberg RD, Lorch Y (1999). Twenty-five years of the nucleosome, fundamental particle of the eukaryote chromosome. Cell 98 285-294. 

Eukaryotic chromosomes, unlike their bacterial counterparts, are linear rather than- circular. Since RNA oligonucleotides prime both prokaryotic and eukaryotic DNA synthesis, the 5 termini of the daughter... 

Sobti RC, Obe G. Eukaryotic Chromosomes Structural and Functional Aspects, Springer-Verlag, New York, 1991. 

Eukaryotes, defined, 3 757t Eukaryotic cell biology, study of, 26 446 Eukaryotic chromosomes, 17 610 Euler equations, 11 742 Eulerian model, 11 822 Euler integration method, 20 688 Euler number, 11 745 23 190 Euphococcinine, 2 73 Euphorbiaceae, alkaloids in, 2 75 Eurasian Patent Convention, 18 198 Europe... 

Kim MS, Blake M, Baek JH, Kohlhagen G, Pommier Y, Carrier F (2003) Inhibition of histone deacetylase increases cytotoxicity to anticancer drugs targeting DNA. Cancer Res 63(21) 7291—7300 Komberg RD and Lorch Y (1999) Twenty-Five years of die nucleosome, fundamental particle of die eukaryote chromosome. Cell 98(3) 285—294... 

Further condensation occurs to eventually form the chromosome. Each eukaryotic chromosome contains one linear molecule of DNA. 

Each eukaryotic chromosome contains one linear molecule of DNA having multiple origins of replication. Bidirectional replication occurs by means of a pair of replication forks produced at each origin. Completion of the process results in the production of two identical linear molecules of DNA. DNA replication occurs in the nucleus during the S phase of the eukaryotic cell cycle. The two identical sister chromatids are separated om each other when the ceU divides during mitosis. 

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. 

Daban, J.R. (2000) Physical constraints in the condensation of eukaryotic chromosomes. Local concentration of DNA versus linear packing ratio in higher order chromatin structures. Biochemistry 39, 3861-3866. 

Houchmandzadeh, B., Marko, J.F., Chatenay, D., and Libchaber, A. (1997) Elasticity and structure of eukaryote chromosomes studied by micromanipulation and micropipette aspiration. J. Cell. Biol. 139, 1-12. 

Research work with large genomes and the associated need for high-capacity cloning vectors led to the development of yeast artificial chromosomes (YACS Fig. 9-8). YAC vectors contain all the elements needed to maintain a eukaryotic chromosome in the yeast nucleus a yeast origin of replication, two selectable markers, and specialized sequences (derived from the telomeres and centromere, regions of the chromosome discussed in Chapter 24) needed for stability and... 

FIGURE 24-5 Eukaryotic chromosomes, (a) A pair of linked and condensed sister chromatids from a human chromosome. Eukaryotic chromosomes are in this state after replication and at metaphase during mitosis, (b) A complete set of chromosomes from a leukocyte from one of the authors. There are 46 chromosomes in every normal human somatic cell. 

Telomeres (Greek telos, end ) are sequences at the ends of eukaryotic chromosomes that help stabilize the chromosome. The best-characterized telomeres are those of the simpler eukaryotes. Yeast telomeres end with about 100 bp of imprecisely repeated sequences of the form... 

Artificial chromosomes (Chapter 9) have been constructed as a means of better understanding the functional significance of many structural features of eukaryotic chromosomes. A reasonably stable artificial linear chromosome requires only three components a centromere, telomeres at each end, and sequences that allow the initiation of DNA replication. Yeast artificial chromosomes (YACs see Fig. 9-8) have been developed as a research tool in biotechnology. Similarly, human artificial chromosomes (HACs) are being developed for the treatment of genetic diseases by somatic gene therapy. 

Eukaryotic chromosomes have two important special-function repetitive DNA sequences centromeres, which are attachment points for the mitotic spindle, and telomeres, located at the ends of chromosomes. 

Nucleosomes are organized into 30 nm fibers, and the fibers are extensively folded to provide the 10,000-fold compaction required to fit a typical eukaryotic chromosome into a cell nucleus. The higher-order folding involves attachment to a nuclear scaffold that contains histone HI, topoisomerase II, and SMC proteins. 

The termination of replication on linear eukaryotic chromosomes involves the synthesis of special structures called telomeres at the ends of each chromosome, as discussed in the next chapter. 

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