Eukaryotic Chromosomal DNA

Multiple-origin model for eukaryotic chromosomal DNA replication, (a) Autoradiograph of short-term labeling of a eukaryotic chromosome during replication and its interpretation. (b) Overall replication scheme for a eukaryotic chromosome. Only a short region of the chromosome is shown. It is believed that replication origins are relatively free of proteins. 

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Replication of the Escherichia coli Chromosome Initiation and Termination of Escherichia coli Chromosomal Replication DNA Replication in Eukaryotic Cells Eukaryotic Chromosomal DNA SV40 Is Similar to Its Host in Its Mode of Replication... 

Synthesis at the ends of a eukaryotic chromosome. One end of the linear DNA of a eukaryotic chromosome is diagrammed. A flush-ended DNA duplex presents a problem for completing synthesis at the 5 end (a). This is because of the RNA primer requirement for DNA synthesis. When the primer at the 5 end is removed there is no conventional way to fill the gap. A solution to this problem is shown in (b). The ends of eukaryotic chromosomal DNAs consist of highly repetitious tandem repeats (telomeres). These repeats on the 3 end serve as both primer and template for extending the 3 end. The extended 3 end can accommodate a primer RNA, so after chromosomal DNA replication no loss occurs from the 5 end of the DNA. Another process is needed to remove the extension from the 3 end. New synthesis is indicated in red. The zigzag represents primer. 

Fig. 2. Replication of eukaryotic chromosomal DNA. Replication begins at many origins and proceeds bi-directionally at each location. Eventually the replication eyes merge together to produce two daughter DNA molecules, each of which consists of one parental DNA strand (thin line) and one newly synthesized DNA strand (thick line).

Multiple Replicon Model of Eukaryotic Chromosomal DNA Replication. 

In the prokaryotic chromosome there is a protein core to which the circular DNA molecule is attached. In addition, HU protein binds to the DNA and facilitates its bending and super-coiling. In eukaryotic chromosomes, DNA forms complexes with the histones to form nucleosomes. The polyamines are polycationic molecules that bind to negatively charged DNA so the latter molecule can overcome charge repulsions between adjacent coils during the compression process. 

Unlike SV40 DNA, eukaryotic chromosomal DNA molecules contain multiple replication origins separated by tens to hundreds of kilobases. A sIx-subunIt protein called ORC, for origin recognition complex, binds to each origin and associates with other proteins required to load cellular hexameric helicases composed of six homologous MCM proteins. 

Replication begins at a sequence called an origin. Each eukaryotic chromosomal DNA molecule contains multiple replication origins. 

E. coli chromosome contains 4.6 x 10 base pairs while the 23 human chromosomes contain 3 x 10 base pairs (2) Prokaryotic chromosomes are usually circular while eukaryotic chromosomes are linear, (3) Eukaryotic chromosomal DNA is packaged into nucleosomes through specific interactions with histones and other proteins while prokaryotic DNA is not. 

In eukaryotes, chromosomal DNA is associated with a large number of proteins. The most abundant are the histones, very basic proteins of which there are five types H1-H5. The DNA double helix is wound around a core structure formed by histones H2-H5 about 200 base pairs wrapped around such a core comprise a nucleosome. The HI histones are associated with the DNA strand between nucleosomes. There is evidence that the nucleosomes play a role in G.a., in that RNA polymerase evidently cannot bind to DNA within the nucleosome. Thus if the initiation... 

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. 

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... 

Bacteria normally harbour a single, circular chromosome that tends to be tethered to the bacterial plasma membrane and tends to have few if any closely associated proteins. Many bacteria also contain extra-chromosomal DNA in the form of plasmids, as will be discussed later. Eukaryotes (plants, animals and yeasts) posses multiple linear chromosomes contained within a cell nucleus, and these chromosomes are normally closely associated with proteins termed histones (the pro-tein-DNA complex is termed chromatin). Eukaryotes also invariably possess DNA sequences within mitochondria and in chloroplasts in plants. The (usually circular) DNA molecules are much... 

The genetic information of eukaryotic cells is propagated in the form of chromosomal DNA. Besides the nucleic acid component, chromosomes contain architectural proteins as stoichiometric components, which are involved in the protective compaction of the fragile DNA double strands. Together, the DNA and proteins form a nucleoprotein structure called chromatin. The fundamental repeating unit of chromatin is the nucleosome core particle. It consists of about 147 base pairs of DNA wrapped around a histone octamer of a (H3/H4)2 tetramer and two (H2A-H2B) heterodimers. One molecule of the linker histone HI (or H5) binds the linker DNA region between two nucleosome core particles (Bates and Thomas 1981). 

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. 

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. 

Peticolas, W. L., Patapoff, T. W., Thomas, G. A., Postlewait, J., and Powell, J. W. 1996. Laser Raman microscopy of chromosomes in living eukaryotic cells DNA polymorphism in vivo. J. Raman Spectros. 27 571-78. 

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