Replication eukaryotic cell cycle

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. 

The eukaryotic cell cycle (see Fig. 12-41) produces remarkable changes in the structure of chromosomes (Fig. 24-25). In nondividing eukaryotic cells (in GO) and those in interphase (Gl, S, and G2), the chromosomal material, chromatin, is amorphous and appears to be randomly dispersed in certain parts of the nucleus. In the S phase of interphase the DNA in this amorphous state replicates, each chromosome producing two sister chromosomes (called sister chromatids) that remain associated with each other after replication is complete. The chromosomes become much more condensed during prophase of mitosis, taking the form of a species-specific number of well-defined pairs of sister chromatids (Fig. 24-5). 

S The part of interphase of the eukaryotic cell cycle when DNA is replicated. 

Figure 27.34. Eukaryotic Cell Cycle. DNA replication and cell division must take place in a highly coordinated fashion in eukaryotes. Mitosis (M) takes place only after DNA synthesis (S). Two gaps (Gj and G2) in time separate the two processes.

The eukaryotic cell cycle is divided into four phases M (mitosis), Gj (the period between mitosis and the Initiation of nuclear DNA replication), S (the period of nuclear DNA replication), and G2 (the period between the completion of nuclear DNA replication and mitosis) (see Figure 21-1). 

Eukaryotic cell cycle - The processes by which cells divide and DNA is replicated (see here) are somewhat more complicated in eukaryotes than in prokaryotes. DNA replication in bacteria is an almost continuous process, at least during exponential growth. The somatic cells of eukaryotes, on the other hand, typically divide much less frequently, and some, in certain types of mature tissue, do not divide at all. Eukaryotic cells that are dividing in growing tissues exhibit a well-defined cell cycle, which is almost always separated into several distinct phases, as shown in Figure 28.14, Figure 28.15, and Figure 28.16. 

The eukaryotic cell cycle is tightly controlled by regulatory mechanisms that ensine faithful replication of the genome before cell division. These regulatory mechanisms, named cell-cycle checkpoints, are based, to a large extent, on the reversible phosphorylation of enzymes on serine/threonine residues, and the major control sites are Gl/S and G2/M phases. ° ° ... 

The eukaryotic somatic cell cycle is defined by a sequential order of tasks a dividing cell has to complete it must replicate its DNA, segregate its chromosomes, grow, and divide. The cell cycle can be divided into four discrete phases. DNA replication is restricted to S phase (DNA synthesis phase), which is preceded by a gap phase called G1 and followed by a gap phase called G2. During mitosis (M phase) the sister chromatids are segregated into two new daughter nuclei and mitosis is completed by the division of the cytoplasm termed cytokinesis (Fig. 1). 

E. Eukaryotic DNA replication is similar to that of prokaryotes but more complex in scale, and the process is coordinated with the cell cycle. 

The DNA molecules in eukaryotic cells are considerably larger than those in bacteria and are organized into complex nucleoprotein structures (chromatin p. 938). The essential features of DNA replication are the same in eukaryotes and prokaryotes, and many of the protein complexes are functionally and structurally conserved. However, some interesting variations on the general principles discussed above promise new insights into the regulation of replication and its link with the cell cycle. 

Phases in the life cycle of a typical eukaryotic cell. The cell cycle is divided into the resting stage (Go), the prereplication stage (G,), the synthesis or replication stage (S), the postreplication stage (G2), and the mitotic stage (M). 

Multiple replicons In eukaryotes, replication of chromosomal DNA occurs only in the S phase of the cell cycle. As for bacterial DNA (see Topic F3), eukaryotic DNA is replicated semi-conservatively. Replication of each linear DNA molecule in a chromosome starts at many origins, one every 3-300 kb of DNA depending on the species and tissue, and proceeds bi-directionally from each origin. The use of multiple... 

Taxol is a potent inhibitor of eukaryotic cell replication, blocking cells in the late G2, or mitotic, phase of the cell cycle. Interaction of Taxol with cells results in the formation of discrete bundles of stable microtubules as a consequence of reorganization of the microtubule cytoskeleton. Microtubules are not normally static organelles but are in a state of dynamic equilibrium with their components (i.e., soluble tubulin dimers). Taxol alters this normal equilibrium, shifting it in favor of the stable, nonfunctional microtubule polymer. 

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