The Cell Cycle

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 proceeds as follows:

> 1. The G1 phase (or first gap phase), follows cell division. In the G1 phase, the cell contains two copies of each chromosome ("2C" in Figure 28.14); that is, the cell is in the diploid state.

> 2. Late in the G1 phase, the commitment to divide is triggered in an as yet unknown fashion. Because division first requires doubling of the DNA content, and the new DNA needs new histones to make chromatin, synthesis of histones is one of the first indications of incipient DNA replication (see Figure 28.14).

> 3. The cell then enters synthesis, or S phase. During this stage, the DNA is replicated and the histones and nonhistone proteins are deposited on the daughter DNA molecules to reproduce the chromatin structures.

> 4. When replication is complete, the cell enters the G2 phase. It has a DNA content four times the haploid amount (4C). In most eukaryotic cells, the total time required for G1, S, and G2 phases will be many hours. During this whole period, which is termed interphase, the chromatin is dispersed throughout the nucleus and is actively engaged in transcription.

> 5. At the end of G2, the cell is ready to begin to divide (mitosis). Mitosis has been subdivided for descriptive purposes into the phases depicted in Figure 28.15.

> > Prophase - In prophase, the replicated chromosomes condense into the typical metaphase chromosome structures so often pictured. The nuclear membrane disintegrates, and the mitotic spindle forms. The spindle consists of contractile microtubules that pull pairs of chromatids apart so that the daughter cells will each receive identical sets of chromosomes.

> > Metaphase - Each chromosome aligns independently at the metaphase plate.

> > Anaphase - Sister chromatids separate to opposite poles of the cell in anticipation of cell division.

> > Telophase - The nuclear membrane then re-forms about each daughter nucleus.

> > Cytokinesis - The cell divides. After division, the chromosomes of the daughter cells disperse, and a new G1 phase begins.

In many tissues of higher organisms, the G1 phase becomes very prolonged after growth and tissue differentiation are complete. The most extreme examples are fully differentiated nerve cells, most of which never divide again in mature organisms. Nondividing cells in a permanently arrested G1 phase are said to be in G0 phase. On the other hand, some specialized stem cells, such as those found in the bone marrow and intestinal epithelium, undergo continuous division throughout the life of the organism.

See also: Molecular Control of the Cell Cycle, Eukaryotic DNA Replication, Apoptosis

INTERNET LINK: Regulation of the Cell Cycle