Cell cycle

A cell cycle is the progress of events that are required for a eukaryotic cell to pass from one cell division to another. The timespan for a cell cycle can vary for cells of different species and even between different cell types in the same organism. There are several phases that are divided into interphase and mitosis.

Interphase

The cell grows during interphase in order to be large enough to divide again. It is also during this time that the cell synthesizes different proteins and serve its function in the organism.

G₁-Phase

For many cells, this gap phase is the major period of cell growth during its lifespan. During this stage new organelles are being synthesized, so the cell requires both structural proteins and enzymes, resulting in great amount of protein synthesis. The metabolic rate of the cell will be high. The progression through G₁ may be affected by limiting growth factors, nutrient supply, and additional inhibiting factors. A rapidly dividing human cell which divides every 24 hours spends 9 hours in G₁ phase.

There is a "restriction point" present at the end of G₁ phase; a series of check points ensure the DNA is intact and that the cell is functioning normally. This safeguard is controlled by proteins known as cyclin-dependent kinases (CDK's) that can activate the processes responsible for DNA synthesis and allow the cell to enter S phase.

A cell may pause in the G₁ phase before entering the S phase and enter a state of dormancy called the G₀ phase. In order to divide, the cell must re-enter the cycle in S phase.

S-Phase

S-phase is short for synthesis phase. At the beginning of the S-phase, each chromosome is composed of one coiled DNA double helix molecule called a chromatid. After DNA replication, each chromosome has two identical DNA double helix molecules known as sister chromatids (joined at the centromere). When the cell finally divides these two sister chromatids will separate, one into each daughter cells.

Damage to DNA often takes place during this phase, and DNA repair is initiated following the completion of replication. Incompletion of DNA repair may flag further cell cycle checkpoints, which can pause the cell cycle.

G₂-Phase

The last subphase during interphase within the cell cycle in which the cell undergoes a period of rapid growth to prepare for mitosis. At this gap phase in interphase the nucleus is well defined still bound by a nuclear envelope. Although chromosomes have been replicated they cannot yet be distinguished individually because they are still in the form of loosely packed chromatin fibers.

At the end of this phase is another control checkpoint (G₂ checkpoint) to determine if the cell can proceed to M phase and divide. This is the last opportunity before division for DNA repair and stopping the proliferation of damaged cells.

M-Phase

Mitosis is the process in which a eukaryotic cell separates the chromosomes in its cell nucleus, into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle - the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell.

Mitosis occurs exclusively in eukaryotic cells, but occurs in different ways in different species. For example, animals undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae (yeast) undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus.

The process of mitosis is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, anaphase and telophase. During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell. The cell then divides in cytokinesis, to produce two identical daughter cells.