Overview of the cell division cycle

The cell division cycle is a series of events that take place in a cell leading to its division and duplication. In unicellular species (no cell nucleus) such as bacteria, cell division occurs by a process called binary fission and produces a new organism. In multicellular species (with a cell nucleus) somatic cell division occurs in two distinct phases: interphase, during which the cell grows and replicates its DNA, followed by mitosis in which the cell divides to produce two daughter cells. This process is also called proliferation.

Interphase

Interphase consists of 3 phases; G1 phase (growth), S phase (replication) and G2 phase (preparation for division). During interphase, cells actively prepare for mitosis by gathering nutrients and conducting normal cellular functions i.e. they are metabolically active. Since cell division operates in a cycle, interphase is preceded by the previous cycle of mitosis.

The standard cell division cycle in multicellular species.

G1 phase

G1 phase (G = gap) is the gap between completion of mitosis and the beginning of S phase (DNA synthesis). During this time, protein synthesis occurs and the cell grows to about double its original size. More organelles are produced, increasing the volume of the cytoplasm. If the cell is not to divide, it can pause its progress and enter a specialised resting state known as G0 (G zero), where it can remain for days, weeks or even years before resuming proliferation.

S phase

Following the G1 phase, the cell takes a decisive step and commits to DNA replication in a phase known as S phase (S = synthesis). During S phase, DNA is replicated to produce exactly two identical chromosomes. At the end of S phase, all of the chromosomes have been replicated; the amount of DNA in the cell has therefore effectively doubled. Typically, in a population of cells, about 30% will be in S phase at any one time.

G2 phase

Following DNA replication, the cell enters the G2 phase of the cell cycle, which lasts until the cell enters mitosis. Here, the cell undergoes significant biosynthesis to ensure necessary cell growth and production of microtubules required for mitosis. Inhibition of protein synthesis during G2 phase prevents the cell from undergoing mitosis.

Mitosis

When G2 is complete, the cell enters a relatively brief period of nuclear and cellular division, composed of mitosis and cytokinesis; together they define the mitotic phase (M phase) of the cell cycle involving the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle.

The process of mitosis is complex and highly regulated. The M phase (nuclear division) has been characterised into five sequential phases (prophase, metaphase, anaphase and telophase) (PMAT) in which the pairs of chromosomes condense and attach to fibres that pull the sister chromatids to opposite sides of the cell. In a standard mammalian cell, mitosis can typically take around 1 hour to complete.

The cell then divides in cytokinesis, to produce two identical daughter cells.

Prophase

The chromatin, which is diffuse in interphase, slowly condenses into well defined chromosomes. Each chromosome has duplicated during the preceding S phase and consists of two sister chromatids. Towards the end of prophase, the cytoplasmic microtubules disassemble and the mitotic spindle begins to form.

Metaphase

Metaphase starts abruptly with breakdown of the nuclear envelope (prometaphase). Specialised protein complexes called kinetochores attach to some of the spindle microtubules, which are then called kinetochore microtubules. These structures are responsible for aligning the chromosomes in one plane between the poles of the cell. This is called the metaphase plate.

Anaphase

Triggered by a specific signal, anaphase begins as the chromosomes are pulled to opposite ends of cell. The kinetochore microtubules shorten as the chromosomes approach the poles. This typically occurs at around a distance of 1 µm per minute.

Telophase

In telophase, the newly separated daughter chromosomes arrive at opposite ends of the cell and the microtubules disappear. A new nuclear envelope re-forms around each group of chromosomes, and the nucleoli, which disappeared at prophase, begin to reappear. Mitosis is now at an end.

The cytoplasm then divides in a process called cytokinesis. The membrane between the daughter nuclei is drawn inward to form a cleavage burrow, which gradually deepens until it finally breaks at each end, leaving two separate daughter cells.

After the successful completion of these processes, both resulting daughter cells re-enter G1 of interphase and the cycle begins all over again. The standard cell cycle is generally quite long- 12 hours or more for fast growing tissues in mammals. The shortest eukaryotic division cycles are the early embryonic cell cycles that occur in certain animal embryos. In these cycles, no growth occurs and M phase directly follows S phase; there are no G1 or G2 phases.

Reference

Alberts B et al (1994) The cell division cycle. In: Molecular Biology of the Cell 3rd Ed New York, Garland Publishing, p863-910.

Cell Division