Cell cycle is an ordered set of phases in the life of proliferating cells. Cells are born after the division of a progenitor cell, then they increase in size about twice by synthesizing many cell structures and molecules, including DNA, and finally they divide and give rise to two new cells. This cycle is typical of proliferating cells. However, there are other possibilities. For example, many cells never divide, such as neurons, and some other special cells are born from the fusion of two cells, such as zygotes (fusion of two gametes), or by the fusion of many cells, such as skeletal striated muscle cells during development. Finally, some cells die.
There are two major types of cells in multicellular organisms: somatic and germinal cells. Somatic cell are those not giving gametes, whereas germinal cells do. Gametes result from a division cell process known as meiosis where a diploid cell, the germinal cell, divide twice to give haploid cells, the gametes. A somatic cell, and a germinal cell too, may proliferate and divide only once to give two new daughter cells that are also diploids. This division is known as mitosis.
In the next pages we will deal with the cell cycle of proliferating somatic cells. It is also shown some examples of cells that do not complete the cell cycle and become long-living cells or die. Cell death may be caused by damages or it can be a physiological mechanism known as apoptosis.
The cell cycle of different cell types in a tissue must be tightly controlled and coordinated. During embryo development and juvenile periods, most cell types contribute to the animal body growth. However, once in the adult stage, the size of many cell populations remains constant, and there is a decrease or stop of the proliferation rate. The cell cycle is then adjusted to the requirements of the organism, for example, for physiological cellular turnover or to repair tissue damages. Sometimes, there are some errors in some cells that do not follow the rules of the organism and divide without control. These are the cancer cells.
Cell cycle goes through several phases: G1, S, G2 and M (G stands for gap, S for synthesis, and M for mitosis) (Figure 1). This sequence is present in almost every eukaryote proliferating cell and only occasionally some cell cycle phases are skipped. G1, S, and G2 phases are grouped in a larger phase known as interphase.
G1 is the first phase after the cell birth. This is the longest and more variable period of the cell cycle, where the cell grows to reach the optimal size. At the end of the G1 phase, there is a molecular checkpoint that blocks the start of the next phase, S phase, unless all the requirements needed to carry out a successful S phase are accomplished. For instance, a required cell size or not having DNA damages. Not all the cells of the organism are continuously proliferating. Many cells halt the cell cycle in G1 phase in order to carry out their functions. Cells may keep doing their functions for a while and then restart the cell cycle, or they can stay as differentiated cells forever.
S phase is the DNA synthesis phase. During this phase DNA is replicated. This is a complex process because of the long DNA strands that form the eukaryotic chromosomes. The duplication of the DNA must accomplish two requirements: just one copy (one replication) and making as few mistakes as possible. Any error during the replication of DNA may lead to lethal damages in the two new cells and, what is worse, in the organism.
G2 phase is another period for cell growing, shorter than G1. A variety of molecules that will be used during the next phase, M phase, are synthesyzed during G2 phase.
M phase is probably the most complex of the cell cycle phases, involving a large reorganization of cellular components. Organelles and other cellular components are distributed in two groups to form the new daughter cells. In this phase, several molecular processes are triggered, and then they run in parallel. Mitosis is the process of condensing, segregating and decondensing chromosomes to form the two new nuclei. It can be divided in several stages: prophase, metaphase, anaphase and telophase (Figures 2 and 3). The compaction of DNA happens during prophase, chromosomes are lined up into an equatorial plane during metaphase, chromatids separate during anaphase, and decondensation of DNA and rebuilding the two new nuclei happen during telophase. Other parallel processes are the breakdown of the nuclear envelope, mitotic spindle formation, and distribution of cytoplasm components. Cytokinesis begins during the later stages of mitosis. Cytokinesis is the mechanism of splitting the mother cytoplasm y two. In animal cells, an actin filament ring strangles the cytoplasm and separates the two new cells. In plant cell cytokinesis, it is synthesized a new cell wall that divides the cytoplasm in two. M phase ends up with two new cells and G1 phase starts again.