Cell Cycle Cyclins

The cell cycle or cell division varies in length in different types of cells, but is repeated each time a cell divides. It is composed of a series of events that prepare the cell to divide into two daughter cells. The cell cycle consists of four different phases: G1 phase (the presynthetic), S phase (DNA Synthesis), G2 phase (premitotic) collectively known as interphase and M phase (mitotic. M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's chromosomes are divided between the two daughter cells, and cytokinesis, in which the cell's cytoplasm divides forming distinct cells. Activation of each phase is dependent on the proper progression and completion of the previous one. Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G0 phase.

G1 phase or Gap 1 phase usually lasts from hours to several days. This occurs after mitosis and it is the period during which the cell grows and proteins are synthesized. S phase also known as the synthetic phase its lasts 8 to12 hours in most of the cells. DNA is replicated and proteins are synthesized which results to duplication of the chromosomes. In this phase centrosomes are also duplicated. In the G2 phase or the gap two phase it lasts for 2 to 4 hours. In this phase it follows S phase and extends to mitosis. During this phase the cell prepares to divide. In G0 phase it refers to quiescent or senescent cells. This cell is exemplified by hepatocytes. Tissues are divided into three groups according to their proliferative capacity. First, the labile or the continuously dividing: cells proliferate throughout life and replacing those that are destroyed. These are the surface epithelium and bone marrow hematopoietic cells. Some cells are capable of rapid division in response to stimuli (liver, kidney, smooth muscle, and endothelial cells. There are also cells that cannot undergo division like neurons, skeletal muscles and cardiac muscle cell, called permanent of non-dividing cells.

There are several controls factors have been identified, including a category of proteins known as cyclins as well as cyclin-dependent kinases or CDKs in which initiate and induce progression through the cell cycle. These factors may enter during the G1 phase which is exemplified by cyclin D and E. These particular complexes enable the cell to enter and advance through the S phase. There are also CDKs which enable the cell to leave the S phase and enter the G2 phase (cyclin A). Cyclin B binds to its CDK which induce the cell to leave the G2 phase and may enter the M phase.

Mitosis lasts for 1-3 hours and it follows the G2 phase. In other words it completes the cell cycle. There are particular events happening in mitosis like (karyokinesis) division of the nucleus and (cytokinesis) division of cytoplasm. This event results in the production of two identical daughter cells and consists of 5 major stages. The first stage is Prophase, this begins when the chromosomes condense and in this phase the nucleolus and the nuclear envelope begin to disappear. The next phase is the Metaphase; in this phase the duplicated condensed chromosomes align at the equatorial plate of the mitotic spindle and become attached to spindle microtubules at their kinetochore. As the chromatids separates (at the centromere) and the daughter chromosomes move to opposite poles of the cell this is collectively known as the Anaphase. In this phase a cleavage furrow begins to form. The last phase is Telophase, during this phase there is deepening of the furrow and forms two daughter cells. Nuclear envelope reforms, nucleoli reappear, chromosomes disperse to form a new interphase nucleus. The DNA content of each stage varies. In prophase the DNA content was doubled in the S phase of interphase (4n).Double complement of DNA (4n) in metaphase and anaphase. Lastly, in the telophase stage each new daughter cell contains a single compliment of DNA (2n).

In summary of cell cycle it is the series of events that take place in a eukaryotic cell which leads to replication. These events are subdivided in two brief periods: interphase—during which the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA—and the mitotic (M) phase, the cell splits itself into two distinct cells, often called "daughter cells". The cell-division cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process happening to hair, skin, blood cells, and some internal organs are renewed.

The main result of mitosis is the formation of the parent cells into two daughter cells. The parent cells are composed of a specific number of chromosomes which contain tightly-coiled DNA that contains genetic information which is vital for proper cell function. We must understand that each daughter cell that is produce during mitosis should be genetically identical to the parent cell. This event happens during the S phase or DNA synthesis. The new formed chromosomes with identical copies of itself, called sister chromatids. They are attached to a specific region called the centromere. Each sister chromatid is not considered a chromosome in itself, and a chromosome does not always contain two sister chromatids. In most eukaryotic cells their nuclear envelope separates DNA from the cytoplasm. The chromosomes align themselves in a line spanning the cell and microtubules splay out from opposite end of the cell and shorten. As the cell elongates, corresponding sister chromosomes are pulled toward opposite ends. A new nuclear envelope forms around the separated sister chromosomes.

As mitosis completes its cytokinesis the cells pinches inward forming a cleavage furrow separating two developing nuclei. In prokaryotic cells they undergo process similar to mitosis called binary fission but prokaryotes cannot undergo mitosis because they lack a nucleus. They only contain single chromosomes and no centromere.


Leslie P. Gartner, James L. Hiatt and Judy M. Strum. Cell Biology and Histology 4th Edition. Baltimore, Maryland: Board Review Series, 2003.

Robbins and Cotran. Pathologic Basis of Disease 7th Edition.Philadelpia, PA: Pocket, 2006

Luis Carlos Junquiera and Jose Carneiro. Basic Histology 11th Edition. Paolo, Brazil: Text and Atlas, 2007.

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