See also cell cycle control

Introduction/Definitions:

Progression through the cell cycle requires the coordination of  variety of macromolecular syntheses, assemblies and movements. The chromosomes must be replicated, condensed, segregated, and decondensed. The spindle poles must duplicate, separated, and migrate to opposite ends of the nucleus. Coordination of these complex processes is thought to be achieved by  series of changes in the cyclin-dependent kinases (CDKs). The active forms of the CDKs are a complex of at lest two proteins,  kinase and  cyclin. These complexes undergo changes in the kinase and cyclin components that are believed to drive the cell from one stage of the cell cycle to another. According to this paradigm, cell cycle stage is determined by the constellation of proteins activated or inactivated by phosphorylation as  result of the activity of the CDKs during that stage.

The phases of the cell cycle are gap 1 (G1), synthesis (S1), gap 2 (G2), mitosis and cytokinesis (C). G1, S and G2 are collectivley called interphase and mitosis and cytokinesis together are called M phase. 

Mitosis: is the phase of the cell cycle in which the spindle apparatus assembles, binds to the chromosomes, and moves the sister chromatids apart. Chromosomes become highly condensed and then are separated and distributed to the two daughter nuclei. The condensation of interphase chromosomes into mitotic chromosomes requires a class of proteins called condensins which are large protein complexes that contain SMC proteins; these are long, dimeric protein molecules that use the energy of ATP hydrolysis to make large right handed loops in DNA. When the 46 human chromosomes at mitosis are stained with dyes, one can see a reproducible banding patter. The centromere attaches to the duplicated chromosomes to the mitotic spindle so that one copy is distributed to each daughter cell. 

Cytokinesis: is the phase of the cell cycle when the cytoplasm divides, creating two daughter cells. 

Interphase:  

The period when the cell is not in mitosis is referred to as “interphase”. most of th cell cycle is spent in interphase. Chromosomes are replicated beginning at the origins of replication and proceed bidirectionally from the origins across the chromosome.  

Interphase contains the G1, S and G2 phases of the cell cycle. During interphase, the cell grows, replicates chrosomes, organelles and centrioles and syntehsizes components needed for mitosis., including tubulin. Cohesin proteins hold chromatids togetehr at the centromere of each chromosome. :

• G1 (gap phase 1): is the phase between M phase (below) and S phase. For most cells, this is the longest phase. Unlike transit through the S, G2, and M phases, G1 progression normally relies on sitmulation by mitogens and can be blocked by antiproliferative cytokines. Cells often pause in G1 before DNA replication and enter a resting state called the Go phase. some cells, such as muslce and nerve cells, remain there permanently, others, such as liver cells, can resume G1 phase in response to factors released druing injury. The ability to enter Go accounts for the incredible diversity seen in the lenght of the cell cycle in different tissues. Epithelial cells lining the human gut divide more than twice a day, constantly renewing this lining. By contrast, liver cells divide only once every year or two, spending most of their time in the Go phase. Mature neurons and muslces cells as stated usually never leave Go. 

• S (syntehsis): In contrast to bacteria which replicate their DNA continually, DNA replication in most eucaryotic cells occurs only during a specific part of the cell division cycle, called the DNA synthesis phase or S phase. In a mammalian cell, the S phase typically lasts for around 8 hours. 

• G2 (gap phase 2): is the phase between S phase and mitosis. 

M Phase: (Mitosis and Cytokinesis)

Mitosis and cytokineses (below) are usually referred to collectivley as M phase. 

In late interphase (G2), the cytoplasms contains two centrosomes. Within the nucleus, the chromosomes are duplicated, but cannot yet be distinguished individually because they are still in the form of loosely pakced chromatin fibers.

After the chromosomes duplicate during interphase, a series of stages occurs during mitosis: prophase, metaphase, anaphase and telophase as described below. Cytokinesis then divides the cytoplasm, yielding two genetically dientical daugher cells. 

Prophase: In the nucleus, the chromatin fibers coil, so that the chromosomes become thick enough to be seen individually with a light microscope. Each chromosome exsits as two identical sister chromatids joined together at the centromere. In the tyoplasms, the mitotic spindle begins to form. Late in prophase, the nuclear envelope breaks into pieces, the spindle tracks attach to the centromeres of the chromosomes and move toward the center of the cell.

Metaphase: The mitotic spindle is now fully formed. The centromeres of all the chromosomes line up between the two poles of the spindle. For each chromosome, the tracks of the mitotic spindle attached to the two sister chromatids pull toward opposite poles. 

Anaphase: The sister chromatids of each chromosomes separate and move toward opposite poles of the cell. Separation of sister chromatids during mitosis depends on microtubules attaching to proteins found in the kinetochore. 

Telophase: The nuclear envelope forms, the chromosomes uncoil and the spindle disappears. Cytokineses, the division of the cytoplasms occurs. In animals, a cleavage furrow pinches the cell in two, producing two daughter cells. 

Meiosis

Both somatic cells (nonrerpdouctive cells) and germ-line cells are diploid. The cirtical difference is that somatic cells undergo mitosis to form genetically idnetical, diploid daughter cells, but germ-line cells can undergo meiosis to produce haploid gamets. Germ-line cells contained in the sexual organs of the male and female can undergo meiosis to produce haploid gametes (eggs and sperm). As only one set of chromosomes are present in the gametes, they are haploid. Germ cells are diploid at the beginning of their development, but become haploid during the process of gametogenesis.Subsequent to feritzation of the sperm and egg to become diploid, with one set 23 chromosomes from the father and one set of 23 chromosomes form the mother in the human. 

The process of cell division that produces haploid gametes in diploid organisms, resembles mitosis, but with two important differences. First, the number of chromosomes is cut in half. In meiosis, a cell that has duplicated its chromosomes undergoes two consecutive divisions called meiosis I and meiosis II. Because one duplication of the chromosomes is followed by two divisions, each of the 4 daugters cells resulting from meiosis has a haploid set of chromosomes -half as many chromosomes as the starting cell. In other words, meiosis entails two nucler and cytoplasmic divisions (duplication, division in half, then division in half again), yielding four haploid cells. 

Meiosis I: Homologous Chromosomes Separate

–Interphase: As with mitosis, meiosis is preceded by an interphase during which the chromosomes duplicate. Each chromosomes then consists of two identical sister chromatids. 

–Prophase I: As the chromosomes coil up, special proteins cause the homologous chromsomes to stick together in pairs. The resulting struture has 4 chromatids. Within each set, chromatids of the homologous chromosomes exchange corresponding segments -they “cross over“. Crossing over rearranges genetic information. The exchange of segments between nonsister chromatids –one maternal chromatid and one paternal chromatid of a homologous par adds to the genetic variety resulting from sexual reproduction. 

–Methaphase I: The homologous paairs are aligned in the middle of the cell. The sister chromatids of each chromosomes are still attached at their centromeres. 

–Anaphase I: The attachment between the homologous chromosomes of each pair breaks, the chromosomes now migrate toward the poles of the cell. In contrast to mitosis, the sister chromatids migrate as a pair instead of splitting up. They are separated not from each other but from their homologous partners. 

–Telophase I and Cytokinesis: In telophase I, the chromosomes arrive at the poles of the cell. Usually cytokineses occurs along with telophase I, and two haploid daughter cells are formed. 

Meiosis II: is like a mitotic division without DNA replication. 

Meiosis II is essentially the same as mitosis. The important difference is that meiosis II starts with a haploid cell that has not undergone chromosome duplication during the preceding interphase. 

–Prophase II: a spindle forms and moves the chromosomes toward the middle of the cell. 

–Metaphase II: the chromosomes are aligned as they are in mitosis, with the tracks attached to the sister chromatids of each chromosomes coming from opposite poles. 

–Anaphase II: The centromeres of sister chromatids separate and the sister chromatids of each pair move toward opposite poles of the cell.

–Telophase II: nucleus form at the cell poles and cytokinesis occurs. There are now four haplid daughter cells each with single chromosomes.