Differentiated cells that do not divide (e.g., neurons or cells from the cardiac muscle) leave the cell cycle at this point and start the G0 phase.
The first stage of the mitotic phase is called karyokinesis, mitosis or nuclear division and the second step is known as cytokinesis, which is the physical separation of the cytoplasmatic components into the two daughter cells.
During the last step of the mitotic phase, cytokinesis, a contractil ring of actin filaments is produced which divides the cytoplasm into the two daughter cells.
Control of the cell cycle
The time that a cell spends in each phase of the cycle varies too. Thus, for example, those human cells with a 24-hour cycle spend approximately nine hours in the G1 phase, ten hours in the S phase, around four and a half hours in the G2 phase and about 30 minutes in the M phase.
Each step of the cell cycle is controlled by mechanisms both internal and external to the cell.
Regulation of the cell cycle by external mechanisms
For instance, events such as the death of a near cell, the release of hormones that favour growth or the cell reaching a specific size can start cell division.
Regardless of the type of externally received message, a series of internal effects then lead it to the interphase.
From this starting point, each required parameter in each phase must be satisfied so that the cell cycle can continue.
Regulation at internal checkpoints
Si no se cumplen, la célula puede parar el ciclo hasta que las condiciones adversas se solucionen o puede pasar a la fase G0 y esperar señales que indiquen que las condiciones han mejorado.
The control system of the cell cycle could be compared to the program of a washing machine, where the washing machine only carries out its functions when the signals from its sensors (lock door, levels of water and detergent, power supply) indicate that it is ok to move forward during the diverse stages of the washing cycle.
Regulator molecules of the cell cycle
Retinoblastoma protein is a suppressor protein for tumours that is altered in many kinds of cancer (prostate, breast), but it is retina cancer from where it takes its name.
P21 inhibits the cell cycle, with its levels controlled, in turn, by p53, which regulates cell growth and controls DNA damage. This protein can halt the cycle if needed, and is able to cause apoptosis too.
 Set of microtubules whose function is to enable the migration and correct separation of the chromosomes during mitosis.
 Protein structures whose function it is to initiate, control and supervise the movement of the chromosomes during cell division.
 Proteins synthesised during the interphase and destroyed at the end of the mitosis. They regulate the enzymatic activity of CDKs.
 Enzymes that activate or inhibit other proteins by phosphorylating them (adding phosphate groups).