Mitosis and the Cell Cycle
The most fundamental role of the cell cycle is to duplicate the DNA in the chromosomes, and distribute the duplicated DNA into two identical daughter cells. Cell cycle duration significantly varies depending on the type of the cell. Early frog embryo cells divide every 30 minutes, whereas, human liver cells divide once a year on average. The cell cycle consists of a sequence of steps that take place in order to make an identical copy of the original cell. There are four steps that make up the cell cycle which include the G1 phase, S phase, G2 phase, and M phase. Each phase consists of checkpoints that allow a cell to move further with the cell cycle only if the cell has the accurate encoding information to carry on. If a cell is not being able to pass a checkpoint, it leads the cell to apoptosis; where it orders cell death or the cell stays in G0 phase where it tries to correct its malfunction.
G1 phase is the interval between the end of M phase and beginning of S phase. G1 phase includes checkpoint which make sure that the cell has enough resources for it to continue onto S phase and finish the cycle. Next, the DNA is duplicated in the S phase. Then in G2, the checkpoints make sure the entire DNA is replicated accurately, and that there are no damages in the cell. The G1, G2, and S phase all together are known as interphase. If the cell passes the G2 checkpoint, it goes into the M phase. M phase consists of mitosis and cytokinesis. The nucleus divides first in the process of mitosis followed by the division of the cytoplasm known as cytokinesis.
M phase is an integral step of the cell cycle. The daughter cells produced at the end of mitosis are genetically identical to the original parent cell. M phase is subdivided into 5 stages of mitosis followed by cytokinesis.
Stage 1- Prophase: replicated chromosomes (each consisting of two sister chromatids) condense. In the cytoplasm, the mitosic spindle starts to form.
Stage 2- Prometaphase: the nuclear envelope breaks, and the chromosomes can now attach to the spindle microtubules through kinetochores.
Stage 3- Metaphase: the spindle fully develops and the chromosomes line up in the middle of the two spindle poles at the metaphase plate.
Stage 4- Anaphase: sister chromatids are pulled apart toward the spindle poles attached to them.
Stage 5- Telophase: chromosomes arrive at the poles of the spindle. Nuclear envelope reassembles around each set of chromosomes and the two nuclei are now formed.
Mitosis is followed by cytokenisis which divides the cytoplasm into two daughter cells each containing one nucleus. At the end of cytokinesis, we now have two daughter cells genetically identical to the parent cell.