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The neurons that are able to re-enter the cell cycle are much more likely to undergo apoptosis and lead to the disease phenotypes. In Alzheimer’s disease, affected neurons show signs of DNA replication such as phosphorylated Mcm2 and cell cycle regulators cyclin D, Cdk4, phosphorylated Rb, E2F1, and cyclin E.
The eukaryotic cell cycle consists of four distinct phases: G 1 phase, S phase (synthesis), G 2 phase (collectively known as interphase) and M phase (mitosis and cytokinesis). M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's nucleus divides, and cytokinesis, in which the cell's cytoplasm and cell membrane divides forming two daughter cells.
Neuroblasts asymmetrically divide during embryogenesis to create GMCs. [4] GMCs are only present in certain species and only during the embryonic and larval stages of life. Recent research has shown that there is an intermediate stage between a GMC and two neurons. The GMC forms two ganglion cells which then develop into neurons or glial cells. [5]
Mitotic exit is an important transition point that signifies the end of mitosis and the onset of new G1 phase for a cell, and the cell needs to rely on specific control mechanisms to ensure that once it exits mitosis, it never returns to mitosis until it has gone through G1, S, and G2 phases and passed all the necessary checkpoints.
In rodents for example, neurons in the central nervous system arise from three types of neural stem and progenitor cells: neuroepithelial cells, radial glial cells and basal progenitors, which go through three main divisions: symmetric proliferative division; asymmetric neurogenic division; and symmetric neurogenic division.
In this stage there is a cytoplasmic division that occurs at the end of either mitosis or meiosis. At this stage there is a resulting irreversible separation leading to two daughter cells. Cell division plays an important role in determining the fate of the cell. This is due to there being the possibility of an asymmetric division.
Mitosis in an animal cell (phases ordered counter-clockwise), with G 0 labeled at left. Many mammal cells , such as this 9x H neuron , remain permanently or semipermanently in G 0 . The G 0 phase describes a cellular state outside of the replicative cell cycle .
Translocation of cyclin B from the cytoplasm to the nucleus is necessary for cell division, but not sufficient, as its inhibitors do not allow the cell to enter mitosis prematurely. In addition to the back up inhibition of the cyclin B-Cdk1 complex, premature cellular division is prevented by the translocation of the cyclin B itself.