Cell Cycle1
Introduction
As in prokaryotes, Eukaryotic DNA replication is restricted to either Mitosis or Meiosis stages. Mitosis is used for growth and development and in some lower forms it is one of the modes of reproduction. But meiosis is mostly involved in reproductive stages. Whether Mitosis or Meiosis, cell division is highly regulated and precise and exact. Mitosis goes through several stages such as Prophase, Metaphase, Anaphase, Telophase and cytokinesis (not always) and then enters Interphase, which is an intervening stage at which the cell prepares for the next division or goes into resting phase where the cells undergo differentiation to specific cell type.
The above photomicrograph shows yeast S.pombe is going through cell division.
- Among the five stages in a 24hrs cell cycle animal cell culture, Interphase occupies longest period where as the other stage called M-stage lasts just about 0.30 min to 1 hr. But in embryonic stages the cell cycle is just 15 –30 minutes or less; in this the cell after M-phase, enters directly to S-phase. It also means all the inputs required for the next stage are already existing in embryonic cells. In S.cerevisiae, G1 last for 15 min, S-phase requires 30 min and M-takes about 75 min all together require 120 min. While S.pombe (fission yeast), G-1 requires 20 min, S takes 20 min, G2-takes 35 min and M-phase lasts for 45 min, I n all 120 min. In both the above-mentioned forms cell divides without dissolution of nuclear membrane.
This figure shows different stages a cell undergoes during cell division. RESTRICTION (R) is the restriction point or some times it is called START point, which is the major checkpoint. Cell cycle control by cyclin-Cdk; http://csls-text3.c.u-tokyo.ac.jp/
Interphase consists of sub-stages such as G1, S and G2; where, G at earlier times stands for gap in the knowledge about these stages. In 24 hr cell cycle events G1 occupies 10-12 hrs, S-stage about 6-8 hr and G2 stage 4-4.5 hr. The G1 phase is considered as preparatory phase for DNA replication, but the Cells escape from G1 phase in terminally differentiating cells into what is called Go stage, where cells assume specific shape, structure and function. But some of the cells remain embryonic and such cells can be stimulated to become dividing cells by some mitogens, and they can be differentiated depending upon the kind of stimulus they get, or stimulus provided. They are called STEM cells. Such cells are found not only in animal tissues but also in plant tissues, in fact plant cells have greater potentiality to be mitotic. Such cells can be stimulated by mitogens to enter into cell division mode, where they enter again into G1 phase. The S-phase is for DNA replication and G2 stage is a preparatory phase for M-phase, where the nucleus disassembles, chromatids separate and mitotic apparatus assembles, the centromere split, sister chromatids are pulled to their respective poles, daughter nuclei reform and cytokinesis leads to division of cytoplasm into two cells. This is a simplistic description of cell division.
- Each of these phases are regulated by specific molecular events and factors, until each of the events in each of the phases are completed cell won’t enter in to next stage. Thus the entry and exit to and out of stages or phases is tightly controlled by a variety of factors; and the factors themselves undergo changes in their turn over (synthesis and degradation), activation and inactivation of them.
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In classical experiments as shown above shows that certain molecular events during each of the stages generate a set of factors and they are responsible for executing the stage and perhaps provide signals for the next stage. For example when a cell in S-stage is fused with G1 stage, the cell in G1 stage is stimulated to proceed into S-phase. But if a cell in S-stage is fused with a cell at G2 stage, nothing happens, which means the components found in S-phase cells have no effect on G2, because the cells at G2 cells have already achieved what the S-phase components can provide.. Fusion between G1 and G2 does not result in any changes in each of them. But if a an Interphase cell is fused with a cell at M stage the Interphase cells directly enter into M-phase with disastrous consequences. The Interphase cell is not yet competent to enter into M phase, but M phase cells have all the components for chromosomal separation. So there is regulation at each of the entry points called check points, which is tightly regulated.
Cell cycle control by cyclin-CDK; http://csls-text3.c.u-tokyo.ac.jp/
The major checkpoints lie in between G1 and S phase and G2 and M-phase and another control point exists within the M-phase events at anaphase. Not withstanding the said checkpoints, DNA damage can introduce a checkpoint, where until the damage is repaired, cell does not enter M-phase, this can happen at S-phase or at G2 phase; if the damage is beyond repair the cell is signaled for Apoptosis.
Mechanism of cyclin-CDK activity regulation; http://csls-text3.c.u-tokyo.ac.jp/