Growth and reproduction of eukaryotes depend on the cellular life cycle (cell cycle) whereby the cell duplicates its components to physically split into two identical daughter cells. In general, the cell cycle is divided into two phases: interphase where cell growth and DNA replication takes place, and mitosis where the duplicated DNA is divided into two daughter cells.The interphase of the cell cycle is further sub-divided into three discrete phases: G1, S and G2. During interphase, the cell is metabolically active and has distinct biochemical processes that prepares the cells for the cell division.
G1 phase ( or gap 1 phase) of the cell cycle corresponds to the gap between mitosis and initiation of DNA replication in the subsequent S phase (synthesis phase). G1 is most often the longest phase of the cell cycle during which the cell grows continuously and makes proteins that are required for DNA replication. In S phase, all the chromosomes are replicated following which histones and other proteins are recruited to the chromosomes to form sister chromatids. The sister chromatids are joined by a common region known as centromere and to ensure faithful DNA segregation, the sister chromatids remain physically connected to each other with the help of cohesins, from the time of their synthesis in S phase until late M phase (or mitosis) (reviewed in ). Outside nucleus, centrosome, the microtubule organising centre of the cells, also duplicates to form two centrosomes. The S phase of the cell cycle is immediately followed by G2 phase (or gap 2 phase) where the cell prepares itself for mitosis by synthesizing variety of protein that are required during the next phase.
2 Introduction 1.1.1 Mitosis
Mitosis is the most dram...
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...early stages of DNA synthesis and during the late stages of DNA replication, CDK2 is subsequently activated by S-phase cyclins (cyclin A) to drive the transition from S to the G2 phase .
Finally, at the onset of mitosis, S-phase cyclins are degraded and CDK1–cyclin B becomes active as a result of dephosphorylation of specific threonine and tyrosine residues within the adenosine triphosphate (ATP) binding site of CDK1 (reviewed in ). The CDK1–cyclin B complex governs most the mitotic processes like separation of centrosomes, spindle pole formation, chromosome condensation, microtubule kinetochore attachment and cytokinesis and hence this complex is also known as M phase-promoting factor (MPF). Upon, mitotic exit, the CDK1–cyclin B complex is inactivated from the proteolytic destruction of the cyclin B through a ubiquitin-dependent proteolytic degradation process.
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