The first milestone in understanding the regulation of the cell cycle was the discovery in the early 1970s of a protein called maturation promoting factor (MPF). MPF appeared to control the initiation of mitosis. When injected into the nuclei of immature frog oocytes, which are normally arrested in G2, the cells immediately proceeded through mitosis. MPF was eventually found to consist of two proteins:  Cdc2 (also known as Cdk-1), a 32-kilodalton member of the Cdk family of proteins; and  Cyclin B, a 45-kilodalton member of the cyclin family, which are key regulators of the cell cycle. Cyclins are synthe sized as constitutive proteins; however, their levels during the cell cycle are controlled by ubiquitin-mediated degradation.
It is now known that the cyclin–Cdk complex acts at different phases of the cell cycle and targets different proteins to control cell-cycle–dependent functions. Table 3.1 shows the combination of the different types of cyclins with different types of Cdks and how interactions between these two proteins affect cells progressing through the cell cycle. Passage through the cell cycle requires an increase in cyclin–Cdk activity in some phases followed by the decline of that activity in other phases (Fig. 3.11). The increased activity of
FIGURE 3.11 • Regulation of the cell cycle by cyclin–Cdk complexes. This diagram shows the changing pattern of cyclin–Cdk activities during different phases of the cell cycle.
cyclin–Cdk is achieved by the stimulatory action of cyclins and is counterbalanced by the inhibitory action of proteins such as Inks (inhibitors of kinase), Cips (Cdk inhibitory proteins), and Kips (kinase inhibitory proteins).
Cell division is a crucial process that increases the number of cells, permits renewal of cell populations, and allows wound repair.
Mitosis is a process of chromosome segregation and nuclear division followed by cell division that produces two daughter cells with the same chromosome number and DNA content as the parent cell.
The term mitosis is used to describe the equal partitioning of replicated chromosomes and their genes into two identical groups. The process of cell division includes division of both the nucleus (karyokinesis) and the cytoplasm (cytokinesis). The process of cytokinesis results in distribution of nonnuclear organelles into two daughter cells. Before entering mitosis, cells duplicate their DNA. This phase of the cell cycle is called the S or synthesis phase. At the beginning of this phase, the chromosome number is (2n), and the DNA content is also (2d); at the end, the chromosome number remains the same (2n), and the DNA content doubles to (4d).
Mitosis follows the S phase of the cell cycle and is described in four phases.