2. Regulated Protein Phosphorylation and Degradation Control Passage through the Cell Cycle cyclin Any of several related proteins whose concentrations rise and fall during the course of the eukaryotic cell cycle. Cyclins form complexes with cyclin-dependent kinases, thereby activating and determining the substrate specificity of these enzymes. cyclin-dependent kinase (Cdk) A protein kinase that is catalytically active only when bound to a cyclin. Various Cdk-cyclin complexes trigger progression through different stages of the eukaryotic cell cycle by phosphorylating specific target proteins.
In vertebrates and diploid yeasts, cells in G1 have a diploid number of chromosomes (2n), one inherited from each parent.
In haploid yeasts, cells in G1 have one of each chromosome (1n).
D. Molecular Mechanisms for Regulating Mitotic Events E. Cell-Cycle Control in Mammalian Cells
A. The Cell Cycle and Its Control Cell: cycling cell and quiescent cell (G0 state). Cell cycle:
Chapter 9: Regulation of the Eukaryotic Cell Cycle
A. The Cell Cycle and Its Control B. Bioche and Early Embryos
C. Genetic Studies with S. pombe
Where are Postmitotic cells going?
Postmitotic cells in multicellular organisms can "exit" the cell cycle and remain for days, weeks, or in some cases (e.g., nerve cells and cells of the eye lens) even the lifetime of the organism without proliferating further.
1. The Cell Cycle Is an Ordered Series of Events Leading to Replication of Cells
Cell division
Life Cycle of a Cell.
Diagram showing the three sets of microtubules (MTs) in the mitotic apparatus.
Current model for regulation of the eukaryotic cell cycle. ( Illustration )
Passage through the cycle is controlled by G1, S-phase, and mitotic cyclin-dependent kinase complexes (CdkCs) highlighted in green. These are composed of a regulatory cyclin subunit and a catalytic cyclin-dependent kinase subunit. Protein complexes (orange) in the Cdc34 pathway and APC pathway polyubiquitinate specific substrates including the S-phase inhibitor, anaphase inhibitor, and mitotic cyclins, marking these substrates for degradation by proteasomes. These pathways thus drive the cycle in one direction because of the irreversibility of protein degradation. Proteolysis of anaphase inhibitors inactivates the protein complexes that connect sister chromatids at metaphase, thereby initiating anaphase.
Interphase ( I ), comprises the G1, S, and G2 phases. Mitotic phase (M), comprises the prophase, metaphase, anaphase and telophase.
The eukaryotic cell cycle.
There are three classes of cyclin-Cdk complexes that control passage through the cell cycle:
1. the G1 Cdk complexes; 2. the S-phase Cdk complexes; 3. the mitotic Cdk complexes.
cell cycle Ordered sequence of events in which a cell duplicates its chromosomes and divides into two. Most eukaryotic cell cycles can be commonly divided into four phases: G1 before DNA synthesis occurs; S when DNA replication occurs; G2 after DNA synthesis; and M when cell division occurs, yielding two daughter cells. Under certain conditions, cells exit the cell cycle during G1 and remain in the G0 state as nongrowing, nondividing (quiescent) cells. Appropriate stimulation of such cells induces them to return to G1 and resume growth and division. cell division Separation of a cell into two daughter cells. In higher eukaryotes, it involves division of the nucleus (mitosis) and of the cytoplasm (cytokinesis); mitosis often is used to refer to both nuclear and cytoplasmic division.
Most postmitotic cells in vertebrates exit the cell cycle in G1, entering a phase called G0 . G0 cells returning to the cell cycle enter into the S phase; this reentry is regulated, thereby providing control of cell proliferation.
Rapidly replicating human cells progress through the full cell cycle in about 24 hours: mitosis : 30 minutes;
G1: 9 hours;
the S phase : 10 hours; G2 : 4.5 hours. In contrast, the full cycle takes only ≈90 minutes in rapidly growing yeast cells.
Function of the G1 Cdk complexes
When cells are stimulated to replicate, G1 Cdk complexes are expressed first. These prepare the cell for the S phase by activating transcription factors that cause expression of enzymes required for DNA synthesis and the genes encoding S-phase Cdk complexes. The activity of S-phase Cdk complexes is initially held in check by a specific inhibitor. Then, in late G1, G1 Cdk complexes induce the degradation of the S-phase inhibitor, releasing the activity of the S-phase Cdk complexes, which stimulate entry into the S phase.