THE CELL CYCLE AND PROGRAMED CELL DEATH The minimum set of processes that a cell has to perform are those that allow it to pass on its genetic information to the next generation of cells Molecular Biology of Cancer 1

2 Universal characteristics of cell cycle

The two major phases of the cell cycle Molecular Biology of Cancer3

The eucaryotic cell cycle is traditionally divided into four sequential phases Two functional phases, S and M phases Two preparatory phases, G1 and G2 Molecular Biology of Cancer4

Two functional phases, S and M phases S phase:  The phase in which the DNA is replicated.  The time it takes a typical human cell to complete S phase is about 8 hours and is invariant under normal circumstances. M phase:  Fully replicated chromosomes are segregated to each of the two daughter nuclei by the process of mitosis.  The length of M phase is about 1 hour and is also normally invariant. Molecular Biology of Cancer5

Two preparatory phases, G1 and G2 G1 phase precedes S phase, whereas G2 phase precedes M phase.  G1 and G2 phases are required for the synthesis of cellular constituents needed to support the following phase and ultimately to complete cell division.  In mammalian cells, the length of G1 phase is highly variable and can range from about 6 hours to several days or longer.  The length of G2 phase is about 2 hours. Molecular Biology of Cancer6

Two preparatory phases, G1 and G2 The cell monitors the internal and external environment  When conditions are suitable and preparations are complete the cell commits itself to S or M phase. If extracellular conditions are unfavourable: G 0 (G zero).  cells delay progress through G1 and may even enter a specialized resting state known as G 0 (G zero).  Such cells are metabolically active, but are not actively proliferating. If extracellular conditions are favourable: G1G0  cells in early G1 or G0, become committed to DNA replication, even if the extracellular signals that stimulate cell growth and division are removed Molecular Biology of Cancer7

CELL-CYCLE CONTROL A successful cell division cycle requires the orderly and unidirectional transition from one cell-cycle phase to the next. Certain events must be completed before others are begun.  For example, beginning mitosis before the completion of DNA replication would obviously be deleterious to the cell. Molecular Biology of Cancer8

Two classes of cell cycle regulatory circuits exist 1. Intrinsic regulatory pathways: Responsible for the precise ordering of cell-cycle events.  for example, entry into mitosis must always come after DNA replication. A clock, or timer, that turns on each event at a specific time  It provides a fixed amount of time for the completion of each event.  Predominate in the transitions between S, G2, and M phases in mammalian cells because there time is relatively invariant. Molecular Biology of Cancer9

Deregulation of intrinsic regulatory pathways can contribute to cancer For example, errors in the spindle-assembly checkpoint can lead to chromosomal imbalance and aneuploidy, a feature characteristic of virtually all cancers. Misregulation of proteins that control this checkpoint has been detected in human cancer Molecular Biology of Cancer10

Two classes of cell cycle regulatory circuits exist 2. Extrinsic regulatory pathways: Function in response to environmental conditions or in response to detected cell- cycle defects. In these pathways, differences between normal and neoplastic cells are most commonly observed. Molecular Biology of Cancer11

The cell cycle control system is a protein kinase based machine The cell cycle control system is based on two families of proteins: 1. Cyclin-dependent protein kinase (Cdk) Selectively phosphorylates downstream proteins on serines and threonines (cyclins) 2. The specialized activating proteins (cyclins) Cdk They bind to Cdk molecules and control their ability to phosphorylate target proteins They undergo a cycle of synthesis and degradation in each division cycle of the cell Molecular Biology of Cancer12

Classes of cyclins: 1. G1/S-cyclinsCdks G1 1. G1/S-cyclins bind Cdks at the end of G1 and commit the cell to DNA replication. 2. S-cyclinsCdks S 2. S-cyclins bind Cdks during S phase and are required for the initiation of DNA replication. 3. M-cyclins 3. M-cyclins promote the events of mitosis. G1-cyclins 4. In most cells, a fourth class of cyclins, the G1-cyclins, helps promote passage through Start or the restriction point in late G1. Molecular Biology of Cancer13

Classes of Cdk protein Cdk cyclins In yeast cells A single Cdk protein binds all classes of cyclins  It drives all cell-cycle events by changing cyclin partners at different stages of the cycle. In vertebrate cells, by contrast, there are four Cdks. G1-cyclins  Two interact with G1-cyclins G1/S-S-cyclins  One interacts with G1/S- and S-cyclins M-cyclins  One interacts with M-cyclins. Molecular Biology of Cancer14

The names of the individual Cdks and cyclins Molecular Biology of Cancer15CYCLIN-CDKCOMPLEXVERTEBRATES BUDDING YEAST CYCLIN CDK PARTNER CYCLIN G 1 -Cdk cyclin D*Cdk4, Cdk6Cln3Cdk1** G 1 /S-Cdk cyclin ECdk2Cln1, 2Cdk1 S-Cdkcyclin ACdk2Clb5, 6Cdk1 M-Cdkcyclin BCdk1**Clb1, 2, 3, 4Cdk1 * There are three D cyclins in mammals (cyclins D1, D2, and D3).

Regulation of Cdk activity at different stages of cell cycle: Molecular Biology of Cancer16

Phosphorylation/Dephosphorylation at a pair of aa in the roof of the active site: Molecular Biology of Cancer17 M- Cdk This regulatory mechanism is particularly important in the control of M- Cdk activity at the onset of mitosis.

Cdk inhibitor proteins (CKIs) bind to and regulate Cyclin-Cdk complexes Molecular Biology of Cancer18 CKI G1 S There are a variety of CKI proteins that are primarily employed in the control of G1 and S phase.

The Cell-Cycle Control System Depends on Cyclical Proteolysis Two ubiquitin ligases are important in the destruction of cyclins and other cell-cycle regulators: 1. SCFubiquitylation and destruction 1. SCF enzyme complex responsible for the ubiquitylation and destruction of: – G1/S-cyclins CKI proteins S-phase – G1/S-cyclins and certain CKI proteins that control S-phase initiation 2. The anaphase-promoting complex (APC) ubiquitylation and proteolysis 2. The anaphase-promoting complex (APC) In M phase, is responsible for the ubiquitylation and proteolysis of: – M-cyclins and regulators of mitosis – M-cyclins and other regulators of mitosis Molecular Biology of Cancer19

Molecular Biology of Cancer20

Molecular Biology of Cancer21