CELL CYCLE AND CELL CYCLE ENGINE OVERVIEW Fahareen-Binta-Mosharraf MIC-404 1

To make multicellular organisms cell must communicate. This communication is mediated by extracellular signal molecules. Sophisticated mechanisms control which signal molecules are released from a specific type of cell, at what time and concentration they are secreted, and how these signals are interpreted by the target cells Some signalling molecules act over long distances, some act only on the immediate neighbour cells Most cells in higher organisms are both emitters and receivers of signals 2

An animal cell's dependence on multiple extracellular signals. Each cell type displays a set of receptors that enables it to respond to a corresponding set of signal molecules produced by other cells. These signal molecules work in combinations to regulate the behaviour of the cell. As shown here, an individual cell requires multiple signals to survive (blue arrows) and additional signals to divide (red arrow) or differentiate (green arrows). If deprived of appropriate survival signals, a cell will undergo a form of cell suicide known as programmed cell death, or apoptosis. 3

Cell Life Cycle The most basic function of the cell cycle is to duplicate accurately the vast amount of DNA in the chromosomes and then segregate the copies precisely into two genetically identical daughter cells 4

The Cell Cycle -all the events that occur when a cell divides -time required can vary from minutes to days, depending on the cell Salmonella = 29 min Red B. C. = 120 days 5

6

 G1 Phase (Gap 1 Phase) cells undergo normal cell activity can get stuck in this stage (G0-resting stage) active but not dividing the cell increase in size and prepares to copy DNA  S phase (Synthesis Phase) chromosomes get duplicated  G2 Phase (Gap 2 Phase) cell makes proteins needed for mitosis (M phase)  M phase (Mitosis phase) Duplicated chromosomes are separated go into two daughter cells (cytokinesis) 7

INTERPHASE(G1-S-G2) and MITOSIS-M Phases 8

The Only Cell Cycle State that Lacks both Cdk and cyclin 9

Cell Restriction Point(R Phase) Cell decides whether to commit itself to complete the cycle G1 check point G2 check point M check point 10

11

Cell cycle clock Capacity of cell to grow and divide is control is controlled by a set of proteins which integrate the inhibitory and stimulatory signals regulated by or produced within the cell. 12

Cell Cycle Engine Components 13

Major Cell Cycle Regulatory Proteins 1. Cyclin-dependent kinases (Cdk) first discovered in yeast 2. Different kinds of cyclins; levels oscillate at different stages of cycle Control mechanisms A. Availability of cyclins varies B. Cdk must be phosphorylated Cyclin and Cdk must be bound together to be active Initial cyclin-Cdk complex is inactive A series of phosphorylation and dephos-phorylation steps make it active Complex is called MPF (mitosis-promoting factor) 14

MPF - mitotic promoting factor... [ complex of two proteins cdk + cyclin] MPF is a kinase enzyme, one that switches on/off target cell cycle proteins by phosphorylating them 15

cdk - a cell division control protein - cyclin dependent kinase; active only when bound to cyclin; cyclin - a protein whose amount varies cyclically; when in high concentrations, binds to cdk makes MPF 16

Cyclins 1. a G1 cyclins (D cyclins) 2. S-phase cyclins (cyclins E and A) 3. mitotic cyclins (B cyclins) Cyclin-dependent kinases (Cdks) 1. a G1 Cdk (Cdk4,6) 2. The late G1/ S-phase Cdk (Cdk2) 3. an M-phase Cdk (Cdk1) 17

3.Rb protein Transcriptional repressor Its function requires binding of Rb with Histone Deacetylase (Hdac) Important protein whose phosphorylation is required for G1 to S phase transition through restriction point. 18

Rb-Hdac complex 19

Major Cell Cycle Gene Regulatory Proteins 1. E2f-Dp1 Transcriptional activator Coded by Myc (c-Myc) gene promotes transcription of genes (cyclin E,A Cdk 2 and E2f itself) requires for G1/S transition dimerization partner DP1or TFDP1 After the cell pass the G1 restriction point E2f initiate the transcription of Ink4 family proteins which inhibit Cyclin D and Cdk 4 and 6 and block the ability of cell to respond to further environmental response. 20

Rb –E2f sequester activity 21

22

2. p53 Promotes transcription of genes those are involved in cell cycle arrest (Cdk inhibitory proteins-CKI) in minor cell damage Promotes apoptosis in response to DNA damage or cell stress in major case p53 is suppressed by protein Mdm2 23

24

Major cell cycle Cdk inhibitory proteins (CIK) 25

Molecular Switch For a cell to pass through R and enters S a molecular switch must be turned ON from OFF state Mitogens stimulate G1-Cdk and G1/S- Cdk activities A simplified model of one way that mitogens stimulate cell division (cell signalling) 26

ON of molecular switch 1. With appropriate growth signal the myc gene in nucleus is activated. myc is a regulatory gene having multiple functions 2. Myc gene codes for E2f-Dp1(transcriptional activator) control transcription of cyclin D and later cyclin E 3. p53 with Mdm2 complex codes for CIK is degraded by ubiquitylation coded by SCF genes controlled by myc. 27

Ubiquitylation of p53 28

4. Level of cyclin D and later E increase and activate enzymes Cdks 5. MPF - mitotic promoting factor the complex of two proteins cdk + cyclin,grabs phosphate group from ATP and transfer of pRB-master break of cell cycle clock 6.Phosphorylated pRB release E2f-Dp1 complex-the transcription factor 7. Induce the production of proteins required for continued progression to cell cycle 29

30

OFF of molecular switch In case of cell minor damage 1. p53 halts cell cycle until the damage repaired 2. Mdm2 protein is released from p53 3. p53 releases two KIP proteins p21 and p27 and one major INK protein p p16 inhibits the activity of Cdk inG1 phase 5. p21 suppresses activity of G1/S Cdks and S Cdks 6. p27 is the characteristics of quiescent cell. Also suppresses activity of G1/S Cdks and S Cdks 7. Rb protein reamains dephosphorylated and cell cycle halts. 31

Regulation of Cell Cycle by p53 Cell 2004;116: INK4 (Rb) 32 (activation of p21 and p27)

Cell cycle control 33

34

35