Molecular and Cell Biology - The Cell Cycle (GC11) PDF

Molecular and Cell Biology The Cell Cycle Dr. John Th’ng GC11 [email protected] MCB.14: Understand the cell cycle and its regulation during cell division MCB.14.1. Identify the phases of the cell cycle and the proteins that regulate each of the phases MCB.14.2...

Molecular and Cell Biology The Cell Cycle Dr. John Th’ng GC11 [email protected] MCB.14: Understand the cell cycle and its regulation during cell division MCB.14.1. Identify the phases of the cell cycle and the proteins that regulate each of the phases MCB.14.2. Distinguish cell cycle checkpoints from the restriction point and the proteins involved in their regulation MCB.14.3. Describe the role of ATM/ATR kinases in the cell cycle for DNA repair  Cells in development Fertilized Egg Human Being Division ~60 Trillion Cells 1 Cell Zygote Differentiation >200 Types of Cells Cells in development The mammalian cell cycle: - how do cells divide The Cell Cycle: G1 S G2 M The Cell Cycle The phases of the cell cycle: G0,G1,S,G2,M Enzymes that regulate the cell cycle Checkpoint controls Mitosis Checkpoints for DNA damages Cell cycle phases G1 Cell cycle phases: G1: Growth (gap) phase - allow daughter cells to grow after cytokinesis - daughter cells do not end up with uniform sizes - prepare for S phase - monitor environment for favourable growth - monitor intercellular signals for growth - eg. growth factors, steroid hormones, etc. Cell cycle phases: G1: Growth (gap) phase - allow daughter cells to grow after cytokinesis - daughter cells do not end up with uniform sizes - prepare for S phase - monitor environment for favourable growth - monitor intercellular signals for growth - eg. growth factors, steroid hormones, etc. How do intercellular signals trigger the cell cycle? Cell cycle phases: G1: Growth (gap) phase - allow daughter cells to grow after cytokinesis - daughter cells do not end up with uniform sizes - prepare for S phase - monitor environment for favourable growth - monitor intercellular signals for growth - eg. growth factors, steroid hormones, etc. - variable in length between cell types - restriction point - commitment point to enter S phase and the cell cycle - dependant on favourable signals (eg. DNA damages?) Ras-Dependent Signal Transduction Signal transduction pathways Mitogen activation of cell division (EGF, etc.) Mitogen activation of cell division Transcription factor Cell cycle phases: G1: Growth phase - stimulated by external signals (eg. mitogens) - controlled by cyclin-dependent kinases (cdk) - family of >10 genes - only a few involved with regulating cell cycle - cdk2, cdk4, and cdk6 in G1 - regulated by regulatory proteins cyclin D and cyclin E - cdks are enzymatically inactive without cyclins Cell cycle phases: G1: Growth phase - cyclins undergo cycles of synthesis & degradation through the cell cycle to activate cdks Cyclin D Cyclin E Cell cycle phases: G1: Growth phase - cyclins undergo cycles of synthesis & degradation through the cell cycle to activate cdks Cell cycle phases: G1: Growth phase - driven by cyclin-dependent kinases (cdks) - inhibited by phosphorylations at active site - phosphorylated by Wee1 kinase to inactivate - at early G1 - dephosphorylated by cdc25 to activate Cell cycle phases: G1: Growth phase - activated cyclin-dependent kinases drive cell cycle by phosphorylating pRB (retinoblastoma protein) Retinoblastoma protein - pRB retinoblastoma protein - a major tumour suppressor - when pRB is hypophosphorylated, it binds to E2F transcription factor - prevents G1 progression into S - acts as the Restriction point - controls entry into cell cycle Retinoblastoma protein - pRB retinoblastoma protein - phosphorylated by cdk4/cyclin D, cdk6/cyclin D, and cdk2/cyclin E - release transcription factor E2F to bind promoters of genes for DNA replication - initiate transcription of genes for DNA replication - cyclins, PCNA, cdks, histones, etc. Cell cycle phases: G1: Growth phase - activated cyclin-dependent kinases drive cell cycle by phosphorylating pRB (retinoblastoma protein)  Mitogen activation of cell division Transcription factor * * * Cell cycle phases S & G2 Cell cycle phases: S: DNA Synthesis - replication of DNA - regulated by cdk2/cyclin A G2: Growth phase - cells build up cell contents for division - chromatin organize for mitosis - regulated by cdk2/cyclin A - monitor conditions are favorable - topoisomerase II complete untangling of chromosomes - check for DNA damages, chromosomes, etc. Cell cycle phases: G2/M: Checkpoint - cdk1 inactivated by wee1 kinase - activated by cdc25 phosphatase M: Mitosis - entry regulated by cdk1/cyclin B - also referred to as lamin kinase* - nuclear envelope breakdown* - chromosome condensation & cell division *See: MCB.13. Understand the components of the cytoskeleton and how they contribute to the functions of a cell Cell cycle phases M The Cell Cycle - Mitosis Mitosis S Phase APC/C  Mitosis Figure 20.7. Lippincott Cell and Molecular Biology M phase - have 46 pairs of chromosomes - mitosis - nuclear division - chromosome separation and division - cell division [cytokinesis] - division into two daughter cells - 23 pairs of chromosome per cell Telophase & Cytokinesis - dephosphorylation of lamins - reformation of nuclear membrane - decondensation of chromosomes - separation of daughter cells Cell cycle phases G0 Cell cycle phases: G0: Resting state - external signals to stop growth - adequate growth, etc. - shortened telomeres - withdraw from cell cycle - terminally differentiated cells - blood, neurons, muscles, skin, etc. - permanently in most cells - no synthesis of cell cycle proteins - cdks, cyclins - most cells eventually die (apoptosis) Cell cycle phases: G0: Resting state - some cells can re-enter cell cycle - stem cells, B & T lymphocytes, liver - driven by cdk3/cyclin C - phosphorylate pRB initially - followed by cdk4/6-cyclin D MCB.14: Understand the cell cycle and its regulation during cell division MCB.14.2. Distinguish cell cycle checkpoints from the restriction point and the proteins involved in their regulation Cell cycle phases: G1: Growth phase - controlled by cyclin-dependent kinases (cdk) - regulated by cyclins - phosphorylate pRB Cells have “brakes” to control cycle - “brakes” are the checkpoints What happens if no checkpoints? - uncontrolled cell growth - hyperplasia, neoplasia….. HYPERPLASIA - not cancer growth - reversible CANCER - invasive mode - irreversible Cell cycle phases: G1: Growth phase regulation - checkpoints & restriction points - at the restriction (R) point - commitment point for the cell cycle - if no mitogen, do not proceed with cell cycle - cdks remain inactive - pRB retinoblastoma protein is not phosphorylated - E2F remain bound Cell cycle phases: G1: Growth phase regulation - checkpoints & restriction points - cdks are inactived - pRB retinoblastoma protein is not phosphorylated - E2F remain bound Cell cycle phases: G1: Growth phase regulation - checkpoints & restriction points - p53 transcription factor - a major tumour suppressor - “Guardian of the Genome” - Nature (1992) - stimulate production of p21 (and others) - p21 inhibits cdk2 to arrest cell cycle - cdki – cdk inhibitor Cell cycle phases: G1: Growth phase regulation - checkpoints & restriction points - cdk inhibitors (cdki) - bind and inhibit cdk activities - stimulated by external signals to stop cell cycle - eg. DNA damages, differentiation, etc. - allow for DNA repair or apoptosis Cell cycle phases: G1: Growth phase regulation - checkpoints & restriction points - cdk inhibitors (cdki) - bind and inhibit cdk activities - CIP/KIP family [p21,p27,p57] - also referred to as p21WAF/CIP - inhibit cdk2 - INK4 family [p14,p16,p18,p19] - p16 inhibit cdk4/cdk6 Cell cycle phases: G1: Growth phase regulation Cell cycle phases: G1: Growth phase regulation Cell cycle phases: G2/M: entry into mitosis - regulated by cdk1/cyclin B - monitor conditions are favorable - topoisomerase II complete untangling of chromosomes - check for DNA damages, chromosomes, etc. - inactivation of cdc25 M phase Checkpoint: - at metaphase-anaphase transition - chromosomes attached to spindles? - chromosomes are aligned at metaphase plate? - prevent chromosome abnormalities - Anaphase-Promoting Complex (APC/C) - promote degradation of cyclin B - inactive cdk1 - trigger entry into anaphase Anaphase - normal vs. lagging chromosomes Lagging Normal  Cell cycle phases: Summary Figure 17–11 Cyclin–Cdk complexes of the cell-cycle control system.. Molecular Biology of the Cell Summary of regulation of interphase: - activities of cdk/cyclin complexes Lippincott Cell & Molecular Biology Cell Cycle Summary: Cell cycle phases & regulators Cell Cycle Checkpoint Summary Take home: Cell cycle regulatory genes: - conserved through evolution - cell cycle genes: GO proteins - cdks [1,2,3….14+], cyclins [A,B,C…L+] - checkpoints: STOP proteins - cki (p16,p21,p27,p57….) - potential link to cancer? - elevated cdks: cause or effect of cancer? - depressed checkpoints - targets for cancer therapy MCB.14. Understand the cell cycle and its regulation during cell division MCB.14.3. Describe the role of ATM/ATR kinases in the cell cycle for DNA repair Cell cycle checkpoint regulation - how do cells detect DNA damages - induce cell cycle arrest for DNA repair DNA damage checkpoint regulation - detection of DNA damage - sensor ATM/ATR kinase - bind to DNA damages - cell cycle arrest, DNA repair, death, etc. - induce p53 & p21, inactivate cdc25, etc.  Checkpoint regulation Figure 21.8. Lippincott Cell and Molecular Biology  Checkpoint regulation Figure 17-33, Molecular Biology of the Cell, 4th ed.  Checkpoint regulation Figure 17-33, Molecular Biology of the Cell, 4th ed. Summary of DNA repair checkpoints DNA damages - base modifications, replication errors, strand breaks (single & double), etc. - detected by sensors ATM/ATR - arrest the cell cycle at G1 and G2 - through activities of cdc25, p53, p21 - DNA repair or cell death (apoptosis) Summary of the Cell Cycle Cell cycle phases & progression Genes that regulate phases - cdks, cyclins - checkpoints - tumour suppressors p53 & pRB Mitosis DNA repair checkpoints https://educationalgames.nobelprize.org/educati onal/medicine/2001/