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Cell division, cell growth, cell Cycle • Interphase and meiosis I INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I 2. cross over Centrosomes Centromere Sister chromatids (with centriole pairs) (with kinetochore) remain attached Chiasmata Metaphase Sister chromatids plate Spindle Nuclear Microtubule Homologous envelope Tetrad attached to chromosomes Chromatin kinetochore separate Chromosomes duplicate Tertads line up Pairs of homologous Homologous chromosomes chromosomes split up (red and blue) pair and exchange Figure 13.8 segments; 2n = 6 in this example 1. Synapsis (聯會) (synaptonemal complex) • Telophase I, cytokinesis, and meiosis II MEIOSIS II: Separates sister chromatids TELOPHASE I AND PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS CYTOKINESIS Cleavage Haploid daughter cells furrow Sister chromatids forming separate Two haploid cells form; chromosomes During another round of cell division, the sister chromatids finally separate; Figure 13.8 are still double four haploid daughter cells result, containing single chromosomes • A comparison of mitosis and meiosis MITOSIS MEIOSIS Parent cell Chiasma (site of MEIOSIS I (before chromosome replication) crossing over) Prophase Prophase I Chromosome Chromosome replication replication Tetrad formed by Duplicated chromosome 2n = 6 synapsis of homologous (two sister chromatids) chromosomes Chromosomes Tetrads positioned at the positioned at the Metaphase I Metaphase metaphase plate metaphase plate Anaphase Sister chromatids Homologues Telophase separate during separate Anaphase I anaphase during Telophase I anaphase I; sister Haploid chromatids Daughter n=3 remain together cells of meiosis I 2n 2n Daughter cells MEIOSIS II of mitosis n n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II Cell cycle: --- the life of a cell from the time it is first formed from a dividing parent cell until its own division into two cells. Smallest unit of life all living things must reproduce Cells replicate for growth, replacement, and repair Cell division functions in reproduction, growth, and renewal. 200 µm 20 µm Cell Cycle The Cell’s Time Clock • Cell division requires Mitosis & Cytokinesis Cytokinesis • Phases of a dividing cell’s life – interphase • cell grows • replicates chromosomes • produces new organelles & biomolecules – mitotic phase • cell separates & divides chromosomes – mitosis • cell divides cytoplasm & organelles – cytokinesis Cell cycle • Cell has a “life cycle” cell is formed from a mitotic division cell grows & matures cell grows & matures to divide again to never divide again G1, S, G2, M liver cells G0 epithelial cells, brain nerve cells blood cells, stem cells • Cell performs normal Interphase function • Three subphases: – G1: cell duplicates most organelles – S: quantity of DNA in the cell is doubled as chromosomes are replicated. Each chromosome has a pair of sister chromatids connected by a centromere that contains a kinetochore – G2: chemical components stockpiled • Nucleolus present Mitosis • Nuclear division • Mitotic events can be without a reduction in categorized into discrete stages based chromosome number on what is happening to • Each new cell structure of the cell (daughter cell) will • Stage include: have the same – Prophase • Prometaphase quantity of DNA as the – Metaphase parental cell – Anaphase • Why is this important? – Telophase Prophase (Including Prometaphase) • Pro • Three things visibly occur – Chromosomes condense (shorten) – Centrosomes migrate to the poles while producing spindle fibers – Nuclear membrane fragments Metaphase Metaphase Plate • Meta • Chromosomes are moved by growing spindle fibers to the equator of the cell (metaphase plate) • Centrosomes are at the poles, nuclear membrane is gone Anaphase • Ana • Centromere splits into two • Spindle fibers shorten from kinetochore end separating sister chromatids • Activated kinetochores “pull” chromatids along the spindle fibers and toward the poles Telophase • Telo • Nuclear membrane reforms around each region of chromosomes • Nucleolus reforms • Cytokinesis (division of the cytoplasm) may occur Cytokinesis May Vary Between Major Taxonomic Groups Cytokinesis divides the cytoplasm * Cleavage furrow * No cleavage furrow Cleavage furrow 100 µm Actin Vesicles Wall of 1 µm + forming patent cell Cell plate New cell wall cell plate Myosin Contractile ring of Daughter cells microfilaments Daughter cells (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM) Regulation of Cell Division 2006-2007 Coordination of cell division • A multicellular organism needs to coordinate cell division across different tissues & organs – critical for normal growth, development & maintenance • coordinate timing of cell division • coordinate rates of cell division • not all cells can have the same cell cycle Activation of cell division • How do cells know when to divide? – cell communication signals • chemical signals in cytoplasm give cue • signals usually mean proteins – activators – inhibitors experimental evidence: Can you explain this? Frequency of cell division • Frequency of cell division varies by cell type – embryo • cell cycle < 20 minute – skin cells • divide frequently throughout life M • 12-24 hours cycle metaphase anaphase – liver cells prophase telophase C • retain ability to divide, but keep it in G2 reserve • divide once every year or two interphase (G1, S, G2 phases) mitosis (M) – mature nerve cells & muscle cells cytokinesis (C) G1 S • do not divide at all after maturity • permanently in G0 There’s no Overview of Cell Cycle Control turning back, now! • Two irreversible points in cell cycle – replication of genetic material – separation of sister chromatids • Checkpoints – process is assessed & possibly halted sister chromatids centromere single-stranded double-stranded chromosomes chromosomes Cell Cycle Regulation • Cell cycle events are triggered by the cell- cycle control system; a set of molecules found in the cytoplasm affected by internal and external controls • Checkpoints in G1, G2, and M phases of the cycle • G1 checkpoint is most critical. May throw cells out of cyclic phase into G0, never to divide again Other Internal and External Factors • Internal – M checkpoint does not proceed until signal is received that all kinetochores are attached to spindle microtubules • External – Growth factors: cycle will not proceed if requirements are not met – Social signals • Density-dependent inhibition: under crowded conditions chemical requirements are insufficient to allow cell growth • Anchorage dependence: some cells must be attached to a substrate in order to replicate – DNA damage inhibits growth External signals: ex. Growth factors ~ Cells fail to divide if an essential nutrient is left out of the culture medium. ~ GFs trigger a signal transduction pathway that allows the cells to pass the G1 checkpoint and divide. PDGF PDGF receptor cell Signal transduction Cell division External signals • Growth factors – coordination between cells – protein signals released by body cells that stimulate other cells to divide • density-dependent inhibition – crowded cells stop dividing – each cell binds a bit of growth factor » not enough activator left to trigger division in any one cell • anchorage dependence – to divide cells must be attached to a substrate » “touch sensor” receptors External signals: physical factor Density-dependent inhibition of cell division ~ Crowded cells stop Cells anchor to dish surface and dividing divide (anchorage dependence). When cells have formed a complete single layer single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the gap and then stop (density- dependent inhibition). 25 µm • Most animal cells exhibit anchorage dependence – In which they must be attached to a substratum to divide Anchorage dependence * Cancer cells: Normal cell ~ single layer ~ Exhibit neither density- Cancer cells do not exhibit anchorage dependence or density-dependent dependent inhibition nor inhibition. anchorage dependence 25 µm 25 µm Growth factor signals growth factor nuclear pore nuclear membrane P P cell division cell surface Cdk receptor protein kinase P E2F cascade chromosome P Rb F P E2 Rb nucleus cytoplasm Internal signal of a Growth Factor • Platelet Derived Growth Factor (PDGF) – made by platelets in blood clots – binding of PDGF to cell receptors stimulates cell division in fibroblast (connective tissue) • heal wounds Don’t forget to mention erythropoietin! (EPO) The sequential events of the cell cycle are directed by a distinct cell cycle control system, a cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle. G1 checkpoint ~ similar to a clock The cell cycle is regulated Control at certain checkpoints by system S both internal and external G1 controls. G2 M M checkpoint G2 checkpoint Checkpoint control system • Checkpoints – cell cycle controlled by STOP & GO chemical signals at critical points – signals indicate if key cellular processes have been completed correctly Checkpoint control system • 3 major checkpoints: – G1/S • can DNA synthesis begin? – G2/M • has DNA synthesis been completed correctly? • commitment to mitosis – spindle checkpoint • are all chromosomes attached to spindle? • can sister chromatids separate correctly? Spindle checkpoint G2 / M checkpoint Chromosomes attached at • Replication completed metaphase plate • DNA integrity Inactive Active Active Inactive Cdk / G2 M APC cytokinesis cyclin (MPF) C G2 mitosis G1 S Cdk / G1 cyclin Inactive MPF = Mitosis Active Promoting Factor G1 / S checkpoint • Growth factors APC = Anaphase • Nutritional state of cell Promoting Complex • Size of cell G1/S checkpoint • G1/S checkpoint is most critical – primary decision point • “restriction point” – if cell receives “GO” signal, it divides • internal signals: cell growth (size), cell nutrition • external signals: “growth factors” – if cell does not receive signal, it exits cycle & switches to G0 phase • non-dividing, working state G0 phase • G0 phase – non-dividing, differentiated state – most human cells in G0 phase § liver cells § in G0, but can be “called back” to cell cycle by external cues § nerve & muscle cells § highly specialized; arrested in G0 & can never divide Cell Cycle Checkpoints • If cell size inadequate – G1 or G2 arrest • If nutrient supply inadequate – G1 arrest • If an essential external stimulus is lacking – G1 arrest (at R) • If the DNA is not replicated – S arrest • If DNA damage is detected – G1 or G2 arrest • If the spindle formation is improper, chromosome misalignment R – M-phase arrest “Go-ahead” signals • Protein signals that promote cell growth & division – internal signals • “promoting factors” – external signals • “growth factors” • Primary mechanism of control – phosphorylation • kinase enzymes • either activates or inactivates cell signals inactivated Cdk Cell cycle signals • Cell cycle controls – cyclins • regulatory proteins • levels cycle in the cell – Cdk’s • cyclin-dependent kinases activated Cdk • phosphorylates cellular proteins – activates or inactivates proteins – Cdk-cyclin complex • triggers passage through different stages of cell cycle Types of Cyclins and Cdks • There are many types of cyclins, but the 4 main ones are: – Cyclin D (G1 cyclin) – Cyclin E (S-phase cyclin) – Cyclin A (S-phase and mitotic cyclin) – Cyclin B (mitotic cyclin) • These are the 3 main cdks – Cdk4 (G1 Cdk) – Cdk2 (S-phase Cdk) – Cdk1 (mitotic Cdk) • The complex of Cdk1 and cyclin B is called mitosis promoting factor (MPF) a.k.a maturation promoting factor Rise and fall of cyclins Cyclin Concentration Mitosis Cdks and cyclins Cyclin-dependent kinases (Cdks) are enzymes that are present in the cell cytoplasm at all times. However, they are inactive unless they are bound by a specific partner-protein called a cyclin to form a Cdk-cyclin complex The amount of cyclins in the cell changes – because they get degraded A Cdk-cyclin complex will push the cell cycle forward. Figure 19-35 Phosphorylation and Dephosphorylation in the Activation of a Cdk-Cyclin Complex MPF: M-phase Promoting Factor • MPF is composed of two key subunits: Cdc2 and Cyclin B. – Cdc2 is the protein that encoded by genes which are required for passage through START as well as for entry into mitosis. – Cyclin B is a regulatory subunit required for catalytic activity of the Cdc2 protein kinase. What does MPF do? The complex of Cdk1 and cyclin B is called mitosis promoting factor (MPF) MPF activity is dependent upon Cyclin B • The cyclins were identified as proteins that accumulate throughout interphase and are rapidly degraded toward the end of mitosis. • It is suggested that they might function to induce mitosis, with their periodic accumulation and destruction controlling entry and exit from M phase. MPF activity is dependent upon Cyclin B • Accumulation and degradation of cyclins Figure 19-34 Fluctuating Levels of Mitotic Cyclin and MPF During the Cell Cycle MPF regulation • Cdc2 forms complexes with cyclin B during S and G2. • Cdc2 is then phosphorylated on threonine-161, which is required for Cdc2 activity, as well as on tyrosine-15 (and threonine-14 in vertebrate cells), which inhibits Cdc2 activity. Dephosphorylation of Thr14 and Tyr15 activates MPF at the G2 to M transition. • MPF activity is then terminated toward the end of mitosis by proteolytic degradation of cyclin B. MPF regulation • Demonstration of regulation of MPF Figure 19-40 A General Model for Cell Cycle Regulation 1970s-’80s | 2001 Cyclins & Cdks • Interaction of Cdk’s & different cyclins triggers the stages of the cell cycle Leland H. Hartwell Tim Hunt Sir Paul Nurse checkpoints Cdks cyclins • external signals is density-dependent inhibition, in which crowded cells stop dividing but lost of contact inhibition and outgrowth in cancer cells Tumors • Mass of abnormal cells – Benign tumor • abnormal cells remain at original site as a lump – p53 has halted cell divisions • most do not cause serious problems & can be removed by surgery – Malignant tumors • cells leave original site – lose attachment to nearby cells – carried by blood & lymph system to other tissues – start more tumors = metastasis • impair functions of organs throughout body Tumors • Benign - A spontaneous growth of tissue which forms an abnormal mass is called a tumor. A tumor that is noninvasive and noncancerous is referred to as a benign tumor. • Malignant - A tumor that invades neighboring cells and is cancerous is referred to as a malignant tumor. • Matastasis – Cancer that has spread to other tissues. Development of Cancer • Cancer develops only after a cell experiences ~6 key mutations (“hits”) – unlimited growth • turn on growth promoter genes – ignore checkpoints • turn off tumor suppressor genes – escape apoptosis • turn off suicide genes It’s like an – immortality = unlimited divisions out of control • turn on chromosome maintenance genes car! – promotes blood vessel growth • turn on blood vessel growth genes – overcome anchor & density dependence • turn off touch censor gene Cancer & Cell Growth • Cancer is essentially a failure of cell division control – unrestrained, uncontrolled cell growth • What control is lost? – checkpoint stops – gene p53 plays a key role in G1 checkpoint • p53 protein halts cell division if it detects damaged DNA p53 is the Cell Cycle – stimulates repair enzymes to fix DNA Enforcer – forces cell into G0 resting stage – keeps cell in G1 arrest – causes apoptosis of damaged cell • ALL cancers have to shut down p53 activity p53 discovered at Stony Brook by Dr. Arnold Levine p53 — master regulator gene NORMAL p53 p53 allows cells with repaired DNA to divide. p53 protein DNA repair enzyme p53 protein Step 1 Step 2 Step 3 DNA damage is caused Cell division stops, and p53 triggers the destruction by heat, radiation, or p53 triggers enzymes to of cells damaged beyond repair. chemicals. repair damaged region. ABNORMAL p53 abnormal p53 protein cancer Step 1 Step 2 cell DNA damage is The p53 protein fails to stop Step 3 caused by heat, cell division and repair DNA. Damaged cells continue to divide. radiation, or Cell divides without repair to If other damage accumulates, the chemicals. damaged DNA. cell can turn cancerous. Growth Factors and Cancer • Growth factors influence cell cycle – proto-oncogenes • normal genes that become oncogenes (cancer- causing) when mutated • stimulates cell growth • if switched on can cause cancer • example: RAS (activates cyclins) – tumor-suppressor genes • inhibits cell division • if switched off can cause cancer • example: p53 What causes these “hits”? • Mutations in cells can be triggered by u UV radiation u cigarette smoke u chemical exposure u pollution u radiation exposure u age u heat u genetics How we naturally fight cancer cells • Tumor suppressor genes like p53 – Can arrest the cell cycle – Can launch the apoptotic pathway, causing the rogue cells to lyse A mutation in the p53 gene can lead to cancer • Immune cells (WBCs) such as NK cells can attack and lyse tumor cells – Some immune cells can signal the rogue cells to launch the apoptotic pathways Traditional treatments for cancers • Treatments target rapidly dividing cells – high-energy radiation • kills rapidly dividing cells – chemotherapy • stop DNA replication • stop mitosis & cytokinesis • stop blood vessel growth New “miracle drugs” • Drugs targeting proteins (enzymes) found only in tumor cells – Gleevec • treatment for adult leukemia (CML) & stomach cancer (GIST) • 1st successful targeted drug Any Questions?? Signal Transduction Pathways • What are they? – Signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another. – A large number of proteins, enzymes and other molecules participate in a "signal cascade“ • What is the end result? – Either the activation or inhibition of a certain enzyme in the cytoplasm – Either the expression or suppression of a particular gene Just a few examples of Signal Transduction Pathways • Cell Division signals • Apoptotic signals • Insulin pathways Apoptotic Pathways Insulin Signaling Pathway The binding of insulin to its receptor on a cell starts a cascade of cellular events which finally leads to the uptake of glucose and the lowering of blood glucose levels. “Go-ahead” signals • Protein signals that promote cell growth & division – internal signals • “promoting factors” – external signals • “growth factors” • Primary mechanism of control – phosphorylation • kinase enzymes • either activates or inactivates cell signals
"Regulation of Cell Division"