The Cell Cycle PDF

The Cell Cycle  Session Learning Outcomes (SLO) SLO# 1 : Describe the overall purposes of cell division and the cell cycle. SLO# 2 : Describe the structural organization of a eukaryotic genome. SLO# 4 : Explain cytokinesis and its importance and purpose SLO# 3 : List the phases o...

The Cell Cycle  Session Learning Outcomes (SLO) SLO# 1 : Describe the overall purposes of cell division and the cell cycle. SLO# 2 : Describe the structural organization of a eukaryotic genome. SLO# 4 : Explain cytokinesis and its importance and purpose SLO# 3 : List the phases of the cell cycle and describe the sequence of events that occurs during each phase.  The functions of cell division The continuity of life is based upon the reproduction of cells, or cell division 100 µm 200 µm (a) Reproduction (Anamoeba) (b) Growth and development (a sand dollar embryo) Embryonic development 20 µm (c) Tissue renewal. bone marrow cells  The cell cycle ▪ The cell division process is an integral part of the cell cycle. ▪ A cell reproduces by performing an orderly sequence of events in which it duplicates its contents and then divides in two Each daughter cell receives an exact copy of the genetic material, DNA Cells duplicate their genetic material before they divide.  Phases of the Cell Cycle The cell cycle consists of: 1- The mitotic phase: Mitosis (division of the nucleus) Cytokinesis (division of the cytoplasm). 2- Interphase: L G1 phase S phase G2 phase DNA synthesize replicates The stages of mitotic cell division in an animal cell Mitosis consists of five distinct phases – Prophase – Prometaphase – Metaphase – Anaphase – Telophase  Prophase: sad 1. The chromatin fibers become more tightly coiled. chromosomes 2. The nucleoli disappears. 3. Each duplicated chromosome appears as two identical sister TheDNAis already duplicated chromatids joined together. 4. The centrosomes move a away from each other. 5. The mitotic spindle begins to form. fortheattachment thine pnaMiater  Prometaphase: 1. The nuclear envelope fragments. t.IN 2. The microtubules of the spindle isinvade the nuclear area and chromosomeattached arealready interact with condensed tothespindle chromosomes. Twotypesoffibers 3. Each of the two chromatids of a Kinetochore thechromos whichhaskinetochorewill chromosomes has a kinetochore to thespindle located at the centromere region. nonkinetochore fiber inthespind neckingation intkell 4. Some of the microtubules attach to the kinetochores.  Metaphase: Is 1. The centrosomes are at the opposite poles of the cell. 2. The chromosomes convene on the metaphase plate. 3. The centromeres of the chromosomes are all on the metaphase plate. Emes 4. The kinetochores of the sister iii chromatids are attached to microtubules.  chromosome willnotmovealong Anaphase: withthespindlethere is shortness in thedegradation 1. The paired centromeres of each chromosomes separate, liberating 3ps the sister chromatids from each other. 2. The once joined sisters, each now considered a full-fledged go chromosome, begin moving toward opposite poles of the cell. 3. The kinetochore microtubules get shorter, and the spindle poles also move apart. to help inseparating of chromatides 4. The two poles of the cell have equivalent and complete collections of chromosomes. helpsforthe degradation  Telophase: 1. The nonkinetochore microtubules elongate the cell. 2. The daughter nuclei form at the two poles of the cell. 3. The nuclear envelopes arise. 4. The chromatin fiber of each chromosome becomes less tightly coiled. 5. The division of one nucleus into two genetically identical nuclei is complete. EiT I 6. The cytokinesis is well under way by this time, involves the formation of a cleavage furrow, which pinches the cell in two. Cytokinesis During cytokinesis of an animal cell, the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches in the cell to create two daughters, each with one nucleus.  Interphase ▪ G1 phase: It is the post-mitotic phase and takes place at the end of cell division. The newly formed cells accumulate the energy and prepare themselves for the synthesis of DNA. During it, active synthesis of RNA and protein takes place. ▪ S phase: it is the synthesis phase during this phase duplication of DNA and centriole takes places. The duplication of DNA results in the duplication of chromosomes. ▪ G2 phase: It is pre-mitotic gap phase the synthesis of RNA and protein continues in this phase. The formation of macromolecules for spindle formation takes place and the cell prepare itself to go into the mitotic phase. ▪ During interphase, the cell increases in size, the DNA of the chromosomes is replicated, and the centrosome is duplicated. moveawayfromeach other in prophase being attheoppositepoles ofthecellinmetaphase The cell-cycle control system  Session Learning Outcomes (SLO) SLO# 1 : Describe the roles of checkpoints, cyclin, Cdks, and MPF in the cell cycle control system. SLO# 2 : Describe the internal and external factors that influence the cell cycle control SLO# 3 : Explain how the abnormal cell division of cancerous cells escapes normal cell  The frequency of cell division The frequency of cell division varies with cell type: – Some human cells divide frequently throughout life (skin cells). p – Others have the ability to divide, but keep it in reserve (liver cells). p served a in – Mature nerve and muscle cells do not appear to divide at all after maturity.Tedbloodcell These cell cycle differences Result from regulation at the molecular level Notimportant  The Cell Cycle Control System The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock. A chekpoint in the cell cycle is a critical control point where stop and go chemical signals regulate the cycle. Three major checkpoints are found in the G1, G2, and M phases. Check for cell size Check for Chromosomes And DNA damage attachment to spindle Illnehand afterDNA replication ualityof DNA Check for cell size Nutrients Growth factors DNA damage growth It needs factor checkthequality ofDNA Figure 18-3 Essential Cell Biology (© Garland Science 2010)  the G1 checkpoint G1 is most critical ▪ Primary decision point Iowa For many cells, the G1 checkpoint is the “restriction point” ▪ If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues on in the cell cycle. ecked Internal signals: cell growth (size), Cell nutrition y6 External signals: growth factors ▪ If a cell does not receive a go-ahead signal at the G1 G0 checkpoint, the cell exits G1 checkpoint the cell cycle and goes into G0, a nondividing state. G1 G1 presence of damages aftermaturation waiting for  Stop and Go Signs: Internal and External Signals at the Checkpoints External and internal factors regulate cell division. – External factors: factors or signals that come from the outside of the cell that help to control the division process – Internal factors: factors or signals from the inside of the cell that control the division process External Factors that can Influence Cell Division 1. Chemical factors: Edibility a. Lack of nutrients inhibit cell division b. Presence of specific growth factors are needed for cell division  fifelets Growth Factors Growth factors are a broad group of proteins that act as chemical signals released from other cells to tell other cells to divide. Growth factors bind to receptors that activate specific genes to trigger cell growth. Cells will usually respond to a combination of several growth factors not just one.  Ñ hair weenthe cells Eells Example: separate from other each The effect of a growth factor on cell division increase inthe fight  External Signals – Platelet-derived Growth Factor (PDGF) is required for division of fibroblasts used in healing – Receptors on plasma membrane bind PDGF and trigger pathway to signal cell division  2. Physical factors: a. Density-dependent inhibition Crowded cells stop dividing r b. Anchorage-dependent inhibition or attach toa Cells must attach to substratum (surface) adhere supporter Anchorage is signaled to cell-cycle control system by linkage between membrane proteins and elements of cytoskeleton Cancer cells – Exhibit neither density-dependent inhibition nor anchorage dependence  layers formingmany causetumors (b) Cancer cells. Cancer cells usually (a) Normal mammalian cells. Contact continue to divide well beyond a single with neighboring cells and the layer, forming a clump of overlapping availability of nutrients, growth cells. They do not exhibit anchorage factors, and a substratum for dependence or density-dependent attachment limit cell density to a inhibition. single layer.  Cell to Cell Contact Once a cell touches other cells it stops dividing. The exact reason is considered a phenomenon and is unknown. Irs One hypothesis suggests that receptors on neighboring cells bind to each other and trigger the cytoskeleton to block signals that will trigger growth.  Internal Factors Enzymes – Kinases Phosphorylation activation and Cyclins  pÉTce attive inactive Kinases and Cyclins Bletsent allways Kinases are enzymes that when activated transfer a phosphate group from one molecule to a target molecule. This gives energy or changes the shape of the target molecule. Some kinases help control the cell cycle. Cyclins are proteins that are rapidly made and broken down that activate kinases. For example MPF maturationPromotingfactors  Evidence for Cytoplasmic Signals Molecules present in the cytoplasm regulate progress through the cell cycle EXPERIMENTS In each experiment, cultured mammalian cells at two different phases of the cell cycle were induced to fuse. RESULTS Experiment 1 Experiment 2 is S G1 M G1 When a cell in the M phase was fused with a cell in G1, the G1 ed When a cell in the S phase was fused with a cell in G1, the G1 at nucleus immediately began mitosis (a spindle formed and chromatin nucleus immediately entered condensed, even though the the S phase (DNA was chromosome had not been synthesized). f S S M M duplicated). immediately those phase gotoM ing CONCLUSION The results of fusing cells at two different phases of the cell cycle suggest that molecules present in the cytoplasm of cells in the S or M phase control the progression of phases. A maturation promoting factor (MPF) triggers entry into M phase. 3 I  I The activity of MPF oscillates during the cell cycle in Xenopus embryos. stable stay decreaseon increasing activity in Figure 18-8 Essential Cell Biology (© Garland Science 2010) ▪ MPF activity increased rapidly just before the start of mitosis and fell rapidly to zero toward the end of mitosis. ▪ MPF a strong candidate for a component involved in cell-cycle control. ▪ MPF contain a protein kinase that was required for its activity. But the kinase portion of MPF did not act alone. It had to have a specific protein bound to it in order to function.  G2 Checkpoint Sequence of cell cycle events are coordinated by rhythmic changes in activity of certain protein kinases Sactivating in Mitosis (phosphorylation) inactivate in Phosphorylation causes change in shape that activates/inactivates target protein which affect the progression through cell cycle. Changes in kinase activity regulated by regulatory proteins – The protein kinase, cyclin-dependent kinase (Cdks) are regulated by concentration of cyclin present  The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases Cyclin-dependent kinases (Cdks) are present in constant amounts but require attachment of a second protein, a cyclin, to become activated. Because of the requirement for binding of a cyclin, the kinases are called cyclin-dependent kinases, or Cdks. Cyclin levels rise sharply throughout interphase, and then fall abruptly during mitosis.  The Cyclin-Cdk complexes: the major components of the cell cycle control system Cyclin-dependent kinases (Cdks) as are inactive unless bound to cyclins Active complex phosphorylates key proteins in the cell that are required to initiate particular steps in the cell cycle Figure 18-4 Essential Cell Biology (© Garland Science 2010) Cyclin helps to direct Cdks to the target proteins that the Cdk phosphorylates  The cyclins Four classes of cyclins: 1. G1-cyclins: help to promote passage through “Start”or the restriction point in late G1 2. G1/S-cyclins: bind Cdks at the end of G1 and commit the cell to DNA replication 3. S-cyclins: binds Cdks during S phase and are required for the initiation of DNA replication 4. M-cyclins: acts in G2 to trigger entry into M phase  The cyclins Distinct cyclins partner with distinct Cdks to trigger different events of the cell cycle mitosi Pepitation Figure 18-10 Essential Cell Biology (© Garland Science 2010) The activation of the Cdk requires phosphorylation and dephosphorylation as well as cyclin binding. Cellular levels of (mitotic) cyclin rises and falls during the cell cycle cyclin levels are low during interphase but gradually increases to a peak level during mitosis cdk activity is, likewise, low in interphase but increases in mitosis the increase in the cyclin concentration helps form the active cyclin–Cdk complex that drives entry into M phase  Regulation of the iwr mosijxiom TF.ie cyclin-Cdk complex was in After ▪ Post-translational modification ▪ Transcriptional regulation ▪Inhibitors of Cdks ▪ Cyclical proteolysis Regulation of the Cyclin-Cdk complex: Post-translational modification Activation of the cyclin-Cdk complex Partial activation of Cdks by cyclin association Full activation by Cdk-activating kinase (CAK) phosphorylation Figure 18-9 Essential Cell Biology (© Garland Science 2010)  Activation of the cyclin-Cdk complex ▪ When the cyclin–Cdk complex first forms, it is not phosphorylated and is inactive. ▪ For a Cdk to be active, it must be phosphorylated at one site and dephosphorylated at two other sites. ▪ The Cdk is phosphorylated at a site that is required for its activity and at two other sites that inhibit its activity. ▪ This phosphorylated complex remains inactive until it is finally activated by a protein phosphatase that removes the two inhibitory phosphate groups.  Regulation of the Cyclin-Cdk complex In order to move on to the next phase, cyclins from the previous phase need to be removed from the system  Regulation of the Cyclin-Cdk complex The abundance of cyclins (and the activity of Cdks) is regulated by protein degradation peptide peptidesbinds to cyclin degraded in someorgans mechanism bywhichFigure 18-11 Essential Cell Biology (© Garland Science 2010) thecyclin isdegraded regulation Ubiquity Ubiquitylation of cyclins to target them for destruction. This in turn inactivates the corresponding Cdk i an Molecular control of the cell cycle at the G2 checkpoint MPF = maturation promoting factor - Triggers M phase to start by phosphorylating a variety of proteins. - Acts at the G2 checkpoint as a go-ahead signal,  Cell cycle Control System Checkpoints Inhibitors ofCdks ▪ Before the cell can progress through the cell cycle, it needs to go through a series of chechpoints. ▪ If the cell decides it can’t move on, it can stop the cell cycle and wait ▪ This is done by Cdk inhibitor proteins Inhibit the cell cycle progression of the cyclin/Cdk complexes by inhibiting the Cdk in response to DNA damage, intracellular processes that have not been completed, or an unfavorable extracellular environment. Figure 18-13 Essential Cell Biology (© Garland Science 2010)  DNA damage arrests the cell cycle in G1 ▪ DNA damage can arrest the cell cycle at a checkpoint in G. when DNA is 1 damaged ▪ Specific protein kinases respond bystop activating the p53 protein and halting its normal rapid degradation. ▪ Activated p53 protein accumulates and binds to DNA. ▪ P53 stimulates the transcription of the gene that encodes the Cdk inhibitor protein, p21. ▪ p21 protein binds to G /S-Cdk and S- 1 Cdk and inactivates them, so that the cell cycle arrests in G. 1 Figure 18-16 Essential Cell Biology (© Garland Science 2010)  A checkpoint in G1 offers the cell a crossroad. the cell can commit to completing another cell cycle, pause temporarily until conditions are right, or withdraw from the cell cycle altogether and enter G0. Thecellhas 3options proceedto phase continue through s DNA e cellcycletoreplicate Sause temporarily Haltprogress 6until conditionsare favorable withdrawpermanently toGoExit hecellcycleandenteraquiescent state Figure 18-12 Essential Cell Biology (© Garland Science 2010) maturenerve musclecells In some cases, cells in G0 can re-enter the cell cycle when 9 conditions improve, but many cell types permanently withdraw from the cell cycle when they differentiate, persisting in G0 for the lifetime of the animal. i The G1, G1-S, and S phase Control of the initiation for DNA v replication (S phase) (formed by Orc proteins) Cell division cycle 6 Chromatin licensing and DNA replication factor 1 Regulation of pre-replicative complex (pre-RC) by S-CDK Origin recognition complex (ORC): Binds to the replication origin through the cell cycle. Cdc6, Cdt1 associate with ORC at early G1 help to recruit Mcm proteins, a DNA helicase S-Cdk causes: Start of DNA replication, by ORC phosphorylation, MCM activation, and replication firing Control only one replication, by Cdc6 degradation and by Mcm export from nucleus  Cohesins help hold the sister chromatids of each replicated chromosome together ▪ After the chromosomes have been duplicated in S phase, the two copies of each replicated chromosome remain tightly bound together as identical sister chromatids. ▪ The sister chromatids are held together by protein complexes called cohesins, which assemble along the length of each sister chromatid as the DNA is replicated in S phase. ▪ The cohesins form protein rings that surround the two sister chromatids, keeping them united. ▪ This cohesion between sister chromatids is crucial for proper chromosome segregation, and it is broken completely only in late mitosis to allow the sister chromatids to be pulled apart by the mitotic spindle. Figure 18-15 Essential Cell Biology (© Garland Science 2010) Regulation of entry into mitosis The M phase ▪ M-Cdk triggers the condensation of the replicated chromosomes into compact, rod-like structures, readying them for segregation. ▪ M-Cdk induces the assembly of the mitotic spindle that will separate the condensed chromosomes and segregate them into the two daughter cells. ▪ M-Cdk activation begins with the accumulation of M cyclin. ▪ Synthesis of M cyclin starts immediately after S phase; its concentration then rises gradually and helps time the onset of M phase. ▪ The increase in M cyclin protein leads to a corresponding accumulation of M-Cdk complexes. But those complexes, when they first form, are inactive. ▪ The sudden activation of the M-Cdk stockpile at the end of G2 is triggered by the activation of a protein phosphatase (Cdc25) that removes the inhibitory phosphates holding M-Cdk activity in check  m-Cdk drives entry into m phase and mitosis Figure 18-17 Essential Cell Biology (© Garland Science 2010) For m-Cdk to be active, it must be phosphorylated at one site and dephosphorylated at other sites. the M cyclin–Cdk complex is enzymatically inactive when first formed. The Cdk is phosphorylated at one site that is required for its activity by an enzyme called Cdk-activating kinase, Cak It is also phosphorylated at two other, overriding sites that inhibit its activity by an enzyme called wee1.  Cdk activates itself indirectly via a positive feedback loop Activated m-Cdk indirectly activates more m-Cdk, creating a positive feedback loop. Once activated, M-Cdk phosphorylates, and thereby activates, more Cdk- activating phosphatase (Cdc25). The phosphatase can now activate more M-Cdk by removing the inhibitory phosphate groups from the Cdk subunit. Figure 18-18 Essential Cell Biology (© Garland Science 2010) Activated M-Cdk also inhibits the inhibitory kinase Wee1, further promoting the activation of M-Cdk. The overall consequence is that, once the activation of M- Cdk begins, there is an explosive increase in M-Cdk activity that drives the cell abruptly from G2 into M phase.  Cyclical proteolysis regulates cyclin-Cdk activity Regulated by SCF complex in G1 and S phase SCF complex(Ubiquitin ligase) Targets:G1/S cyclins, some CKIs  The spindle-attachment checkpoint Ensure that all chromosomes are properly attached to the spindle before sister-chromatid separation occurs A sensor mechanism monitors the state of the kinetochore, the specialized region of the chromosome that attaches to microtubules of the spindle Improper attachment of kinetochore to the spindle sends out a negative signal to the cell-cycle control system, blocking Cdc20- APC activation and sister chromatid separation Several proteins, including Mad2, are recruited to unattached kinetochores. Mad2 binding results in Inhibition of Cdc20-APC and blocking securin destruction  Checkpoint: spindle assembly Mitosis must not complete unless all the chromosomes are attached to the mitotic spindle Mitotic checkpoint delays metaphase to anaphase transition until all chromosomes are attached Prolonged activation of the checkpoint -->cell death Mechanism of many anti-cancer drugs  At the beginning of anaphase, each pair of sister chromatids separates. the resulting daughter chromosomes are then pulled to opposite poles of the cell by the mitotic spindle. s Figure 18-21 Essential Cell Biology (© Garland Science 2010)  The APC triggers the separation of sister chromatids by promoting the destruction of cohesins. Activated anaphase-promoting complex (APC) indirectly triggers the cleavage of the Cohesins that hold sister chromatids together. 1. Activated APC catalyzes the ubiquitylation and destruction of an inhibitory protein called Securin. 2. Securin inhibits the activity of a proteolytic enzyme called Separase 3. Separase cleaves the Cohesin complexes when become free from Securin 4. Clivage of Cohesin allowing the mitotic spindle to pull the sister chromatids apart  An overview of the cell-cycle control system Checkpoints ensure the cell cycle proceeds without errors