Cell Cycle PDF

CELL CYCLE Radhika Bhardwaj, PhD Department of Biotechnology, AIU  Outline - The Eukaryotic Cell Cycle - Regulators of Cell Cycle Progression - Cell cycle checkpoints - The Events of M Phase  The cell cycle The series of events that leads to the duplication and d...

CELL CYCLE Radhika Bhardwaj, PhD Department of Biotechnology, AIU  Outline - The Eukaryotic Cell Cycle - Regulators of Cell Cycle Progression - Cell cycle checkpoints - The Events of M Phase  The cell cycle The series of events that leads to the duplication and division of a cell In bacteria, The cell growth and DNA replication take place throughout most of the cell cycle, and duplicated chromosomes are distributed to daughter cells in association with the plasma membrane. In eukaryotes, however, - The cell cycle is more complex - Although cell growth is usually a continuous process, DNA is synthesized during only one phase of the cell cycle - The replicated chromosomes are then distributed to daughter nuclei by a complex series of events preceding cell division.  The Eukaryotic Cell Cycle Consists of 4 coordinated processes: 1) Cell growth 2) DNA replication 3) Distribution of the duplicated chromosomes to daughter cells 4) Cell division  Phases of the Cell Cycle Eukaryotic cell cycles are divided into four discrete phases: M, G1, S, and G2. M phase consists of mitosis, which is usually followed by cytokinesis. The cell grows throughout interphase, which includes G1, S, and G2 ❖ A typical eukaryotic cell cycle is illustrated by human cells in culture, which divides approximately every 24 hours. ❖ As viewed in the microscope, the cell cycle is divided into two basic parts: mitosis and interphase. However, mitosis and cytokinesis last only about an hour, so approximately 95% of the cell cycle is spent in interphase-the period between mitosis.  INTRODUCTION TO THE CELL-CYCLE PHASES A, Diagrams of cellular morphology and chromosome structure across the cell cycle B, Length of cell-cycle phases in cultured cells C, Time scale of cell-cycle phases For a typical rapidly proliferating human cell with a total cycle time of 24 hours, G1 phase might last about 11 hours, S phase about 8 hours, G2 about 4 hours, M about 1 hour. The duration of these cell cycle phases varies considerably in different kinds of cells. Other types of cells, however, can divide much more rapidly. Budding yeasts, for example, can progress through all 4 stages of the cell cycle in about 90 minutes. Even shorter cell cycles (30 minutes or less) occur in early embryo cells shortly after fertilization of the egg. In this case, however, cell growth does not take place. Instead, these early embryonic cell cycles rapidly divide the egg cytoplasm into smaller cells. There is no G1 or G2 phase, and DNA replication occurs very rapidly in these early embryonic cell cycles, which therefore consist of very short S phases alternating with M phases  Embryonic cell cycles Early embryonic cell cycles rapidly divide the cytoplasm of the egg into smaller cells. The cells do not grow during these cycles, which lack G1 and G2 and consist simply of short S phases alternating with M phases. In contrast to the rapid proliferation of embryonic cells, some cells in adult animals cease division altogether (e.g., nerve cells) Many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. Cells of the latter type include skin fibroblasts, as well as the cells of some internal organs, such as the liver. These cells exit G1 to enter a quiescent stage of the cycle called G0 , where they remain metabolically active but no longer proliferate unless called on to do so by appropriate extracellular signals  Regulation of the Cell Cycle by Cell Growth and Extracellular Signals The progression of cells through the division cycle is regulated by extracellular signals from the environment, as well as by internal signals that monitor and coordinate the various processes that take place during different cell cycle phases.  Animal cell The availability of growth factors controls the animal cell cycle at a point in late G1 called the restriction point. If growth factors are not available during G1, the cells enter a quiescent stage of the cycle called G0. Regulation of animal cell cycles by growth factors  Cell Cycle Checkpoints Cell cycle checkpoints are control mechanisms in the eukaryotic cell cycle which ensure its proper progression.  Checkpoints detect damage to the DNA due to external agents or problems that arise during DNA replication and trigger the DNA damage response. Other checkpoints detect problems that arise during attachment of chromosomes to the spindle Several checkpoints function throughout the cell cycle to ensure that complete genomes are transmitted to daughter cells. In most cells, this coordination between different phases of the cell cycle is dependent on a series of cell cycle checkpoints that prevent entry into the next phase of the cell cycle until the events of the preceding phase have been completed 1.G1/S Checkpoint (Start or Restriction checkpoint) 2. Intra S Checkpoint 3. G2/M Checkpoint 4. Spindle Assembly Checkpoint Cell cycle checkpoints Several checkpoints function to ensure that complete genomes are transmitted to daughter cells. DNA damage checkpoints in G1, S, and G2 lead to cell cycle arrest in response to damaged or unreplicated DNA. Another checkpoint, called the spindle assembly checkpoint, arrests mitosis if the chromosomes are not properly aligned on the mitotic spindle 3 Major Cell cycle checkpoints Progression through these checkpoints is largely determined by the activation of cyclin- dependent kinases (CDKs) by regulatory protein subunits called cyclins, different forms of which are produced at each stage of the cell cycle to control the specific events that occur therein  Restricting DNA Replication to Once per Cell Cycle It is equally important to ensure that the genome is replicated only once per cell cycle. Restriction of DNA replication is restricted to once per cell cycle by the MCM helicase proteins MCM proteins bind to origins of replication together with ORC (origin recognition complex) proteins MCM proteins are required for the initiation of DNA replication. MCM proteins are only able to bind to DNA in G1 allowing DNA replication to initiate in S phase. Once initiation has occurred, the MCM proteins are displaced so that replication cannot initiate again until after mitosis. The association of MCM proteins with DNA during the S, G2 and M phases of the cell cycle is blocked by activity of the protein kinases that regulate cell cycle progression Regulators of Cell Cycle Progression Protein Kinases and Cell Cycle Regulation Families of Cyclins and Cyclin-Dependent Kinases Growth Factors and the Regulation of G1 Cdk’s DNA Damage Checkpoints  Cyclins Cyclin is a family of proteins that controls the progression of a cell through the cell cycle by activating cyclin-dependent kinase enzymes (CDks) or group of enzymes required for the synthesis of cell cycle  Cyclin-Dependent Kinases Humans have more than 10 distinct protein kinases related to p34cdc2, although only a few are involved in cell-cycle control. To be active, each enzyme must associate with a regulatory subunit called a cyclin. Thus, they have been termed cyclin-dependent kinases (Cdks).  Cyclin-Dependent Kinases p34cdc2, now termed Cdk1, seems to function primarily in the regulation of the G2 → M transition in animal cells. Cdk2 (plus Cdk4 and Cdk6 in some cell types) is involved in passage of the restriction point during G1. Cdk2 also contributes to the G2 → M transition, although Cdk1 is the only Cdk absolutely essential for this step. Cdk7 is important for activation of other Cdks, and also appears to participate in transcribing RNA and repairing damaged DNA. Other Cdks participate in diverse processes ranging from transcriptional regulation to neuronal differentiation.  Protein Kinases and Cell Cycle Regulation Cyclins, Cdks, and the MPF is a dimer Cell Cycle Maturation consisting of cyclin promoting factor (MPF)- B and the Cdkl consisting of cyclin B protein kinase. and Cdkl-regulates the transition from G2 to M phase of the cell cycle. Structure of MPF  Animal cells Families of Cyclins and Cyclin-Dependent Kinases In animal cells, progression through the G1 restriction point is controlled by complexes of Cdk4 and Cdk6 with D-type cyclins. Cdk2/cyclin E complexes function later in G1 and are required for the G1 to S transition. Cdk2/ cyclin A complexes are then required for progression through S phase. Cdkl I cyclin A regulates progression to G2 , and Cdkl/cyclin B complexes drive the G2 to M transition.  G1/S Checkpoint (pRb/ E2F) CDK 4/CDK6 (CycD) To move from G1 to S, E2F is required E2F is a protein acts as transcription factor which allows the synthesis of proteins required for DNA replication.  G1/S Checkpoint (pRb/ E2F) CDK 4/CDK6 (CycD) By default, in cell, E2F is blocked or masked by pRb. In G1/S checkpoint, CDK4/6 along with Cyclin D phosphorylates pRb (retinoblastoma sensitivity protein) protein. The phosphorylation of pRb makes it inactive and degrade the pRb. E2F is now free means its active and can do the functions like production of replication enzymes. So, cell will be ready to go from G1 to S phase  Growth Factors and the Regulation of G1 Cdk's Cell cycle regulation of Rb and E2F In its underphosphorylated form, Rb binds to members of the E2F family, repressing transcription of E2F- regulated genes. Phosphorylation of Rb by Cdk4, 6/cyclin D complexes results in its dissociation from E2F in late G1. E2F then stimulates expression of its target genes, which encode proteins required for cell cycle progression.  Cdk2/ cyclin E and entry into S phase ln early G1, Cdk2/cyclin E complexes are inhibited by the Cdk inhibitor p27. Passage through the restriction point induces the synthesis of cyclin E via activation of E2F. In addition, growth factor signaling reduces the levels of p27 by inhibiting its transcription and translation. The resulting activation of Cdk2/ cyclin E leads to activation of the MCM helicase and initiation of DNA replication. Cell cycle arrest at the DNA damage checkpoints The ATM and ATR protein kinases are activated in complexes of proteins that recognize unreplicated or damaged DNA. ATM is activated principally by double-strand breaks and ATR by single-stranded or unreplicated DNA. ATM and ATR then phosphorylate and activate the CHK2 and CHKI protein kinases, respectively. CHKI and CHK2 phosphorylate and inhibit the Cdc25A and Cdc25C protein phosphatases. Cdc25A and Cdc25C are required to activate Cdk2 and Cdkl, respectively, so their inhibition leads to arrest at the DNA damage checkpoints in G1, S, and G2 P53: Tumor suppressor protein, also known as guardian of the cell cycle Role of p53 in G1 arrest The protein p53 plays a key role in cell cycle arrest at the G1 checkpoint in mammalian cells. Phosphorylation by ATM and CHK2 stabilize p53, resulting in rapid increases in p53 levels in response to DNA damage. The protein p53 then activates transcription of the gene encoding the Cdk inhibitor p21, leading to inhibition of Cdk2/ cyclin E complexes and cell cycle arrest.  The Events of M Phase Stages of Mitosis Cdkl/Cyclin B and Progression to Metaphase The Spindle Assembly Checkpoint and Progression to Anaphase Cytokinesis Stages of mitosis in an animal cell  Stages of mitosis in an animal cell During prophase the chromosomes condense and centrosomes move to opposite sides of the nucleus, initiating formation of the mitotic spindle. Breakdown of the nuclear envelope then allows spindle microtubules to attach to the kinetochores of chromosomes. During prometaphase the chromosomes shuffle back and forth between the centrosomes and the center of the cell, eventually aligning in the center of the spindle (metaphase). At anaphase, the sister chromatids separate and move to opposite poles of the spindle. Mitosis then ends with re-formation of nuclear envelopes and chromosome decondensation during telophase, and cytokinesis yields two interphase daughter cells. Note that each daughter cell receives one centrosome, which duplicates prior to the next mitosis.  Stages of mitosis in an animal cell Prometaphase- a transition period between prophase and metaphase. During prometaphase the microtubules of the mitotic spindle attach to the kinetochores of condensed chromosomes. The kinetochores of sister chromatids are oriented on opposite side of the chromosome, they attach to microtubules emanating from opposite poles of the spindle. The chromosomes shuffle back and forth until they eventually align on the metaphase plate in the center of the spindle. At this stage, the cell has reached metaphase. Cdkl/Cyclin B and Progression to Metaphase Targets of Cdkl/ cyclin B The Cdkl/cydin B complex induces multiple nuclear and cytoplasmic changes at the onset of M phase both by activating other protein kinases and by phosphorylating proteins such as condensins, components of the nuclear envelope, Golgi matrix proteins, and proteins associated with centrosomes and microtubules. The action of cohesins and condensins Cohesins bind to DNA during S phase and maintain the linkage between sister chromatids following DNA replication in S and G2 As the cell enters M phase, the cohesins are replaced by condensins along most of the chromosome, remaining only at the centromere. Phosphorylation by Cdkl activates the condensins, which drive chromatin condensation. Breakdown of the nuclear envelope Cdkl/cyclin B phosphorylates the nuclear lamins as well as proteins of the nuclear pore complex and inner nuclear membrane. Phosphorylation of the lamins causes the filaments that form the nuclear lamina to dissociate into free lamin dimers. The Spindle Assembly Checkpoint and Progression to Anaphase  The metaphase spindle (A)The spindle consists of four kinds of microtubules Kinetochore and chromosomal microtubules are attached to chromosomes; polar microtubules overlap in the center of the cell; and astral microtubules radiate from the centrosome to the cell periphery. (B) A whitefish cell at metaphase. (B, Michael Abbey /Photo Researchers, Inc.) The spindle assembly checkpoint monitors the alignment of chromosomes on the metaphase spindle. Once this has been accomplished, the cell proceeds to initiate anaphase and complete mitosis. The progression from metaphase to anaphase results from ubiquitin-mediated proteolysis of key regulatory proteins, triggered by activation of an E3 ubiquitin ligase called the anaphase-promoting complex (APC). Activation of the anaphase-promoting complex is induced at the beginning of mitosis, so the activation of Cdkl/ cyclin B ultimately triggers its own destruction. The anaphase-promoting complex remains inhibited, however, until the cell passes the spindle assembly checkpoint, after which activation of the ubiquitin degradation system brings about the transition from metaphase to anaphase and progression through the rest of mitosis. The spindle assembly checkpoint is remarkable in that the presence of even a single unaligned chromosome is sufficient to inhibit activation of the anaphase- promoting complex. The checkpoint is mediated by a complex of proteins, called the Mad/Bub proteins, that bind to Cdc20-a required component of the anaphase-promoting complex  The spindle assembly checkpoint Progression to anaphase is mediated by activation of the anaphase- promoting complex (APC) ubiquitin ligase. Unattached kinetochores lead to the assembly of a complex of Mad/Bub proteins in which Mad proteins are activated and prevent APC activation by inhibiting Cdc20. The spindle assembly checkpoint Once all chromosomes are aligned on the spindle, the Mad/Bub complex dissociates, relieving inhibition of Cdc20 and leading to APC activation. APC ubiquitinates cyclin B, leading to its degradation and inactivation of Cdkl. In addition, APC ubiquitinates securin, leading to activation of separase. Separase degrades a subunit of cohesin, breaking the link between sister chromatids and initiating anaphase.  The Cell Cycle - YouTube Cell Cycle & Regulation, Mitosis, Cyclins, RB, P53 & Tumor Suppressors (USMLE Esssentials) - YouTube