Lec-6-Cell division cycle-2024-stud.pdf

2024-09-09 The Cell-Division Cycle 1 Introduction The most basic function of the cell cycle is to duplicate accurately the vast amount of DNA in the chromo...

2024-09-09 The Cell-Division Cycle 1 Introduction The most basic function of the cell cycle is to duplicate accurately the vast amount of DNA in the chromosomes and then to segregate the DNA into genetically identical daughter cells such that each cell receives a complete copy of the entire genome. In most cases, a cell also duplicates its other macromolecules and organelles and doubles in size before it divides; otherwise, each time a cell split it would get smaller and smaller. In picture is shown a hypothetical eukaryotic cell—with only one copy each of two different chromosomes—to illustrate how each cell cycle produces two genetically identical daughter cells. Each daughter cell can divide again by going through another cell cycle, and so on for generation after generation. 2 2 1  2024-09-09 Introduction The duration of the cell cycle varies greatly from one cell type to another. In an early frog embryo, cells divide every 30 minutes, whereas a mammalian fibroblast in culture divides about once a 3 day. 3 The Eukaryotic Cell Cycle Usually Includes Four Phases Only two events in the cell cycle are visible through a microscope: 1. when the nucleus divides, a process called mitosis, and 2. when the cell later splits in two, a process called cytokinesis. These two processes together constitute the M phase of the cycle. The period between one M phase and the next is called interphase. Interphase, however, is a very busy time for a proliferating cell, and it encompasses the remaining three phases of the cell cycle. During S phase (S = synthesis), the cell replicates its DNA. S phase is flanked by two “gap” phases—called G1 and G2—during which the cell continues to grow. During these gap phases, the cell monitors both its internal state and external environment. This monitoring ensures that conditions are suitable for reproduction and that preparations are complete before the cell commits to the major upheavals of S phase and mitosis. During all of interphase, a cell generally continues to transcribe genes, synthesize proteins, and grow in mass. Together with S phase, G1 and G2 provide the time needed for the cell to grow and duplicate its cytoplasmic organelles. If interphase lasted only long enough for DNA replication, the cell would not have time to double its mass before it divided and would consequently shrink with each division. 4 4 2  2024-09-09 A Cell-Cycle Control System Triggers the Major Processes of the Cell Cycle The cell-cycle control system regulates progression through the cell cycle at three main transition points. At the transition from G1 to S phase, the control system confirms that the environment is favorable for proliferation before committing to DNA replication. Cell proliferation in animals requires both sufficient nutrients and specific signal molecules in the extracellular environment; if these extracellular conditions are unfavorable, cells can delay progress through G1 and may even enter a specialized resting state known as G0 (G zero). At the transition from G2 to M phase, the control system confirms that the DNA is undamaged and fully replicated, ensuring that the cell does not enter mitosis unless it’s DNA is intact. Finally, during mitosis, the cell-cycle control machinery ensures that the duplicated chromosomes are properly attached to a cytoskeletal machine, called the mitotic spindle, before the spindle pulls the chromosomes apart and segregates them into the two daughter cells. 5 5 The Cell-Cycle Control System Depends on Cyclically Activated Protein Kinases called Cdks The cell-cycle control system governs the cell-cycle machinery by cyclically activating and then inactivating the key proteins and protein complexes that initiate or regulate DNA replication, mitosis, and cytokinesis. This regulation is carried out largely through the phosphorylation and dephosphorylating of proteins involved in these essential processes. The protein kinases at the core of the cell-cycle control system are present in proliferating cells throughout the cell cycle. Switching these kinases on and off at the appropriate times is partly the responsibility of another set of proteins in the control system—the cyclins. Cyclins have no enzymatic activity themselves, but they need to bind to the cell-cycle kinases before the kinases can become enzymatically active. The kinases of the cell-cycle control system are therefore known as cyclin- dependent protein kinases, or Cdks. 6 6 3  2024-09-09 The Cell-Cycle Control System Depends on Cyclically Activated Protein Kinases called Cdks Cyclins are so-named because, unlike the cyclin-dependent protein kinases [Cdks], their concentrations vary in a cyclical fashion during the cell cycle. The cyclical changes in cyclin concentrations help drive the cyclic assembly and activation of the cyclin– cyclin-dependent protein kinases [Cdk] complexes. Once activated, these complexes help trigger various cell-cycle events, such as entry into S phase or M phase. The figure shows the changes in cyclin concentration and cyclin-dependent protein kinases [Cdk] activity responsible for controlling entry into M phase. Increasing concentration of the relevant cyclin (called M cyclin) helps direct the formation of the active cyclin–Cdk complex (M–Cdk) that drives entry into M phase. Although the enzymatic activity of each type of cyclin-Cdk complex rises and falls during the course of the cell cycle, the concentration of the Cdk component does not (not shown). 7 7 Different Cyclin–Cdk Complexes Trigger Different Steps in the Cell Cycle There are several types of cyclins and, in most eukaryotes, several types of Cdks involved in cell-cycle control. Different cyclin–Cdk complexes trigger different steps of the cell cycle. The cyclin that acts in G2 to trigger entry into M phase is called M cyclin, another cyclin S, help launch S phase late in G1. Cyclins form active complexes when they binds to appropriate cyclin-dependent protein kinases. Here you see M-Cdk and S-Cdk complexes 8 8 4  2024-09-09 Different Cyclin–Cdk Complexes Trigger Different Steps in the Cell Cycle Each of these cyclin–Cdk complexes phosphorylates a different set of target proteins in the cell. G1-Cdks, for example, phosphorylate regulatory proteins that activate transcription of genes required for DNA replication. By activating different sets of target proteins, each type of complex triggers a different transition step in the cycle. 9 9 Cyclin Concentrations are Regulated by Transcription and by Proteolysis The abrupt degradation of M and S cyclins part way through M phase depends on a large enzyme complex called— anaphase-promoting complex (APC). This complex tags these cyclins with a chain of ubiquitin. Proteins marked in this way are directed to proteasomes where they are rapidly degraded. The ubiquitylation and degradation of the cyclin returns its Cdk to an inactive state. The loss of cyclin renders its Cdk partner inactive. Cyclin destruction can help drive the transition from one phase of the cell cycle to the next. For example, M-cyclin degradation and the resulting inactivation of M-Cdk leads to the molecular events that take the cell out of mitosis. 10 10 5  2024-09-09 The Cell-Cycle Control System Can Pause the Cycle in Various Ways At the G1-to-S transition, it uses Cdk inhibitors to keep cells from entering S phase and replicating their DNA. At the G2-to-M transition, it suppresses the activation of M-Cdk by inhibiting the phosphatase required to activate the Cdk. And it can delay the exit from mitosis by inhibiting the activation of anaphase- promoting complex [APC], thus preventing the degradation of M cyclin. 11 11 Introduction G1 PHASE is an important point of decision-making for the cell. Based on intracellular signals that provide information about the size of the cell and extracellular signals reflecting the environment, the cell-cycle control machinery can either hold the cell transiently in G1, or allow it to prepare for entry into the S phase of another cell cycle. Once past this critical G1-to-S transition, a cell usually continues all the way through the rest of the cell cycle quickly— typically within 12–24 hours in mammals. The transition from G1 to S phase 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—either temporarily in G0, or permanently in the case of terminally differentiated cells. 12 12 6  2024-09-09 Mitogens Promote the Production of the Cyclins that Stimulate Cell Division As a general rule, mammalian cells will multiply only if they are stimulated to do so by extracellular signals, called mitogens, produced by other cells. If deprived of such signals, the cell cycle arrests in G1; if the cell is deprived of mitogens for long enough, it will withdraw from the cell cycle and enter a nonproliferating state, in which the cell can remain for days or weeks, months, or even for the lifetime of the organism. A crucial negative control is mediated by the Retinoblastoma (Rb) protein. Rb is abundant in the nuclei of all vertebrate cells, where it binds to particular transcription regulators and prevents them from turning on the genes required for cell proliferation. Mitogens release the Rb brake by triggering the activation of G1-Cdks and G1/S-Cdks. These complexes phosphorylate the Rb protein, altering its conformation so that it releases its bound transcription regulators, which are then free to activate the genes required for cell proliferation. 13 13 DNA Damage Can Temporarily Halt Progression Through G1 The cell-cycle control system uses several distinct mechanisms to halt progress through the cell cycle if DNA is damaged. DNA damage in G1 causes an increase in both the concentration and activity of a protein called p53, which is a transcription regulator that activates the transcription of a gene encoding a Cdk inhibitor protein called p21. The p21 protein binds to G1/S-Cdk and S-Cdk, preventing them from driving the cell into S phase. The arrest of the cell cycle in G1 gives the cell time to repair the damaged DNA before replicating it. 14 14 7  2024-09-09 Cells Can Delay Division for Prolonged Periods by Entering Specialized Nondividing States In the absence of appropriate signals, other cell types withdraw from the cell cycle only temporarily, entering an arrested state called G0. They retain the ability to reassemble the cell-cycle control system quickly and to divide again. Most liver cells, for example, are in G0, but they can be stimulated to proliferate if the liver is damaged 15 https://openi.nlm.nih.gov 15 S-Cdk Initiates DNA Replication and Blocks Re-Replication For eukaryotic cells, preparation for replication begins early in G1, when DNA is made replication-ready by the recruitment of proteins to the sites along each chromosome where replication will begin. In the first step of replication initiation, the origin recognition complex [ORC] recruits a protein called Cdc6, whose concentration rises early in G1. Together these proteins load the DNA helicases that will open up the double helix and ready the origin of replication. Once this prereplicative complex is in place, the replication origin is loaded and ready to “fire.” At the second step the signal to commence replication comes from S-Cdk, the cyclin–Cdk complex that triggers S phase. S-Cdk is assembled and activated at the end of G1. During S phase, it activates the DNA helicases in the prereplicative complex and promotes the assembly of the rest of the proteins that form the replication fork. S-Cdk not only triggers the initiation of DNA synthesis at a replication origin; it also helps prevent re- replication. It does so by helping phosphorylate Cdc6, which marks that protein for degradation. Eliminating Cdc6 helps ensure that DNA replication cannot be reinitiated later in the same cell cycle. 16 16 8  2024-09-09 Cohesins and Condensins Help Configure Duplicated Chromosomes for Separation When the cell enters M phase, the duplicated chromosomes condense, becoming visible under the microscope as threadlike structures. Protein complexes, called condensins, help carry out this chromosome condensation, which reduces mitotic chromosomes to compact bodies that can be more easily segregated within the crowded confines of the dividing cell. The assembly of condensin complexes onto the DNA is triggered by the phosphorylation of condensins by M-Cdk. 17 17 Cohesins and Condensins Help Configure Duplicated Chromosomes for Separation Immediately after a chromosome is duplicated during S phase, the two copies remain tightly bound together. These identical copies—called sister chromatids are held together by protein complexes called cohesins, which assemble along the length of each chromatid as the DNA is replicated. 18 18 9  2024-09-09 Different Cytoskeletal Assemblies Carry Out Mitosis and Cytokinesis After the duplicated chromosomes have condensed, two complex cytoskeletal machines assemble in sequence to carry out the two mechanical processes that occur in M phase. The mitotic spindle carries out nuclear division (mitosis), and, in animal cells and many unicellular eukaryotes, the contractile ring carries out cytoplasmic division (cytokinesis). Both structures disassemble rapidly after they have performed their tasks. The mitotic spindle is composed of microtubules and the various proteins that interact with them, including microtubule-associated motor proteins. The contractile ring consists mainly of actin filaments and myosin filaments arranged in a ring around the equator of the cell. 19 19 M Phase Occurs in Stages M phase proceeds as a continuous sequence of events, it is traditionally divided into a series of stages. The first five stages of M phase—prophase, prometaphase, metaphase, anaphase, and telophase— constitute mitosis, which was originally defined as the period in which the chromosomes are visible (because they have become condensed). Cytokinesis, which constitutes the final stage of M phase, begins before mitosis ends. 20 20 10  2024-09-09 Introduction Before nuclear division, or mitosis, begins, each chromosome has been duplicated and consists of two identical sister chromatids, held together along their length by cohesin proteins. During mitosis, the cohesin proteins are cleaved, the sister chromatids split apart, and the resulting daughter chromosomes are pulled to opposite poles of the cell by the mitotic spindle. 21 21 Centrosomes Duplicate To Help Form the Two Poles of the Mitotic Spindle The centrosome duplicates so that it can help form the two poles of the mitotic spindle and so that each daughter cell can receive its own centrosome. Centrosome duplication begins at the same time as DNA replication. As mitosis begins, however, the two centrosomes separate, and each nucleates a radial array of microtubules called an aster. The process of centrosome duplication and separation is known as the centrosome cycle. Centrosome duplication begins at the start of S phase and is complete by the end of G2. Initially, the two centrosomes remain together, but, in early M phase, they separate, and each nucleates its own aster of microtubules. 22 11  2024-09-09 The Mitotic Spindle Starts to Assemble in Prophase Some of the microtubules growing from one centrosome interact with the microtubules from the other centrosome. This interaction stabilizes the microtubules, preventing them from depolymerizing, and it joins the two sets of microtubules together to form the basic framework of the mitotic spindle, with its characteristic bipolar shape. The two centrosomes that give rise to these microtubules are now called spindle poles, and the interacting microtubules are called interpolar microtubules. 23 23 Chromosomes Attach to the Mitotic Spindle at Prometaphase Prometaphase starts abruptly with the disassembly of the nuclear envelope, which breaks up into small membrane vesicles. The spindle microtubules, which have been lying in wait outside the nucleus, now gain access to the duplicated chromosomes and capture them. The spindle microtubules grab hold of the chromosomes at kinetochores, protein complexes that assemble on the centromere of each condensed chromosome during late prophase. Each duplicated chromosome has two kinetochores—one on each sister chromatid— which face in opposite directions. 24 24 12  2024-09-09 Chromosomes Attach to the Mitotic Spindle at Prometaphase The three classes of microtubules that form the mitotic spindle are differently colored in this Figure: astral microtubules, kinetochore microtubules, and interpolar microtubules. 25 25 Chromosomes Line Up at the Spindle Equator at Metaphase The duplicated chromosomes align at the equator of the spindle, halfway between the two spindle poles, thereby forming the metaphase plate. This event defines the beginning of metaphase. Although the forces that act to bring the chromosomes to the equator are not completely understood, both the continual growth and shrinkage of the microtubules and the action of microtubule motor proteins are required. This fluorescence micrograph shows multiple mitotic spindles at metaphase in a fruit fly (Drosophila) embryo. The microtubules are stained red, and the chromosomes are stained green. 26 26 13  2024-09-09 Proteolysis Triggers Sister-Chromatid Separation at Anaphase Anaphase begins abruptly with the breakage of the cohesin linkages that hold sister chromatids together This release allows each chromatid—now considered a chromosome—to be pulled to the spindle pole to which it is attached. This movement segregates the two identical sets of chromosomes to opposite ends of the spindle. 27 27 Chromosomes Segregate During Anaphase The movement is the consequence of two independent processes that depend on different parts of the mitotic spindle. The two processes are called anaphase A and anaphase B, and they occur more or less simultaneously. In anaphase A, the kinetochore microtubules shorten and the attached chromosomes move poleward. The driving force for the movements of anaphase A is thought to be provided mainly by the loss of tubulin subunits from both ends of the kinetochore microtubules. In anaphase B, the two spindle poles move apart as the result of two separate forces: (1) the elongation and sliding of the interpolar microtubules past one another pushes the two poles apart, and (2) forces exerted on the outward- pointing astral microtubules at each spindle pole pull the poles away from each other, toward the cell cortex. Both forces are thought to depend on the action of motor proteins associated with the microtubules. 28 28 14  2024-09-09 The Nuclear Envelope Re-forms at Telophase During telophase, the final stage of mitosis, the mitotic spindle disassembles, and a nuclear envelope reassembles around each group of chromosomes to form the two daughter nuclei. Once the nuclear envelope has re-formed, the pores pump in nuclear proteins, the nucleus expands, and the condensed chromosomes decondense into their interphase state. As a consequence of this decondensation, gene transcription is able to resume. A new nucleus has been created, and mitosis is complete. 29 29 Introduction Whereas mitosis depends on a transient microtubule-based structure, the mitotic spindle, cytokinesis in animal cells depends on a transient structure based on actin and myosin filaments, the contractile ring. Both the plane of cleavage and the timing of cytokinesis, however, are determined by the mitotic spindle. 30 30 15  2024-09-09 The Contractile Ring of Animal Cells Is Made of Actin and Myosin Filaments The contractile ring assembles at anaphase and is attached to membrane-associated proteins on the cytoplasmic face of the plasma membrane. Much of this force is generated by the sliding of the actin filaments against the myosin filaments. 31 31 The Contractile Ring of Animal Cells Is Made of Actin and Myosin Filaments Once cytokinesis is complete, the daughter cells reestablish their strong contacts with the substratum and flatten out again. When cells divide in an animal tissue, this cycle of attachment and detachment presumably allows the cells to rearrange their contacts with neighboring cells and with the extracellular matrix, so that the new cells produced by cell division can be accommodated within the tissue. Note how the cell rounds up as it enters mitosis; the two daughter cells then flatten out again after cytokinesis is complete. 32 32 16  2024-09-09 Apoptosis Is Mediated by an Intracellular Proteolytic Cascade Cells that die as a result of acute injury typically swell and burst, spilling their contents all over their neighbors, a process called cell necrosis. This eruption triggers a potentially damaging inflammatory response. 33 33 Apoptosis Is Mediated by an Intracellular Proteolytic Cascade By contrast, a cell that undergoes apoptosis dies neatly, without damaging its neighbors. A cell in the throes of apoptosis may develop irregular bulges—or blebs—on its surface; but it then shrinks and condenses. The cytoskeleton collapses, the nuclear envelope disassembles, and the nuclear DNA breaks up into fragments. Most importantly, the cell surface is altered in such a manner that it immediately attracts phagocytic cells, usually specialized phagocytic cells called macrophages. The large vacuoles seen in the cytoplasm of the cell in picture are a variable feature of apoptosis. 34 34 17  2024-09-09 Apoptosis Is Mediated by an Intracellular Proteolytic Cascade Macrophages engulf the apoptotic cell before it spills its contents. This rapid removal of the dying cell avoids the damaging consequences of cell necrosis, and it also allows the organic components of the apoptotic cell to be recycled by the cell that ingests it the cell sown in (C) died in a developing tissue and has been engulfed by a phagocytic cell. 35 35 Apoptosis Is Mediated by an Intracellular Proteolytic Cascade The molecular machinery responsible for apoptosis, which seems to be similar in most animal cells, involves a family of proteases called caspases. These enzymes are made as inactive precursors, called procaspases, which are activated in response to signals that induce apoptosis. Two cleaved fragments from each of two procaspase molecules associate to form an active caspase, which is formed from two small and two large subunits; 36 36 18  2024-09-09 Apoptosis Is Mediated by an Intracellular Proteolytic Cascade Two types of caspases work together to take a cell apart. Initiator caspases cleave, and thereby activate, downstream executioner caspases. Some of these executioner caspases then activate additional executioners, kicking off an amplifying, proteolytic cascade; others dismember other key proteins in the cell Each activated initiator caspase molecule can cleave many executioner procaspase molecules, thereby activating them, and these can activate even more procaspase molecules. In this way, an initial activation of a small number of initiator caspase molecules can lead, via an amplifying chain reaction (a cascade), to the explosive activation of a large number of executioner caspase molecules. 37 37 The Intrinsic Apoptotic Death Program Is Regulated by the Bcl2 Family of Intracellular Proteins All nucleated animal cells contain the seeds of their own destruction: in these cells, inactive procaspases lie waiting for a signal to destroy the cell. Bax and Bak are death promoting members of the Bcl2 family of intracellular proteins that can trigger apoptosis by releasing cytochrome c from mitochondria. When Bak or Bax proteins are activated by an apoptotic stimulus, they aggregate in the outer mitochondrial membrane, leading to the release of cytochrome c by an unknown mechanism. The cytochrome c is released into the cytosol from the mitochondrial intermembrane space (along with other proteins in this space—not shown). Cytochrome c then binds to an adaptor protein, causing it to assemble into a seven-armed complex. This complex then recruits seven molecules of a specific initiator procaspase (procaspase-9) to form a structure called an apoptosome. The procaspase-9 proteins become activated within the apoptosome and then go on to activate executioner procaspases in the cytosol, leading to a caspase cascade and apoptosis. 38 38 19  2024-09-09 Extracellular Signals Can Also Induce Apoptosis Sometimes the signal to commit suicide is not generated internally, but instead comes from a neighboring cell. Some of these extracellular signals activate the cell death program by affecting the activity of members of the Bcl2 family of proteins. Others stimulate apoptosis more directly by activating a set of cell-surface receptor proteins known as death receptors. One particularly well-understood death receptor, called Fas, is present on the surface of a variety of mammalian cell types. Fas is activated by a membrane-bound protein, called Fas ligand, present on the surface of specialized immune cells called killer lymphocytes. These killer cells help regulate immune responses by inducing apoptosis in other immune cells that are unwanted or are no longer needed. 39 39 Animal Cells Require Extracellular Signals to Survive, Grow, and Divide. Most of the extracellular signal molecules that influence cell survival, cell growth, and cell division are either soluble proteins secreted by Survival factors other cells or proteins that are bound to the surface of other cells or to the extracellular matrix. The positively acting signal proteins can be classified, on the basis of their function, into three major categories: 1. Survival factors Mitogens promote cell survival, largely by suppressing apoptosis.2. Mitogens stimulate cell division, primarily by overcoming the intracellular braking mechanisms that tend to block progression through the cell cycle. 3. Growth factors stimulate Growth factors cell growth (an increase in cell size and mass) by promoting the synthesis and inhibiting the degradation of proteins and other macromolecules. 40 40 20  2024-09-09 Essential Cell Biology-4e Chapter 18 The Cell-Division Cycle 603 The Eukaryotic Cell Cycle Usually Includes Four Phases A Cell-Cycle Control System Triggers the Major Processes of the Cell Cycle The Cell-Cycle Control System The Cell-Cycle Control System Depends on Cyclically Activated Protein Kinases called Cdks Different Cyclin–Cdk Complexes Trigger Different Steps in the Cell Cycle Cyclin Concentrations are Regulated by Transcription and by Proteolysis G1 PHASE Cdks are Stably Inactivated in G1 DNA Damage Can Temporarily Halt Progression Through G1 S Phase S-Cdk Initiates DNA Replication and Blocks Re-Replication M Phase M-Cdk Drives Entry Into M Phase and Mitosis Cohesins and Condensins Help Configure Duplicated Chromosomes for Separation Different Cytoskeletal Assemblies Carry Out Mitosis and Cytokinesis Mitosis Centrosomes Duplicate To Help Form the Two Poles of the Mitotic Spindle The Mitotic Spindle Starts to Assemble in Prophase Chromosomes Attach to the Mitotic Spindle at Prometaphase Chromosomes Line Up at the Spindle Equator at Metaphase Proteolysis Triggers Sister-Chromatid Separation at Anaphase Chromosomes Segregate During Anaphase The Nuclear Envelope Re-forms at Telophase Cytokinesis The Mitotic Spindle Determines the Plane of Cytoplasmic Cleavage The Contractile Ring of Animal Cells Is Made of Actin and Myosin Filaments Membrane-Enclosed Organelles Must Be Distributed to Daughter Cells When a Cell Divides Control of Cell Numbers and Cell Size Apoptosis Helps Regulate Animal Cell Numbers Apoptosis Is Mediated by an Intracellular Proteolytic Cascade The Intrinsic Apoptotic Death Program Is Regulated by the Bcl2 Family of Intracellular Proteins Extracellular Signals Can Also Induce Apoptosis 41 Animal Cells Require Extracellular Signals to Survive, Grow, and Divide 41 21