Mitosis Lecture Notes PDF

Unit IV Cell cycle, Events in cell cycle Interphase, Mitotic phase,stages of mitosis Mitotic blockage, Stimulators of mitosis, Significance of mitosis. Dr. Manikandan Kathirvel M.Sc., Ph.D., (NET) Assistant Professor, Department of Life Sciences, Kristu Jayanti College (Autonomou...

Unit IV Cell cycle, Events in cell cycle Interphase, Mitotic phase,stages of mitosis Mitotic blockage, Stimulators of mitosis, Significance of mitosis. Dr. Manikandan Kathirvel M.Sc., Ph.D., (NET) Assistant Professor, Department of Life Sciences, Kristu Jayanti College (Autonomous), (Reaccredited with "A" Grade by NAAC) Affiliated to Bengaluru North University, K. Narayanapura, Kothanur (PO) Bengaluru 560077  Cell cycle (cell division cycle) is an Mitosis ordered sequence of events occurring in a cell. Cell cycle results in cell growth and DNA replication thereby forming two daughter cells. It is an essential process for the formation of a mature organism from single-celled fertilized eggs. The process of cell cycle promotes renewal and regeneration of hair, blood cells, skin, and certain internal organs. Stages of Cell Cycle I = Interphase, M = Mitosis; inner ring: M = Mitosis, G1 = Gap 1, G2 = Gap 2, S = Synthesis; not in ring: G0 = Gap 0/Resting. Cells in actively growing tissue go through a cycle of metabolic activity, DNA replication, chromosome segregation and cell division known as the cell cycle. Different Stages of a Cell Cycle 1. The events occurring in the life of the cell constitute cell cycle. 2. There are two phases of cell cycle: Interphase and Mitotic phase. 3. The duration of cell varies from hours to years. 4. A typical human cell cycle has duration of 24-90 hours. Mitosis, the process of chromosome condensation and separation into new daughter nuclei, Interphase: In this phase, the cell grows and produces a copy of the genetic material (DNA). 1. It is the longest phase in a cell. Out of 90 hours, the interphase lasts for 89 hours. 2. It is the resting phase of the cell. It is the longest phase. 3. The nucleus is enlarged and intact and nuclear membrane is also intact of nucleolus. 4. There is no cell division occurs in this phase. This stage prepares the cell for cell division. 5. The metabolic activity is high in this phase. 6. The mRNA and rRNA are synthesized during this phase. 7. The chromosomes duplicates into 2 chromatids. 8. The centriole duplicates into 2, to form the centriole microtubule and microfilaments start arising and this microtubules form asters. 9. Interphase can be further subdivided into three distinct phases: G1 phase, S phase (synthesis), G2 phase. G0 –phase: Resting phase of cell. Every phase will be successfully activated on proper progression and completion of the previous phases. However, if a cell is temporarily stopped progressing or somehow stopped dividing then the cell enter into another state termed as G0 phase, also called a “state of quiescence.” Gap 0 (G0) Phase: At times the cell will leave the cycle and temporarily stop dividing. This is called a resting period. It can be for a short time or long more permanent period. For example neurons after reaching the end stage of development stop dividing and enter into a more permanent resting phase. Sub-phases of Interphase G1 phase: 1. Stands for gap phase. 2. It is the first growth period phase. 3. It is the longest phase. 4. The cell may last for years at this phase. 5. Daughter cell grows and increases in size. 6. The stage last for 25-50% of total interphase and proteins. 7. During this phase mRNA, rRNA and tRNA are synthesized in this phase. Gap 1 (G1) Phase: It is also termed as the first gap phase. In this phase, the cell starts growing and enlarges physically. It forms the copy of organelles, produces all the necessary molecular building blocks such as RNA and also synthesizes proteins that are essential in later stages. During this phase, the cell is metabolically active and continues to grow without replicating its DNA At this point, a control mechanism is activated to ensure proper DNA synthesis. The control mechanism is termed as the G1 checkpoint.  S-phase:  S stands for synthesis phase.  During this phase DNA synthesis occurs.  The DNA molecule duplicates.  It last for 35-40% of interphase. S Phase: In this phase, a cell produces a complete copy of DNA in the nucleus to produce two similar daughter cells. DNA replication begins in the S phase or the synthesis phase. The microtubule-organizing structure (centrosome) is also copied in this phase. The centrosome is the structure that helps in dividing the DNA during M phase.  G2 phase: 1. It is gap period between S phase and mitotic phase of the cell cycle. 2. The nucleus increases in volume increases in metabolic activity. 3. Transcription and translation of missionary needed for cell division. 4. G2 phase is followed by Mitotic phase. Gap 2 (G2) phase: In G2 phase the cell grows further, produce proteins and organelles and starts rearranging the constituents of the cell for mitosis phase. During this phase, the RNA, proteins, other macromolecules required for multiplication of cell organelles, spindle formation, and cell growth are produced as the cell prepares to go into the mitotic phase. At the end of the G2 phase, another checkpoint is activated called as G2 Checkpoint. G2 Checkpoint ensures everything is ready for division and M phase. The G2 phase ends when the mitosis process begins. 2. Mitotic phase or M phase: This is the division phase. M stands for mitosis. During this phase the cell divides. This phase has a short duration. A typical human cell cycle has duration of 24-90 hours. The M phase has the duration of 45 min to 1 hour and two phases: A) Karyokinesis refers to the division of Nucleus into two daughter nuclei. The cell’s “nuclear DNA” is condensed into chromosomes. These visible chromosomes are pulled apart with the help of mitotic spindles (the special structures formed from microtubules). Karyokinesis has four sub stages, namely prophase, metaphase, anaphase and telophase. B) Cytokinesis refers to the division of the cytoplasm resulting in two daughter cells. In this phase, the cytoplasm of the cell is divided into two daughter cells. Duration of the Cell Cycle o The cell cycle duration will vary in different types of cells. o The G1 phase will continue for approximately 11 hours, o S phase will continue for 8 hours, o G2 phase for nearly 4 hours and o the M phase for nearly one hour in a rapidly dividing human cell with cell cycle duration of 24 hours. o Some cells may divide faster than human cells whereas some cells may take more time to complete an entire cell cycle. o For example “budding yeast” will complete the entire cell cycle (4 stages of the cell cycle) in about 90 minutes. Karyokinesis Prophase 1. It is the first stage of mitosis. 2. Cell becomes spheroid and viscous 3. Nuclear membrane disintegrates and disappears into a cytoplasm. 4. The chromosomes become shortened and thickened. 5. Each chromosome is formed of two chromatids. The two chromatids of a chromosome are connected by a centromere. 6. The nucleolus starts to disappear. Metaphase 1. The chromosomes lie at equatorial plane. 2. Fibres of spindle attach with centromere of each chromosome and are known as chromosomal fibres. 3. The fibres which occurs in between the chromosomes are called interzonal fibres. 4. The centromere of each chromosome divides into two. Anaphase 1. The chromatids of each chromosomes are separated and move towards the opposite poles forming daughter chromosomes. 2. The daughter chromosomes achieved by the contraction of centrosomal givers and the stretching of the interzonal fibres. Telophase 1. The chromosome with their centromeres at the poles begins to uncoil and lengthens. They aggregate together to form a mass at the poles. 2. The nucleolus begins to reappear. 3. New nuclear membrane develops around the chromosomes. 4. Two daughter nucleus are formed. B) Cytokinesis: The division of the cytoplasm into two daughter cells is called Cytokinesis. The division starts as a constriction. This constriction depends and thus daughter cells are formed from a single parent cell. A unit membrane develops between the two cells. This plane of division of cytoplasm is perpendicular to the spindle. In cytokinesis, the cytoplasm of the cell is split in two, making two new cells. Cytokinesis usually begins just as mitosis is ending, with a little overlap. Importantly, cytokinesis takes place differently in animal and plant cells. In animals, cell division occurs when a band of cytoskeletal fibers called the contractile ring contracts inward and pinches the cell in two, a process called contractile cytokinesis. The indentation produced as the ring contracts inward is called the cleavage furrow. Animal cells can be pinched in two because they’re relatively soft and squishy. Plant cells are much stiffer than animal cells; they’re surrounded by a rigid cell wall and have high internal pressure. Because of this, plant cells divide in two by building a new structure down the middle of the cell. This structure, known as the cell plate, is made up of plasma membrane and cell wall components delivered in vesicles, and it partitions the cell in two. Q: Which is the longest phase of the cell cycle? 1.G2 2.G1 3.S 4.G0 Sol: The correct answer is option “B”. G1 phase is usually the longest phase of cell cycle. G1 phase is the first gap phase where the cell is preparing for the other stages of cell cycle. Moreover, G1 phase follows the mitosis cell division. It is the time for the newly formed cells to grow before the DNA replication. So, the G1 phase is the longest. G1 phase can vary in the different type of cells. It can last for minutes such as prokaryotic cells, hours such as yeast or sometimes for years such as liver cells. Mitotic blockage, or the interruption of cell division during mitosis, can occur due to various factors: 1. Anti-Microtubule Drugs: These drugs interfere with microtubules, which are essential for spindle formation and chromosome segregation during mitosis. Such interference leads to mitotic arrest. Paclitaxel (Taxol). Paclitaxel stabilizes microtubules, preventing their depolymerization. This inhibits the normal dynamic rearrangement of microtubules required for proper chromosome segregation during mitosis. Vincristine/Vinblastine:Mechanism: These vinca alkaloids disrupt microtubule assembly by binding to tubulin, preventing the formation of the mitotic spindle. 2. DNA Damage-Induced Mitotic Block: Cells have checkpoints in the cell cycle to detect and respond to DNA damage. If DNA damage occurs, the cell can block progression into mitosis or arrest mitosis if the damage is detected at this stage. Radiation-Induced DNA Damage Chemotherapy (e.g., Doxorubicin, Cisplatin) 3. Targeting Cell Cycle Regulatory Proteins: Inhibition of proteins that regulate mitosis can also lead to mitotic blockage. Cdc25 Phosphatase Inhibitors: Cdc25 is a phosphatase that activates cyclin-dependent kinases (CDKs) required for entry into mitosis. Inhibiting Cdc25 prevents the activation of CDK1/cyclin B, necessary for mitotic entry. 4. Nutrient Deprivation or Cellular Stress: Stress conditions, such as nutrient deprivation such as Amino Acid Starvation, hypoxia, or oxidative stress, can trigger cell cycle arrest to prevent mitotic progression in unfavorable conditions. 5. Aneuploidy-Causing Agents: These agents such as Nocodazole increase the likelihood of chromosome missegregation, causing mitotic checkpoint activation. Nocodazole disrupts microtubules, preventing the proper attachment of chromosomes to the mitotic spindle. Stimulation of cell division The stimulation of cell division during mitosis (in somatic cells) is a highly regulated process that ensures proper DNA replication and segregation into two daughter cells. The cell division cycle, leading to mitosis, is divided into several phases: G1, S, G2, and M (mitosis), with multiple checkpoints to ensure the fidelity of division. Here's a detailed breakdown of the key processes involved in the stimulation of cell division specifically during mitosis: 1. Activation of Cyclins and CDKs The Cyclin-CDK complex is essential for driving the cell through the phases of the cell cycle, especially into mitosis. Cyclin B binds to CDK1 (Cyclin-dependent kinase 1) to form the Maturation Promoting Factor (MPF). This complex is crucial for the cell to enter mitosis (M phase) from the G2 phase. MPF phosphorylates several target proteins that promote: Chromosome condensation (via histone phosphorylation). Nuclear envelope breakdown. Spindle assembly. The activity of this complex is tightly regulated by phosphorylation and dephosphorylation events. Cyclins and Cyclin-Dependent Kinases (CDKs) Cyclins and CDKs are essential in regulating the cell cycle. Cyclins activate CDKs, which then phosphorylate target proteins to drive the cell through different phases of the cell cycle. Specific cyclins control each stage of the cycle: Cyclin D/CDK4 or CDK6 controls the G1/S transition. Cyclin E/CDK2 promotes entry into the S phase (DNA replication). Cyclin B/CDK1 drives the cell into mitosis (M phase). Entry into Mitosis (M Phase) The cell enters mitosis in response to the activation of the Cyclin B/CDK1 complex. Mitotic kinases (such as Aurora kinases and Polo-like kinases) help ensure that proper spindle assembly, chromosome segregation, and cytokinesis take place. 2. Growth Factors Growth factors are proteins or hormones that bind to receptors on the cell surface and stimulate cells to enter the cell cycle. They activate signaling cascades that promote the transition from the G0 or G1 phase to the S phase. Examples: Epidermal Growth Factor (EGF) Platelet-Derived Growth Factor (PDGF) Fibroblast Growth Factor (FGF) 3. Nutritional Status and Energy Sensing Cells require a sufficient supply of nutrients and energy to divide. The AMPK and mTOR pathways play crucial roles in sensing energy levels. The mTOR pathway promotes protein synthesis and cell growth, thus stimulating cell division when conditions are favorable. The mTOR (mechanistic target of rapamycin) pathway senses nutrient and energy status. When conditions are favorable (i.e., sufficient nutrients, energy, and growth factors), mTOR promotes protein synthesis and cell growth, ensuring that the cell has the necessary resources to complete mitosis. Activation of the mTOR pathway further stimulates mitosis by promoting the production of proteins required for spindle assembly and chromosome segregation. 4. Anaphase Promoting Complex (APC/C) The Anaphase Promoting Complex/Cyclosome (APC/C) is a ubiquitin ligase that triggers the transition from metaphase to anaphase by marking specific proteins for degradation. One of its targets is securin, which inhibits the enzyme separase. Upon securin degradation, separase is activated, leading to the cleavage of cohesin proteins that hold sister chromatids together, allowing them to separate and move to opposite poles of the cell. APC/C also targets cyclin B for degradation, leading to the inactivation of CDK1, which is necessary for the exit from mitosis and the completion of cytokinesis. 5. Mitotic Checkpoints The Spindle Assembly Checkpoint (SAC) ensures that all chromosomes are correctly attached to the spindle microtubules before proceeding to anaphase. This checkpoint prevents the premature separation of sister chromatids and ensures that mitosis only proceeds when the conditions are optimal for correct chromosome segregation. Proteins like BubR1, Mad2, and Cdc20 are key players in regulating this checkpoint. 6. Cytokinesis Following mitosis, cytokinesis divides the cytoplasm and organelles between the two daughter cells. The process is mediated by a contractile ring composed of actin and myosin filaments that constrict the cell membrane to form two separate cells. Cytokinesis is regulated by the Rho family of GTPases, which control the organization of the actin cytoskeleton during this process. Significance of mitosis: Mitosis is an essential biological process with several critical functions for the growth, maintenance, and survival of multicellular organisms. Its significance spans various biological contexts, from normal tissue growth to healing and even its implications in disease processes such as cancer. 1.Mitosis helps maintain the number of chromosomes during cell division. Mitosis ensures that each daughter cell receives an identical set of chromosomes. This is crucial for maintaining the diploid chromosome number (2n) in somatic cells. Errors in mitosis can lead to an abnormal number of chromosomes (aneuploidy), which may result in developmental disorders or diseases like cancer. 1.This process is important for the growth and development of organisms Mitosis is the primary mechanism by which multicellular organisms grow. It allows a single fertilized egg (zygote) to develop into a fully formed organism by producing billions of cells through successive divisions. It ensures that each daughter cell receives an identical set of genetic information, maintaining consistency across the body's tissues. 3. It helps repair damaged tissues. Mitosis plays a crucial role in wound healing and tissue regeneration. 4. It helps the cell to maintain proper size 5. Unregulated Mitosis and Cancer: While mitosis is usually a well-regulated process, when the regulatory mechanisms fail, uncontrolled cell division can occur, leading to tumor formation and cancer. Cancer cells bypass normal cell cycle controls and divide rapidly through mitosis. Understanding mitosis is key in cancer research for developing therapies that target cell division. 6. Maintenance of Multicellular Organisms Homeostasis: Mitosis plays a critical role in maintaining tissue homeostasis by balancing cell death (apoptosis) and cell division. For example, in tissues like the intestines, where cells are regularly lost, mitosis ensures a constant supply of new cells to maintain normal function. 7. Replacement of Senescent Cells Over time, cells undergo senescence and lose their ability to divide. Mitosis ensures that new, healthy cells replace these senescent cells, thus maintaining the integrity of tissues and organs. 8. Role in Immune Response During an immune response, certain white blood cells like T cells and B cells proliferate rapidly through mitosis. This clonal expansion is necessary for mounting a strong defense against pathogens like bacteria and viruses. Summary of Mitosis Significance: Growth: Allows organisms to increase in size and complexity. Tissue Repair: Replaces damaged or dead cells, facilitating healing. Genetic Stability: Ensures daughter cells have identical genetic information. Asexual Reproduction: Enables single-celled organisms and some plants to reproduce without genetic variation. Homeostasis: Maintains cellular equilibrium in tissues. Disease Implications: Abnormal mitosis can lead to cancer, making it a key target for therapies.