Cell Cycle and Cell Division PDF

Cell Cycle and Cell Division Objectives ▪ List the phases in the cell cycle ▪ Describe the checkpoints at each stage in the cell cycle and be able to list at least one consequence of failure of a checkpoint ▪ Describe the interactions of cyclin-Cdk complexes with cell cycle control proteins i...

Cell Cycle and Cell Division Objectives ▪ List the phases in the cell cycle ▪ Describe the checkpoints at each stage in the cell cycle and be able to list at least one consequence of failure of a checkpoint ▪ Describe the interactions of cyclin-Cdk complexes with cell cycle control proteins including p53 and pRb ▪ List the steps of mitosis ▪ Be able to identify prophase, metaphase, anaphase, and telophase in Giemsa stained or electron micrograph images ▪ Compare and contrast the steps of meiosis 1 with mitosis ▪ Describe the end results of mitosis vs. meiosis Overview of the Cell Cycle ▪ Cell cycle is the process through which cells replicate and make two new cells ▪ We can also define cell cycle as a series of events that takes place in a cell as it grows and divides ▪ The cell cycle is generally divided into two phases: ▪ Interphase and ▪ Mitosis ▪ During interphase, the cell spends most of its time performing the functions that make it unique ▪ Mitosis is the phase of the cell cycle during which the cell divides into two daughter cells Stages of Cell Cycle ▪ A cell spends most of its time in what is called the interphase: ▪ During this time: ▪ it grows, ▪ replicates its chromosomes, ▪ and prepares for cell division ▪ The cell then leaves interphase, undergoes mitosis, and completes its division ▪ The resulting cells, known as daughter cells, ▪ each enter their own interphase and begin a new round of the cell cycle Stages of Cell Cycle cont. ▪ The interphase stage of the cell cycle includes three distinctive phases: ▪ G1 (presynthesis) ▪ S (DNA replication) ▪ The DNA replication that occurs during interphase cannot be seen under the microscope ▪ G2 (post-DNA duplication) ▪ Cells not actively dividing are temporarily or permanently suspended in G0 Cell Cycle Phases G1 Phase: ▪ Cell gathers nutrients and synthesizes RNA and proteins required for DNA synthesis and chromosome replication S Phase: ▪ DNA synthesis occurs ▪ New chromatids are formed – process takes about 7.5-10 hours ▪ Chromosome replication is initiated at many sites called replicons along the chromosomal DNA ▪ Un-replicated areas are segregated by anaphase bridges  Cell Cycle Phases cont. G2 Phase: ▪ Cell examines replicated DNA in preparation for cell division ▪ Cell growth and cytoplasmic organelle reorganization can also occur ▪ Very short phase in rapidly dividing cells M Phase: ▪ Mitosis occurs here – takes about 1 hour ▪ Concludes with the separation of two daughter cells: ▪ Karyokinesis (division of the nucleus) ▪ Cytokinesis (division of the cell cytoplasm) Cell Cycle Phases cont. G0 Phase: ▪ While some cells are constantly dividing, some cell types are quiescent ▪ These cells exit G1 and enter a resting state called G0 ▪ In G0 , a cell is performing its function without actively preparing to divide ▪ G0 is a permanent state for some cells, while others may restart division if they get the right signals Cell Cycle Phases Summary G2 phase M phase Continued cell Cell separates replicated growth chromosomes and divides into DNA checked for two cells (mitosis) errors in replication S phase G1 phase DNA replication Cell growth – organelles and commences at cytoplasm doubles ‘G0’ ‘origins of replication’. Cells that have stopped dividing Results in exact enter ‘G0’; a ‘holding point’ within ‘semi-conserved’ G1. copies of double Stem cells hang out here stranded chromosome G1-S-G2 is called ‘Interphase’ Slide credit: Dr. Roger Young Cell Cycle Checkpoints ▪ Cell cycle checkpoints are used by the cell to monitor and regulate the progress of the cell cycle ▪ The cell can’t proceed to the next phase until checkpoint requirements have been met ▪ There are three main checkpoints: ▪ G1/S checkpoint (before cell enters S phase) ▪ G2/M checkpoint (after S phase) ▪ M/G1 checkpoint (during mitosis)  Cell Cycle Checkpoints cont. G1/S checkpoint (before cell enters S phase): ▪ All the checkpoints require the services of complex of proteins ► Checks for cell size ▪ The levels of these proteins are increased ► Checks for nutrients in damaged cells: ▪ They allow time to repair DNA by ► Checks for DNA damage blocking the cell cycle ► Checks for all the preparations (all proteins, ATP etc. required in S phase) ▪ P53 is a protein which senses DNA ► Checks whether S phase Cyclins and Cdk complex is activated damage and can halt the progression of to initiate DNA replication the cell cycle in the G1 phase by ▪ Then the cell passes to the next S phase blocking the activity of Cdk2 until the damage can be repaired G2/M checkpoint (after S phase): ▪ If the damage is so severe that it can’t be ► Checks for proper DNA replication repaired, then the cell destructs itself by ► Checks for all the preparations apoptosis (all proteins, ATP etc. required in M phase) ▪ In case of damage to DNA after the S ► Checks for Tubulin synthesis phase, the action of Cdk1 is inhibited, ► Checks whether M phase Cyclins and Cdk complex is activated to thus stopping the progression of the cell initiate mitosis from G2 to mitosis ▪ Then the cell passes to the next M phase Cell Cycle Checkpoints cont. ▪ M/G1 checkpoint: ► Checks the assembly of mitotic spindles ► Prevents premature entry into anaphase ► Checks chromosome segregation ► Prevents cytokinesis until all of the chromosomes have correctly separated https://openoregon.pressbooks.pub/mhccbiology112/chapter/10-3-control-of-the-cell-cycle/ Cell Cycle Regulation ▪ Cell cycle does not occur in unchecked manner ▪ The preparations of the cell are checked by regulatory molecules which are proteins that are responsible about cell cycle regulation ▪ This includes: ▪ Detection and repair of genetic damage ▪ Prevention of uncontrolled cell division ▪ There are two key classes of regulatory molecules (cell cycle regulators) that determine the mechanism of cell cycle regulation: ▪ Cyclins ▪ Cyclin dependent kinases (CDK) Cyclins ▪ Determine the Activity of G1 Cyclins (Cyclin D): CDKs ▪ Coordinate the cell cycle with extracellular events ▪ Four classes defined by their ▪ Their activity is subject to regulation by signal transduction presence and activity during pathways that sense the presence of growth factors or cell the cell cycle: proliferation inhibitory signals ▪ Interact with CDK4 and CDK6 ▪ G1 Cyclins (Cyclin D) ▪ Promote the entry into the cell cycle ▪ G1/S Cyclin (Cyclin E) ▪ S-phase Cyclins (Cyclins E G1/S cyclin (Cyclin E): and A) ▪ Accumulates during late G1 ▪ M-phase cyclins (Cyclin B) ▪ Bind to CDK 2 ▪ Cyclin E -CDK 2 complex: ▪ Triggers the G1-S phase transition ▪ This transition is known as START: ▪ The point at which cells are irreversibly committed to cell division ▪ and can no longer return to the G1 state Cyclins cont.. ▪ Determine the Activity of S-phase cyclins (Cyclin A and Cyclin E): CDKs ▪ Synthesized at the end of G1 ▪ Levels remain high throughout the S phase and do not decline ▪ Four classes defined by their until early mitosis presence and activity during ▪ Trigger S-phase the cell cycle: ▪ Two types: ▪ Cyclin E: ▪ G1 Cyclins (Cyclin D) ▪ Promotes entry into the cell cycle: ▪ and is therefore also a G1/S cyclin ▪ G1/S Cyclin (Cyclin E) ▪ Cyclin A: ▪ S-phase Cyclins (Cyclins E ▪ Both cyclins bind CDK 2 and are directly responsible for DNA and A) synthesis ▪ M-phase cyclins (Cyclin B) Mitotic cyclins (B Cyclin): ▪ Bind CDK1 to promote entry into and progression through mitosis ▪ The most important regulatory ▪ Mitotic cyclin-CDK complexes are synthesized during S-phase control that restricts cyclins to the and G2 appropriate cell cycle stage is: ▪ but their activities are held in check until DNA synthesis is completed ▪ Ubiquitin-mediated protein degradation Cyclin Dependent Kinases (CDKs) Cyclin-dependent kinases ▪ CDKs play important roles in cell cycle regulation: (CDKs) are protein kinases ▪ They control cell division and modulate characterized by needing a transcription in response to several extra- and separate subunit – a cyclin – intracellular cues that provides domains essential ▪ The evolutionary expansion of the CDK family in for their enzymatic activity mammals led to the division of CDKs into three cell-cycle-related sub-families : ▪ G1 Cdk (Cdk4 or Cdk6) ▪ S-phase Cdk (Cdk2) ▪ M-phase Cdk (Cdk1) Cyclin Dependent Kinases (CDKs) cont. ▪A CDK binds a regulatory ▪ The levels of CDKs in the cell remain stable protein (cyclin) ▪ Remain inactive ▪ Without cyclin, CDK has little kinase activity ▪ Bind to the appropriate cyclin in order to be ▪ Only the cyclin-CDK complex activated is an active kinase Function: ▪ but its activity can be typically ▪ Provide a phosphate group to a number of further modulated by: ▪ phosphorylation and other proteins that control processes in the cell cycle binding proteins: ▪ like p27 ▪ The E2 factor (E2F) family of transcription factors are downstream effectors of the retinoblastoma (RB) protein pathway and are believed to play a pivotal role in cell division control ▪ The RB/E2F pathway regulates apoptosis, and RB inhibition of apoptosis is an important mechanism of tumor suppression whereby cells deficient for RB function can be eliminated by apoptosis Fast facts About Cell Cycle ▪ Cells in S phase have more DNA than cells in G1 phase ▪ Cells in G2 will have approximately twice DNA content as cells in G1 When each successive set of activities is complete, the cyclin controlling that cell cycle phase is ubiquitinated and removed rapidly by proteasomes and a new cyclin that promotes activities for the next phase takes over ▪ The retinoblastoma protein (pRb) is a tumor suppressor protein that is dysfunctional in several major cancers ▪ One function of pRb is to prevent excessive cell growth by inhibiting cell cycle progression until a cell is ready to divide (pRb prevents expression of E2F and cell cycle progression) ▪ which means that it regulates cell growth and keeps cells from dividing too fast or in an uncontrolled way ▪ dysfunctional pRb can cause malignancies such as: ▪ mesothelioma, osteosarcoma, or ependymoma Cell Division A crucial process that: ▪ Increases cell numbers ▪ Permits renewal of cell populations ▪ Allows wound repair Two types: ▪ Mitosis: ▪ Common type ▪ Occurs in all cells ▪ Meiosis: ▪ Occurs only in germ cells in the gonads  ▪Mitosis goes through 4 phases: Mitosis ▪Prophase ▪Metaphase ▪Anaphase ▪ Mitosis refers to a type of ▪Telophase cell division where replicated chromosomes and their genes are partitioned equally into two identical groups: ▪ Includes: ▪ Karyokinesis: ▪ Division of the nucleus ▪ Cytokinesis: ▪ Division of cytoplasm and organelles equally into the two daughter cells Prophase Prophase: ▪ The nucleolus disappears ▪ Replicated chromatin condenses into discrete threadlike chromosomes: ▪ each consisting of duplicate sister chromatids joined at the centromere ▪ Centrosomes and duplicated centrioles separate ▪ migrate to opposite poles of the cell ▪ and organize the microtubules of the mitotic spindle ▪ Late in prophase, lamins and inner nuclear membrane are phosphorylated, causing the nuclear lamina and nuclear pore complexes to disassemble and disperse in cytoplasmic membrane vesicles Centrosomes A centrosome (or Microtubule Organizing Center): ▪ Contains two centrioles: ▪ that produce microtubules ▪ that make up the spindle fibers ▪ that connect to the kinetochore on the centromere of a chromosome Slide credit: Dr. Roger Young Mitosis cont. Metaphase: ▪ Chromosomes condense further ▪ Large protein complexes called kinetochores (Gr. kinetos, moving), located at the centromere DNA region of each chromosome, attach to the mitotic spindle ▪ The cell is now more spherical ▪ And microtubules move the chromosomes into alignment at the equatorial plate Anaphase: ▪ Sister chromatids (now called chromosomes themselves) separate and: ▪ Move toward opposite spindle poles by: ▪ Microtubule motor protein action and ▪ Changes in the lengths of the microtubules as the spindle poles move farther apart Metaphase Nucleus TEM of a sectioned metaphase cell ▪ The figure shows the two centrioles in each pole ▪ The mitotic spindles formed by microtubules ▪ The chromosomes in the equatorial plane ▪ The arrows show the insertion of microtubules in the kinetochore Mitosis cont. Telophase: ▪ The two sets of chromatids are at the spindle poles and begin reverting to their uncondensed state ▪ Microtubules of the spindle depolymerize ▪ The nuclear envelope begins to reassemble around each set of daughter chromosomes ▪ A belt-like contractile ring of actin filaments associated with myosins develops in the cortical cytoplasm at the cell’s equator ▪ Constriction of this ring: ▪ Produces a cleavage furrow and progresses until the cytoplasm and its organelles are divided into two daughter cells, each with one nucleus Mitosis Summary  Mitosis: Review of Images Metaphase Early Telophase What stages of Mitosis are visible in this image? What stages of Mitosis are visible in this image? Mitotic Catastrophe The improper distribution of chromosomes during mitosis: ▪ Compromises Cellular functions ▪ Reduces cellular fitness or ▪ Contributes to malignant transformation As a countermeasure, eukaryotes have developed strategies for eliminating mitosis-incompetent cells: ▪ one of which is mitotic catastrophe Mitotic catastrophe: ▪ A form of cell death related to mitosis ▪ An onco-suppressive mechanism that precedes (and is distinct from) apoptosis, necrosis or senescence ▪ Disruption of mitotic catastrophe precipitates cancer formation and cancer progression ▪ Its induction constitutes a therapeutic endpoint Mitotic Catastrophe cont. ▪ Stimulation of mitotic catastrophe appears to be a promising strategy in cancer treatment ▪ Several chemotherapeutic drugs are currently used at concentrations that can trigger mitotic catastrophe ▪ Drugs such as vinca alkaloids induce mitotic catastrophe ▪ Vinca alkaloids induce mitotic catastrophe by: ▪ Disrupting the dynamics of microtubule polymerization and depolymerization ▪ The use of DNA damaging agents and radiation ▪ Cells that do not undergo mitotic catastrophe may form aneuploid cells exposure in combination with inhibitors of DNA (cells that contain abnormal chromosome repair systems stimulates mitotic catastrophe, numbers) enhancing the therapeutic effect ▪ Aneuploid cells lead to oncogenesis Meiosis ▪ Meiosis is a specialized process involving two unique and closely associated cell divisions ▪ Occurs only in the cells that will form sperm or egg cells ▪ Homologous chromosomes of each pair come together: ▪ Synapsis ▪ During synapsis double-stranded breaks and repairs occur in the DNA: ▪ Result in reciprocal DNA exchanges: ▪ Crossovers ▪ New combinations of genes in the chromosomes ▪ The cells produced are haploid ▪ Having just one chromosome from each pair Human Chromosomes (Normal Human Karyotype): 46, XY Genotype Meiosis cont. ▪ Occurs in two consecutive cell divisions (PMAT-PMAT) known as: ▪ Meiosis I and Meiosis II ▪ Meiosis I separates homologous chromosomes ▪ Meiosis II separates sister chromatids 1st division – Meiosis I ▪ Meiosis I: Following S phase in humans, each cell contains 46 chromosomes (23 homologous pairs), each of which consists of two sister chromatids. ▪ Meiosis I separates these homologous pairs into two cells using the PMAT system. Each cell contains one chromosome of each homologous pair. Each chromosome still consists of two sister chromatids joined at the centromere Homologous pairs Slide credit: Dr. Roger Young  The 5 Stages of Prophase 1 ▪ Leptotene: Chromosomes thin. Nuclear envelope intact ▪ Zygotene: Homologous chromosomes align (synapsis) via ‘synaptonemal complex’ ▪ Pachytene: Chromosomes condense. Localized breakage of DNA between non-sister chromatids enables DNA exchange ▪ Diplotene: Crossed over locations causes chiasma (junctions between the chromosomes) which then slide to ends of chromosome. (Developing oocytes in fetus are held in diplotene until puberty) Diakinesis: Nuclear membrane disintegrates. Chromosomes condense fully. Slide credit: Dr. Roger Young  Meiosis I Following recombination, homologous chromosomes (but not sister chromatids) are separated into separate cells in meiosis I PMAT of Meiosis I Results in two cells containing one of each homologous chromosome Slide credit: Dr. Roger Young  Meiosis II ▪ Meiosis II occurs in each of the two cells produced from meiosis I ▪ The process of PMAT aligns and separates the two sister chromatids of each chromosome in a manner similar to that of mitosis ▪ The four cells produced from telophase and cytokinesis are haploid gametes ▪ The correct separation of homologous chromosomes in Anaphase I or sister chromatids in Anaphase II is called ‘disjunction’ ▪ (Incorrect separation of homologous chromosomes in Anaphase I or sister chromatids in Anaphase II is called ‘non-disjunction’) Slide credit: Dr. Roger Young Spermatogenesis Spermatogenesis is the process of meiosis by a spermatogonium that produces 4 haploid sperm cells from one primary spermatocyte Slide credit: Dr. Roger Young Oogenesis ▪ Oogenesis is the process of meiosis by an oogonium that produces a primary Fetal oocytes held in oocyte which then diplotene of divides into one Prophase 1 haploid egg and 3 polar bodies ▪ In human females, all future egg cells are present at birth and are held at prophase I until released Slide credit: Dr. Roger Young Cell Renewal ▪ Static cells/Non-dividing cells/Terminally differentiated cells ▪ Sometimes called post-mitotic cells, they are cells that no longer divide ▪ Examples: central nervous system cells, cardiac muscle cells, skeletal muscle cells ▪ Stable cell populations/Facultative mitotic cells ▪ In G0 phase ▪ Divide regularly and slowly to maintain organ structure and function ▪ Examples: Cells in bone and cartilage, smooth muscle cells, endothelial cells in blood vessels, fibroblasts in connective tissue ▪ Renewing cell populations/Labile cells/Mitotic cells ▪ May be slow or fast, may produce daughter cells, or cells that remain as stem cells ▪ Rapidly renewing – blood cells, epithelial stem cells of the skin ▪ Slow renewing – epithelial cells in the lens of the eye, smooth muscle in GI tract Identify the phase of mitosis. Identify the phase of mitosis. Identify the phase of mitosis. Identify the phase of mitosis. Identify the phase of mitosis. Identify the phase of mitosis. Identify the phase of mitosis. References ▪ Histology, A Text and Atlas, M.H. Ross & Pawlina, 8th Ed. Wolters Kluwer, 2020 ▪ Junqueira’s Basic Histology, Text and Atlas, Anthony L. Mescher, 16th Ed. Mc Graw Hill, 2021 ▪ Netter’s Essential Histology, William K. Ovalle, Patrick C, Nahirney, 3rd Ed. Elsevier, 2021 ▪ Color Atlas of Histology, L.P. Gartner, J.L. Hiatt, 7h Ed. Wolters Kluwer, 2018 ▪ Atlas of Human Histology: A Guide to Microscopic Structure of Cells, Tissues and Organs, Robert L. Sorenson, T. Clark Brelje, 3rd Ed., 2014 ▪ Wheater’s Functional Histology, B. Young, J.S. Lowe, A. Stevens, J.W. Heath, 5th Ed. Churchill and Livingstone, 2006