Cell Cycle_BSED PDF

THE CELL CYCLE MARIBEL D. GANEB, Ph.D College of Education NUCLEUS  THE CELL NUCLEUS  Cell is a fundamental component of life containing different organelles.  Nucleus ― A cell organelle.  It is known as nerve center or control center of cell.  CHROMOSOMES  Geneticma...

THE CELL CYCLE MARIBEL D. GANEB, Ph.D College of Education NUCLEUS  THE CELL NUCLEUS  Cell is a fundamental component of life containing different organelles.  Nucleus ― A cell organelle.  It is known as nerve center or control center of cell.  CHROMOSOMES  Geneticmaterial  Heterochromatin - highly condensed.  Euchromatin - less condensed. CHROMOSOMES CELL CYCLE CELL CYCLE CELL GROWTH and REPRODUCTION The Cell Life Cycle – A series of changes a cell goes through from the time it is formed to the time it reproduces, encompasses two major periods: Interphase: in which the cell grows and carries on its usual activities Cell Division (mitotic phase): during which it divides into two cells Interphase Period from cell formation to cell division during which it is metabolically very active and growing (metabolic phase or growth phase) while preparing for the next cell division Interphase Subphases Interphase is the period from cell formation to cell division, and has three subphases: In all three subphases, the cell grows by producing proteins and organelles, however, chromatin is reproduced only during the S phase – G1 (gap 1): cell is synthesizing proteins and actively growing: Metabolically very active Time variable depending on the cell type – Minutes to years » Cells that permanently cease dividing are said to be in the Go phase During this phase virtual no activity is related to cell division – However, as this phase ends, the centrioles start to replicate in preparation for cell division Interphase Subphases S (synthetic) phase: DNA is replicated – Ensuring that the two future cells being created will receive identical copies of the genetic material – New histones are made and assembled into chromatin DNA REPLICATION Interphase Subphases G2 (gap 2): Final phase of Interphase Very brief Enzyme and other proteins needed for division are synthesized and distributed throughout the cell By the end of G2, centriole replication (begun in G1) is complete – The cell is now ready to divide Throughout S and G2, the cell continues to grow and carries on with business as usual CHROMOSOMES  CELL DIVISION  Process necessary for growth and tissue repair  In most body cells, cell division, which is called the M (mitotic) phase of the cell life cycle, involves two distinct events:  Mitosis: division of the nucleus  Cytokinesis: division of the cytoplasm Mitosis Series of events that parcel out the replicated DNA of the mother cell to two daughter cells Four phases: actually a continuous process, with one phase merging smoothly into the next: – Prophase – Metaphase – Anaphase – Telophase  MITOSIS Is nuclear division plus cytokinesis, and produces two identical daughter cells Mitosis occurs in all somatic cells --- diploid (2n) cells PHASES OF MITOSIS prometaphase Spindle Fibers Cell Plate MITOSIS MITOSIS  MITOSIS  1879 - Walther Flemming described the motion of what he saw under microscope as “threads” (Greek, mitos) moving in an actively dividing cell The events of mitosis. Meiosis Why We Are Who We Are MEIOSIS Meiosis is the cell division that takes place in germ cells only. Requires two cell divisions Diploid germ cells give rise to haploid (n) gametes.  Meiosis Takes place in the Gametes of an organism People have a Chromosome count of 46 When an egg joins a sperm, the count must stay at 46 to remain human So, the egg can only have 23 chromosomes, and the sperm can only have 23 chromosomes But the integrity of the organism must be maintained. How does this happen?  During Meiosis gamete (sex) cells undergo a “double division”, maintaining the DNA, but reducing the chromosomal count to 23 + = Sperm (23) + Egg (23) = Fertilized Cell (46)  Primary oocyte or Pair of primary homologous spermatocyte dyad (tetrad) 23 double structured chromosomes SPECIAL EVENTS IN MEIOSIS  Pairing of homologous  As homologous chromosomes length chromosomes separate wise is called synapsis. points of interchange  Pairing is exact and are temporarily united point to point except and form an X like for X & Y chromosome structure called  Cross overs or chiasma. interchange of chromatid segments between paired homologous chromosomes  No DNA synthesis or replication 23 single structured chromosomes Chromosome Chromosomes at after S Phase beginning of Mitosis After After Mitosis Meiosis  Original Gamete Metaphase Anaphase Telophase Cytokinesis 2 Daughter Cells Metaphase 2 Anaphase 2 Telophase 2 Cytokinesis – 4 Gametes SPERMATOGENESIS OOGENESIS ❖Prophase-22 days 22+Y 22+X 51 SIGNIFICANCE OF MEIOSIS: Provides constancy of the chromosome number from generation to generation by reducing the chromosome number from diploid to haploid, thereby producing haploid gametes. Allows random assortment of maternal and paternal chromosomes between the gametes. Relocates segments of maternal and paternal chromosomes by crossing over of chromosome segments, which "shuffles" the genes and produces a recombination of genetic material. CONTROL OF CELL DIVISION  Two other mechanisms that influence when cells divide are:  Chemical signals released by other cells:  Growth factors, hormones, etc.  Availability of space  Normal cells stop proliferating when they begin touching, a phenomenon called contact inhibition  Cancer cells lack many of the normal controls and therefore divide wildly, which makes them dangerous to their host CONTROL OF CELL DIVISION A Cdk-cyclin (cyclin-dependent kinase) complex called MPF (M-phase promoting factor) acts at the G2 phase to trigger mitosis  (a): the graph shows how MPF activity fluctuates with the level of cyclin in the cell  The cyclin level rises throughout interphase (G1, S, and G2 phases), then falls abruptly during mitosis (M phase)  The peaks of MPF activity and cyclin concentration correspond CELL CYCLE CONTROL OF CELL DIVISION  (b)(1): By the G2 checkpoint (black bar), enough cyclin is available to produce many molecules of MPF (M-phase promoting factor)  (2): MPF promotes mitosis by phosphorylating various proteins, including other enzymes CONTROL OF CELL DIVISION CONTROL OF CELL DIVISION  3. One effect of MPF is the initiation of a sequence of events leading to the breakdown of its own cyclin  4. The Cdk (cyclin-dependent kinase) component of MPF (M-phase promoting factor) is recycled  Its kinase activity will be restored by association with new cyclin that accumulates during interphase  CELL DIVISION  There are three main events of cell division  Mitosis is the process of nuclear division in which cells contain all genes  Meiosis is the process of nuclear division found only in egg and sperm cells in which the cells have half the genes found in other body cells  Cytokinesis is the process of dividing the cytoplasm  Controlof cell division depends on surface-volume relationship, chemical signaling, and contact inhibition  CELL DIVISION  There are three main events of cell division  Mitosis is the process of nuclear division in which cells contain all genes  Meiosis is the process of nuclear division found only in egg and sperm cells in which the cells have half the genes found in other body cells  Cytokinesis is the process of dividing the cytoplasm  Controlof cell division depends on surface-volume relationship, chemical signaling, and contact inhibition Thank you for listening! Maribel D. Ganeb, Ph.D