BIO211 - USEK 2024-2025 The Cell Cycle PDF

BIO211 - USEK 2024-2025 Chapter 9 1 The Cell Cycle BIO211 - USEK 2024-2025 2 Overview: The Key Roles of Cell Division  The ability of organisms to produce more o...

BIO211 - USEK 2024-2025 Chapter 9 1 The Cell Cycle BIO211 - USEK 2024-2025 2 Overview: The Key Roles of Cell Division  The ability of organisms to produce more of their own kind best distinguishes living things from nonlivingmatter  The continuity of life is based on the reproduction of cells, or cell division  In unicellular organisms, division of one cell reproduces the entire organism  Multicellular organisms depend on cell division for  Development from a fertilized cell  Growth  Repair  Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division 1 BIO211 - USEK 2024-2025 3 BIO211 - USEK 2024-2025 4 CHAPTER 9 OUTLINE I. The Daughter Cells II. The Cell Cycle III. Cell Cycle Regulation 2 BIO211 - USEK 2024-2025 5 I- The Daughter Cells Most cell division results in genetically identical daughter cells BIO211 - USEK 2024-2025 6  Most cell division results in daughter cells with identical genetic information, DNA  Exception: meiosis: production of gametes (sperm and egg cells) 3 BIO211 - USEK 2024-2025 7 I.1- Cellular Organization of the Genetic Material  All the DNA in a cell constitutes the cell’s genome  can consist of a single DNA molecule (common in prokaryotic cells)  or a number of DNA molecules (common in eukaryotic cells)  DNA molecules in a cell are packaged into chromosomes BIO211 - USEK 2024-2025 8 I.1- Cellular Organization of the Genetic Material  Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division  Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus.  In diploidspecies:  Somatic cells (non-reproductive cells) have two sets of chromosomes (2n)  Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells (n) 4 BIO211 - USEK 2024-2025 9 I.1- Cellular Organization of the Genetic Material BIO211 - USEK 2024-2025 10 I.2- Distribution of Chromosomes During Eukaryotic Cell Division  In preparation for cell division, DNA is replicated and the chromosomes condense  Each duplicated chromosome has two sister chromatids (joined copies of the original chromosome), which separate during cell division  The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached 5 BIO211 - USEK 2024-2025 11 I.2- Distribution of Chromosomes During Eukaryotic Cell Division BIO211 - USEK 2024-2025 12 I.2- Distribution of Chromosomes During Eukaryotic Cell Division  During cell division, the two sister chromatids of each duplicated chromosome separate and move into two nuclei  Once separate, the chromatids are called chromosomes 6 BIO211 - USEK 2024-2025 13 BIO211 - USEK 2024-2025 14 I.2- Distribution of Chromosomes During Eukaryotic Cell Division  Eukaryotic cell division consists of  Mitosis, the division of the genetic material in thenucleus  Cytokinesis, the division of the cytoplasm  Gametes are produced by a variation of cell division called meiosis  Meiosis yields non-identical daughter cells that have only one set of chromosomes, half as many as the parent cell 7 BIO211 - USEK 2024-2025 15 II- The Cell Cycle The mitotic phase alternates with interphase in the cell cycle BIO211 - USEK 2024-2025 16  1892: Walther Flemming developed dyes allowing to observe behavior of chromosomes during mitosis and cytokinesis 8 BIO211 - USEK 2024-2025 17 II.1- Phases of the cell cycle  The cell cycle consists of  Mitotic (M) phase (mitosis and cytokinesis)  Interphase (cell growth and copying of chromosomes in preparation for cell division)  Interphase (~ 90% of the cell cycle) can be divided into subphases  G1 phase (“first gap”)  S phase (“synthesis”)  G2 phase (“second gap”)  The cell grows during all three phases, but chromosomes are duplicated only during the S phase BIO211 - USEK 2024-2025 18 9 BIO211 - USEK 2024-2025 19 II.1- Phases of the cell cycle  Mitosis is conventionally divided into five phases  Prophase  Prometaphase  Metaphase  Anaphase  Telophase  Cytokinesis overlaps the latter stages of mitosis BIO211 - USEK 2024-2025 20 II.1- Phases of the cell cycle Exploring Mitosis in an Animal Cell The fluorescence micrographs show dividing lung cells from a newt; this species has 22 chromosomes. Chromosomes appear blue, microtubules green, and intermediate filaments red. For simplicity, the following drawings show only 6 chromosomes. 10 BIO211 - USEK 2024-2025 21 BIO211 - USEK 2024-2025 22 11 BIO211 - USEK 2024-2025 23 II.2- The Mitotic Spindle  The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis  In animal cells, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center (MTOC)  replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase  An aster (a radial array of short microtubules) extends from each centrosome  The spindle includes the centrosomes, the spindle microtubules, and the asters BIO211 - USEK 2024-2025 24 II.2- The Mitotic Spindle  During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes  Kinetochores are protein complexes associated with centromeres  At metaphase, the chromosomes are all lined up at the metaphase plate, an imaginary structure at the midway point between the spindle’s two poles 12  11 - USEK 2022-2023 25 BIO211 - USEK 2024-2025 26 II.2- The Mitotic Spindle  In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell  The microtubules shorten by depolymerizing at their kinetochore ends 13 BIO211 - USEK 2024-2025 27 BIO211 - USEK 2024-2025 28 II.2- The Mitotic Spindle  In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell  The microtubules shorten by depolymerizing at their kinetochore ends  Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell  In telophase, genetically identical daughter nuclei form at opposite ends of the cell  Cytokinesis begins during anaphase or telophase and the spindle eventually disassembles 14 BIO211 - USEK 2024-2025 29 II.3- Cytokinesis  In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow  In plant cells, a cell plate forms during cytokinesis BIO211 - USEK 2024-2025 30 II.3- Cytokinesis 15 BIO211 - USEK 2024-2025 31 Mitosis in a plant cell. These light micrographs show mitosis in cells of an onion root. BIO211 - USEK 2024-2025 32 16 BIO211 - USEK 2024-2025 33 II.4- Binary Fission in Bacteria  Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission  The chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart  The plasma membrane pinches inward, dividing the cell into two BIO211 - USEK 2024-2025 34 17 BIO211 - USEK 2024-2025 35 III- Regulation of The Cell Cycle The eukaryotic cell cycle is regulated by a molecular control system BIO211 - USEK 2024-2025 36  The frequency of cell division varies with the type of cell  These differences result from regulation at the molecular level  Cancer cells manage to escape the usual controls on the cell cycle 18 BIO211 - USEK 2024-2025 37 III.1- The Cell Cycle Control System  The cell cycle appears to be driven by specific chemical signals present in the cytoplasm  Evidence for this hypothesis from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei BIO211 - USEK 2024-2025 38 19 39 III.1- The Cell Cycle Control System  The sequential events of the cell cycle are directed by a distinct cell cycle control system, similar to a clock  regulated by both internal and external controls  specific checkpoints where the cell cycle stops until a go-ahead signal is received 40 Mechanical analogy for the cell cycle control system. In this diagram, the flat “stepping stones” around the perimeter represent sequential events. Like the control device of a washing machine, the cell cycle control system proceeds on its own, driven by a built-in clock. However, the system is subject to internal and external regulation at various checkpoints; three important checkpoints are shown (red). 20 BIO211 - USEK 2024-2025 41 III.1- The Cell Cycle Control System  For many cells, the G1 checkpoint seems to be the most important  If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide  If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a non- dividing state called the G0 phase BIO211 - USEK 2024-2025 42 21 BIO211 - USEK 2024-2025 43 III.1- The Cell Cycle Control System The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases  Two types of regulatory proteins are involved in cell cycle control:  cyclins  concentrations vary with the cell cycle  cyclin-dependent kinases (Cdks)  Controlled by cyclins => activity fluctuates during the cell cycle  MPF (maturation-promoting factor or M-phase promoting factor)  Cdk1-cyclin B complex  triggers a cell’s passage past the G2 checkpoint into the M phase BIO211 - USEK 2024-2025 44 22 BIO211 - USEK 2024-2025 45 BIO211 - USEK 2024-2025 46 III.1- The Cell Cycle Control System Internal and external signals at the checkpoints  Internal signals –example:  kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase  External signals – examples:  Growth factors:  proteins released by certain cells that stimulate other cells to divide  E.g. platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture  Density-dependent inhibition: crowded cells stop dividing  Anchorage dependence: cells must be attached to a substratum in order to divide 23 BIO211 - USEK 2024-2025 47 BIO211 - USEK 2024-2025 48 24 BIO211 - USEK 2024-2025 49 III.2- Loss of Cell Cycle Controls in Cancer Cells  Cancer cells do not heed the normal signals that regulate the cell cycle  do not respond normally to the body’s control mechanisms  exhibit neither density-dependent inhibition nor anchorage dependence  may not need growth factors to grow and divide  may make their own growth factor  may convey a growth factor’s signal without the presence of the growth factor  may have an abnormal cell cycle control system BIO211 - USEK 2024-2025 50 25 BIO211 - USEK 2024-2025 51 III.2- Loss of Cell Cycle Controls in Cancer Cells  A normal cell is converted to a cancerous cell by a process called transformation  Cancer cells that are not eliminated by the immune system form tumors  = masses of abnormal cells within otherwise normal tissue  If abnormal cells remain only at the original site, the lump is called a benign tumor  Malignant tumors invade surrounding tissues and can metastasize  = exporting cancer cells to other parts of the body, where they may form additional tumors called metastases  Recent advances in understanding the cell cycle and cell cycle signaling have led to advances in cancer treatment BIO211 - USEK 2024-2025 52 26 BIO211 - USEK 2024-2025 53 You should now be able to: 1. Describe the structural organization of the prokaryotic genome and the eukaryotic genome 2. List the phases of the cell cycle; describe the sequence of events during each phase 3. List the phases of mitosis and describe the events characteristic of each phase 4. Draw or describe the mitotic spindle, including centrosomes, kinetochore microtubules, nonkinetochore microtubules, and asters 5. Compare cytokinesis in animals and plants 6. Describe the process of binary fission in bacteria 7. Explain how the abnormal cell division of cancerous cells escapes normal cell cycle controls 8. Distinguish between benign, malignant, and metastatic tumors 27