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Biology 1A03- Chapter 11 notes.docx

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Lovaye Kajiura

Chapter 11: The Cell Cycle All cells arise from pre-existing cells by the process of cell division. Fig. 11.1 Two Types of Nuclear Division Occur in Many Species In animals, meiosis leads to the production of eggs and sperm. Mitosis is responsible for producing somatic cells. TYPES OF NUCLEAR DIVISION Mitosis Meiosis (covered later in Ch. 12)  Occurs in somatic cells  Occurs only in reproductive cells  Produces gametes  Daughter cells are genetically  Daughter cells (ova, sperm) are identical to parent cells genetically different from the parent cells FUNCTIONS OF MITOTIC CELL DIVISION  Reproduction in asexually reproducing species (unicellular organisms)  Growth and Tissue Repair in multicellular species  Development MITOSIS AND THE CELL CYCLE Bacteria (Box 11.2)  Undergo the process of binary fission  Binary fission produces two genetically identical daughter cells Animals, Fungi and Plants complete The Cell Cycle Fig. 11.5 The Cell Cycle has Four Phases The time required for the G and G phases varies dramatically among cells and 1 2 organisms. 4 CELL CYCLE PHASES 1. G P1ase (Interphase) 2. S Phase (Interphase) 3. G Phase (Interphase) 2 4. Mitotic (or M) Phase Interphase- represents a very large portion of the Cell Cycle What occurs during the G P1ase?  Cell growth and the duplication of organelles. What occurs during the S Phase?  DNA Synthesis (chromosomes replicate) What occurs during the G Phase? 2  Cell growth and duplication of organelles continues to build the protein “machinery”  Continuation of G phase 1 Why do Gap Phases exist?  Before mitosis= the parent cell must be large enough in size and must have made the required organelles, so that daughter cells will operate normally.  Cytokinesis= separation of cytoplasm (slime moulds don’t undergo cytokinesis; only eukaryotes do). What is G 0hase (Gap 0 Phase)?  Not a dividing state (the non-dividing state)  No synthesis, no cell replication  Example: mature nerve cells and muscle cells (fully formed)  Exception: alcoholics that abuse alcohol damage their liver and scarring of the liver occurs. If they get help to stop, then the liver cells can go from th0 G stage back to the cell cycle allowing it to heal. This is triggered by external cues (growth release factors; released upon injury). Mitosis  Is a small portion of the cell cycle  NUCLEAR DIVISION- events in the nucleus and cytoplasm Mitotic Phases Table 11.1 Structures involved in Mitosis  Prophase- distinct chromosomes, spindle formation of microtubules  Prometaphase- chromosomes aren’t aligned, breakdown of nuclear envelope  Metaphase  Anaphase  Telophase  G 2f Interphase- microtubule arrangement (3 bundles of 9)= chromatin duplicated Prophase (The Preparation Phase) In the Nucleus:  Nucleoli begin to disappear  Chromatin fibers contract (DNA condenses) by tightly coiling  Chromosomes are visible and consist of two identical sister chromatids joined together at the centromere. In the Cytoplasm:  Mitotic spindle forms (microtubules polymerize)  Assembly of microtubules begins in the centrosome (animals) or microtubule organizing centre (plants)  In animals, the centrioles begin to move apart to opposite sides of nucleus (2 poles) Prometaphase  Chromosomes do not appear completely aligned or organized  Nucleoli disappear and nuclear envelope breaks down and is recycled  A specialized structure called a kinetochore is formed near the centromere  Spindle fibers attach to sister chromatids at the kinetochore regions  Kinetochore microtubules are polymerized, randomly oriented at first, then they become aligned parallel with microtubules.  Kinetochore microtubules begin moving the chromosomes towards the middle of the cell. Kinetochore Microtubules  Extend from the poles to the kinetochores  Attach to the kinetochores What is the function of the kinetochore microtubules?  Chromosome migration  They are attached to the microtubules Nonkinetochore Microtubules  Radiate from each centrosome towards the metaphase plate without attaching to chromosomes  May overlap with those from the opposite pole What are the functions of the nonkinetochore microtubules?  Don’t attach to kinetochores  Assist in elongating the entire cell during anaphase  Interdigitate= cage-like network  They form a cage-like network, which facilitates the activities of the cell cycle components. Metaphase (The Organizing Phase)  Chromosomes line up along the metaphase plate (zone)  Centromeres are aligned on the metaphase plate, which is located equidistant from the two poles. Anaphase (Separating DNA Copies)  Binding proteins between the sister chromatids break down  Centromeres of sister chromatids disjoin and segregate, this process is called Disjunctional Segregation.  Chromosomes move centromere first (they appear “V-shaped”)  Towards the end of anaphase, the two poles have identical numbers of chromosomes  The cell elongates  Daughter chromosomes  8 chromosomes- single stranded (not replicated); replication’s during the S phase Telophase  Nonkinetochore microtubules further elongate the cell  Two daughter nuclei form  Formation of the nuclear envelopes around each set of chromosomes  Cell division  Cleavage furrow= pinching off- separation of daughter cells  Polysaccharides= how they divide Cytokinesis (Cytoplasm Division)  Telophase is often followed by cytokinesis Fig. 11.9 The Mechanism of Cytokinesis Varies among Eukaryote Groups a) In animals, the cytoplasm is divided by a cleavage furrow that pinches the parent cell in two. b) In plants, the cytoplasm is divided by a cell plate that forms in the middle of the parent cell. Animals, Slime Molds and Fungi- form a Cleavage Furrow Plants- form a Cell Plate Fig. 11.10 Mitosis and Cytokinesis in Action Micrographs showing newt cells in interphase and undergoing mitosis and cytokinesis. Chromosomes are stained blue, microtubules are green and actin filaments are red. Fig. 11.11 During Anaphase, Microtubules Shorten at the Kinetochore A Laser beam experiment is done to determine how microtubules shorten to pull sister chromatids apart at anaphase. The conclusion is that microtubules shorten at one end- at the kinetochore. How do the chromosomes move during the process of Mitosis?  Tubulin subunits of the kinetochore microtubules are depolarized from the kinetochore ends.  Motor proteins (dynein) attach and detach along the kinetochore microtubule’s length= chromosomes move. Fig. 11.12 How do Microtubules Move Chromosomes during Mitosis The kinetochore consists of an inner plate, an outer plate, and associated fibers that contain motor proteins. The fibers are thought to act as “legs” that walk the chromosomes down the length of the kinetochore microtubules as the microtubules shorten, due to loss of tubulin dimers at the kinetochore.  Polarity- chromosomes move in the proper direction  Down syndrome- lack of chromosomes in the nucleus CONTROL OF THE CELL CYCLE Cell Cycle Control Molecules  Heterokaryon- “different nuclei”—cell knows about different phases Fig. 11.13 Experimental Evidence for Cell Cycle Control Molecules  Not about the sequence of the cell cycle  Are there signal factors
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