Biology 1020 -- Chapter 12
Biology 1020 -- Chapter 12 BIOL 1020 (Dr. Overturf)
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This 5 page Class Notes was uploaded by Brittney Champagne on Thursday September 22, 2016. The Class Notes belongs to BIOL 1020 (Dr. Overturf) at University of Louisiana at Monroe taught by Dr. Overturf in Fall 2016. Since its upload, it has received 113 views. For similar materials see Principles of Biology (Dr. Overturf) in Biology at University of Louisiana at Monroe.
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Date Created: 09/22/16
Chapter 12 — The Cell Cycle Key Roles of Cell Division Cell division — • Reproduction (unicellular organisms) • Growth and development (multicellular organisms) Tissue renewal (fully grown cells) • Cell cycle — • The life of a cell from the time it is first formed until its own division to two daughter cells • Passes down crucial genetic information 12.1 Most cell division results in genetically identical daughter cells DNAmolecules are packaged into chromosomes. Each chromosome has one long DNAstrand. Each DNAstrand carries several hundred to a few thousand genes, the units of information that specify an organism’s inherited traits. Chromatin — • The entire complex of DNAand proteins • The building material of chromosomes. One piece of chromatin will likely consist of two long strands of DNAplus proteins. • The nuclei of human somatic cells each contain 46 chromosomes, made up of two sets of 23 (one set inherited from each parent). In order for human chromosomes to be correctly counted, it had to be discovered how to use a hypotonic solution to swell nuclei. Distribution of Chromosomes during Eukaryotic Cell Division Sister chromatids — joined copies the original chromosome Each sister chromatid has a centromere. Centromere — region in which chromatids remain attached to one another until anaphase • 2 chromatids for every centromere 2 centromeres for every chromosome • Centrosome — microtubules of the spindle assemble here during mitosis (in both plant and animal cells); the microtubule organizing center 12.2 The mitotic phase alternates with interphase in the cell cycle Eukaryotic cell divison: • Mitosis — the division of genetic material in the nucleus • Cytokinesis — division of the cytoplasm In some cells, mitosis occurs without cytokinesis, resulting in cells with more than one nucleus. Phases of the Cell Cycle 1. Mitotic (M) phase (mitosis and cytokinesis) 2. Interphase (cell growth and copy of chromosomes) • 90% of the cell cycle • G1 phase (first gap) S phase (synthesis) • • G2 phase (second gap) Steps of Mitosis: 1. Prophase — • Centrioles beginning to move apart (in animal cells) • Chromosomes coil and condense during mitosis in order to allow the chromosomes to move without becoming entangled and breaking. Spindle formation • • Separation of the spindle poles • Mitotic spindle — made of microtubules; control chromosome movement during mitosis 2. Prometaphase — Nuclear envelope fragments • • dinoflagellates and diatoms — groups of eukaryotic organisms in which the nuclear envelope remains intact. • Microtubules invade nuclear area Chromosomes continue to condense • • Kinetochore develops by the combination of microtubules • Spindle microtubules that do not attach to kinetochores maintain the region of overlap of microtubules in the cell’s center 3. Metaphase — • Centrosomes are at opposite poles of the cell • Chromosomes align at middle (metaphase plate) 4.Anaphase — Separation of sister chromatids • • For anaphase to begin, cohesion must be cleaved enzymatically. • Movement of the chromosomes during anaphase would most likely be affected by something that prevents shortening of microtubules. How they move: The chromosomes are “reeled in” by the contraction of spindle • microtubules, and motor proteins of the kinetochores move the chromosomes along the spindle microtubules. • ATP as an energy source is required for motor proteins to function in the movement of chromosomes toward the poles of the mitotic spindle. 5. Telophase — • Chromatids become chromosomes • Nuclear membrane forms around new chromosomes Spindle breaks down • Cytokinesis:ACloser Look Cytokinesis — Begins in late anaphase • • Formation of a cleavage furrow; pinches the cell in two • Cleavage furrow — a groove in the plasma membrane between daughter nuclei In animal cells, cleavage occurs; in plant cells, a cell plate forms. • • Golgi-derived vesicles — responsible for cytokinesis in plant cells but NOT in animal cells. Binary Fission in Bacteria Binary fission — process for prokaryotes to reproduce • In binary fission, the chromosome replicates (beginning at the origin of replication) and the two daughter chromosomes move apart • Plasma membrane pinches inward, dividing the cell in two. Using cell division rather than binary fission allows for the orderly segregation of multiple • chromosomes. • Actin and tubulin — involved in both binary fissionAND eukaryotic mitotic division The Evolution of Mitosis Prokaryotes evolved before eukaryotes; therefore, mitosis probably evolved from binary fission. 12.3 The eukaryotic cell cycle is regulated by a molecular control system The frequency of cell division varies with the type of cell and results from regulation at the molecular level. The Cell Cycle Control System Cell cycle control system — • Driven by specific chemical signals in cytoplasm • Directs sequential events similarly to a clock • Regulated by both internal and external controls and checkpoints The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases Cdk (cyclin dependent kinases) — • Protein maintained at constant levels throughout the cycle & requires cyclin to become catalytically active • Present throughout the whole cell cycle • Enzyme that attaches phosphate groups to other proteins MPF (maturation-promoting factor) — • Triggers the cell’s passage past the G2 checkpoint into mitosis • Turns itself off by activating a process that destroys cyclin components • The cyclin component of MPF is destroyed toward the end of the M phase. The decline of MPF activity at the end of mitosis is due to the degradation of cyclin. • Cyclin — • Protein synthesized at specific times during the cell cycle Associates with a kinase to form catalytically active complex • • Involved in the regulation of the cell cycle • Shows fluctuations in concentration during the cell cycle Activation of a Cdk molecule happens when it is in sufficient concentration • • Destruction of cyclin is initiated by the activity of its complex and causes the decrease in amount of cyclin Stop and Go Signs: Internal and External Signals at the Checkpoints Internal: G1 Checkpoint — • The cell is first committed to continue the cell cycle after this checkpoint Cells will not begin anaphase until all chromosomes are properly attached to the spindle at • the metaphase plate (middle), assuring that daughter cells will have the correct number of chromosomes. M Phase Checkpoint — Allows separase enzyme cleaves cohesins and allows chromatids to separate. • External: Growth factors are released by certain cells and cause others to divide. PDGF — released by platelets in the vicinity of an injury • • Density dependent inhibition — crowded cells will stop dividing • Anchorage dependence — must be attached to a substratum • Density dependent inhibition and anchorage dependence check the growth of cells at an optimal density All cell cycle checkpoints are similar because they activate or inactivate other proteins. Loss of Cell Cycle Controls in Cancer Cells Cancer cells — Continue to divide even when they are tightly packed together Transformation — Normal cell is converted to a cancerous cell Cancer cells not eliminated by immune system collect to form tumors. Benign tumor — abnormal cells stay at original site Malignant tumor — • Invades surrounding tissues and can metastasize (export cancer cells to other parts of the body) • Cells from an advanced malignant tumor most often have very abnormal chromosomes, and often an abnormal total number of chromosomes. This is because chromosomal abnormal cells can still go through cell cycle checkpoints. In order to find which cells are malignant, one must karyotype samples to look for unusual • size and number of chromosomes.
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