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Cell Division

Cell Division. CHAPTER 12. Key Roles. asexual reproduction growth renewal & repair production of gametes (sperm & egg). mitosis. meiosis. More about DNA. cell division involves the distribution of DNA to daughter cells

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Cell Division

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  1. Cell Division CHAPTER 12

  2. Key Roles • asexual reproduction • growth • renewal & repair • production of gametes (sperm & egg) mitosis meiosis

  3. More about DNA • cell division involves the distribution of DNA to daughter cells • in humans, nuclei of body cells (somatic cells) contain 46 chromosomes or 23 homologous pairs; reproductive cells (gametes) contain 23 chromosomes • chromosomes are made of chromatin (DNA + proteins) • a duplicated chromosome has two identical sister chromatids attached at a point called the centromere

  4. Mitosis • process of cell division that produces two daughter cells identical to the parent cell • each new cell receives a full set of chromosomes because sister chromatids separate during cell division

  5. 2 Phases of Cell Cycle Interphase Mitotic Phase • period of cell growth • 3 subphases: • G1 phase – carries out function • S phase – copies chromosomes • G2 phase – prepares for division • period of cell division • 2 parts: • Mitosis – division of the nucleus • Cytokinesis – division of the cytoplasm

  6. Stages of Mitosis - Prophase • chromatin coils up forming visible chromosomes • nucleoli disappear • mitotic spindle forms between centrosomes

  7. Stages of Mitosis - Prometaphase • nuclear envelope breaks down • kinetochore microtubules attach to centromeres of each duplicated chromosome

  8. Stages of Mitosis - Metaphase • chromosomes align at the metaphase plate (cell equator)

  9. Stages of Mitosis - Anaphase • sister chromatids separate & are pulled to opposite ends of cell by shortening kinetochore microtubules

  10. Stages of Mitosis - Telophase • nuclear envelopes reform, nucleoli reappear, chromosomes uncoil, & cytokinesis occurs

  11. Mitotic Spindle – A Closer Look • the mitotic spindle is composed of the centrosomes, microtubules, & asters • assembly of microtubules starts at the centrosome, which contains a pair of centrioles in animal cells • microtubules can lengthen & shorten by adding or removing tubulin subunits • asters are radial arrays of short microtubules extending from the centrosomes • kinetochore microtubules move the chromosomes during mitosis • nonkinetochore microtubules lengthen the cell

  12. Cytokinesis – A Closer Look Animal Cells Plant Cells • process is called cleavage • a protein contractile ring made of actin & myosin pinches the cell in two at the cleavage furrow • vesicles containing cell wall building materials fuse in the middle of the cell creating a cell plate • eventually the vesicles fuse with the plasma membrane & a new cell wall is formed

  13. Cell Division in Prokaryotes – Binary Fission • prokaryotes have a single, circular chromosome • replication of the chromosome occurs at the same time as cell division • as the chromosome is replicated, one copy moves toward the opposite end of the elongating cell • the cell elongates to about 2 its original size and then divides

  14. Evolution of Mitosis – A Hypothesis • binary fission (prokaryotes) • a process that appears to be intermediate between binary fission & mitosis (seen in modern day single-celled eukaryotes) • mitosis (most eukaryotes)

  15. Regulation of the Cell Cycle • cell cycle control system – 3 checkpoints: • G1 – does the cell need to divide? (if not, the cell enters G0 phase) • G2 – is everything in order for cell division? • M – have the chromosomes correctly aligned themselves at the metaphase plate?

  16. How the Checkpoints Work • G1 – regulated by a signal transduction pathway stimulated by growth factors that bind to receptor tyrosine kinases in the plasma membrane • G2 – regulated by cyclin-dependent protein kinases: • cyclins build up during S & G2 and bind to the cyclin-dependent kinases forming a MPF complex that initiates mitosis

  17. Regulation of the Cell Cycle cont. • density-dependent inhibition • crowded cells stop dividing because, when a cell population reaches a certain density, the availability of nutrients becomes insufficient to allow continued cell growth and division • anchorage dependence • cells must be attached to the extracellular matrix of a tissue in order to divide

  18. Cancer • results from a loss of cell cycle control • transformation – when a normal cell converts to a cancer cell • benign tumor – noncancerous lump of abnormal cells • malignant tumor – cancerous lump of abnormal cells • can invade other tissues and impair the function of organs • can spread to other areas of the body (metastasis)

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