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Why Cells divide?

Why Cells divide?. 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 of injured tissues. DNA is the genetic material of the cell.

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Why Cells divide?

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  1. Why Cells divide? • 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 of injured tissues.

  2. DNA is the genetic material of the cell

  3. Most of the DNA of the Eukaryotic Cell is Located in the Nucleus

  4. Cellular Organization of the Genetic Material • A cell’s endowment of DNA (its genetic information) is called its genome • DNA molecules in a cell are packaged into chromosomes. • The number of chromosomes in a eukaryotic cell depends on the species. For example, a human body cell has 46 chromosomes. A dog’s body cell has 78 chromosomes.

  5. What is a Chromosome? • One long DNA molecule and a number of protein molecules attached to it form one chromosome.

  6. Chromatin Fibers& Chromosomes • In nondividing cells, chromosomes exist as a diffuse mass of long and thin fibers called CHROMATIN. • As a cell prepares to divide, its chromatin coils up forming compact and distinct (short and thick) chromosomes that are visible under the light microscope.

  7. Concept 12.1: Cell division results in genetically identical daughter cells • Cells duplicate their DNA before they divide • So one DNA molecule in the mother cell doubles ensuring that each daughter cell receives an exact copy of the genetic material, DNA, and consequently the exact # of chromosomes as in the mother cell.

  8. In preparation for cell division: • DNA is replicated and • the two new DNA molecules and associated proteins stay attached while condensing to form two sister chromatids. • The two sister chromatids form a duplicated chromosome. • The narrow “waist” of the duplicated chromosome, where the two sister chromatids are most closely attached, is called the CENTROMERE

  9. DNA Replication

  10. The Cell Cycle • The cell cycle is an ordered sequence of events in the life of a eukaryotic cell from its origin in the division of a parent cell until its own division into two

  11. Phases of the Cell Cycle: • The Interphase(about 90% of the cell cycle) has three subphases: • G1 phase (“first gap”) centers on the growth of the cell. • S phase (“synthesis”) duplicates the chromosomes. • G2 phase (“second gap”) completes the preparation for cell division before the mitotic phase starts. • The Mitotic (M) phasehas two subphases: • Mitosis (the nuclear division) • Cytokinesis (the cytoplasmic division)

  12. Interphase S (DNA Replication) G1 (Cell Growth) G2 (Cell Preparation for Mitosis) M: Mitosis (Nuclear Division) C: Cytokinesis (Cytoplasmic Division) C M Mitotic Phase Phases of the Cell Cycle

  13. Concept 12.2: The mitotic phase alternates with interphase in the cell cycle INTERPHASE S (DNA synthesis) G1 Mitosis Cytokinesis G2 MITOTIC (M) PHASE

  14. The Cell Cycle

  15. Interphase Two pairs of centrioles • Growth of the baby cell. • Synthesis of cellular components needed for cell division, including replication of DNA and the centrosome Chromatin Nucleolus Nuclear envelope Plasma membrane Nucleus with chromatin

  16. Phases of the Mitotic Phase of the Cell Cycle • 1- Mitosis is conventionally divided into the following phases: • Prophase • Early prophase • Late prophase (Prometaphase) • Metaphase • Anaphase • Telophase • 2- Cytokinesis is well underway by late telophase

  17. Prophase Chromosome (two sister chromatids joined at centromere) Sister chromatids Centromere - Chromosomes appear due to coiling of chromatin. - Nucleolus breaks down. - Spindle fibers begin to form from centrioles. - Centrioles move toward opposing cell poles. - Nuclear envelope breaks down at the end of this stage. G2 OF INTERPHASE Developing spindle Nucleus with dispersed chromosomes PROPHASE PROMETAPHASE

  18. G2 of interphase → Prophase → Prometaphase

  19. Cell Division: The Cell Cycle • Metaphase • Chromosomes aligned on equatorial plate of cell • Spindle fibers extending from centriole attach at centromere of chromosome • Total array termed mitotic spindle Equatorial plate Spindle fibers Chromosomes aligned on equatorial plate Spindle fibers

  20. Anaphase • Centromeres that held chromatid pairs together separate • Spindle fibers move sister chromatids apart toward poles • The microtubules shorten by depolymerizing at their kinetochore ends • Each chromatid is now a chromosome of one DNA molecule Sister chromatids being pulled apart Sister chromatids being pulled apart Spindle fibers (d) Anaphase

  21. Telophase • Arrival of new chromosomes at each pole • Chromosomes begin to uncoil and return to chromatin (long and thin) • A nucleolus reforms within each nucleus. • Spindle fibers break up and disappear. • New nuclear envelope forms around each set of chromosomes. Re-forming nuclear envelope Cleavage furrow of cytokinesis Nucleolus Cytokinesis occurring Cleavage furrow (e) Telophase

  22. Metaphase → Anaphase →Telophase & Cytokinesis

  23. Cytokinesis (Cytoplasmic Division) • In animalcells, cytokinesis occurs by a forming acleavage furrow • In plantcells, cytokinesis occurs by a forming a cell plate.

  24. Cytokinesis in Animal Cells 100 µm Cleavage furrow Daughter cells Contractile ring of microfilaments

  25. Cytokinesis in Plant Cells http://www.youtube.com/watch?v=mzeowbIxgwI&feature=PlayList&p=4DFFFFDF820B4C76&playnext_from=PL&playnext=1&index=7 Vesicles forming cell plate Wall of parent cell 1 µm New cell wall Cell plate Daughter cells Cell plate formation in a plant cell (TEM)

  26. Mitosis

  27. Binary Fission • Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission • In binary fission: • the chromosome replicates • and the two daughter chromosomes actively move apart • The plasma membrane grows inward, and • new cell wall is deposited resulting in two daughter cells

  28. Binary Fission Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Two copies of origin Origin Origin Replication continues. One copy of the origin is now at each end of the cell. Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. Two daughter cells result.

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