Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings DNA is the genetic material of the cell

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Most of the DNA of the Eukaryotic Cell is Located in the Nucleus

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings What is a Chromosome? One long DNA molecule and a number of protein molecules attached to it form one chromosome.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings DNA Replication

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phases of the Cell Cycle: – The Interphase (about 90% of the cell cycle) has three subphases: G 1 phase (“first gap”) centers on the growth of the cell. S phase (“synthesis”) duplicates the chromosomes. G 2 phase (“second gap”) completes the preparation for cell division before the mitotic phase starts. – The Mitotic (M) phase has two subphases: Mitosis (the nuclear division) Cytokinesis (the cytoplasmic division)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phases of the Cell Cycle G1 (Cell Growth) S (DNA Replication) G2 (Cell Preparation for Mitosis) M C Interphase Mitotic Phase M: Mitosis (Nuclear Division) C: Cytokinesis (Cytoplasmic Division)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 12.2: The mitotic phase alternates with interphase in the cell cycle G1G1 G2G2 S (DNA synthesis) INTERPHASE Cytokinesis MITOTIC (M) PHASE Mitosis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Cell Cycle

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Two pairs of centrioles Chromatin Nucleolus Nuclear envelope Plasma membrane Nucleus with chromatin -Growth of the baby cell. -Synthesis of cellular components needed for cell division, including replication of DNA and the centrosome 15 Interphase

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Phases of the Mitotic Phase of the Cell Cycle

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Centromere Chromosome (two sister chromatids joined at centromere) Developing spindle Nucleus with dispersed chromosomes Sister chromatids - 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. 17 Prophase PROPHASE PROMETAPHASE G 2 OF INTERPHASE

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings G2 of interphase → Prophase → Prometaphase

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Chromosomes aligned on equatorial plate Spindle fibers Equatorial plate 19

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 20

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. Nucleolus Cytokinesis occurring Re-forming nuclear envelope Cleavage furrow of cytokinesis Cleavage furrow (e) Telophase 21

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Metaphase → Anaphase →Telophase & Cytokinesis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Cytokinesis (Cytoplasmic Division) In animal cells, cytokinesis occurs by a forming a cleavage furrow In plant cells, cytokinesis occurs by a forming a cell plate.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cytokinesis in Animal Cells Cleavage furrow 100 µm Contractile ring of microfilaments Daughter cells

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1 µm Daughter cells Cell plate formation in a plant cell (TEM) New cell wall Cell plate Wall of parent cell Vesicles forming cell plate Cytokinesis in Plant Cells be.com/watch?v =mzeowbIxgwI& feature=PlayList &p=4DFFFFDF 820B4C76&play next_from=PL& playnext=1&ind ex=7

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mitosis

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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Origin of replication Cell wall Plasma membrane Bacterial chromosome E. coli cell Two copies of origin Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Replication continues. One copy of the origin is now at each end of the cell. Origin Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. Two daughter cells result. Binary Fission