Cell division The continuity of life is based on the reproduction of cells or cell division In unicellular organisms division of one cell reproduces the entire organism All the DNA in a cell constitutes the cell s genome Cell division is an integral part of the cell cycle the life of a cell from formation to its own division Multicellular organisms depend on cell division for Development from a fertilized cell Growth Repair Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 12 2 100 m a Reproduction 20 m 200 m b Growth and development c Tissue renewal DNA molecules in a cell are packaged into chromosomes Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus Mitosis is the process of cell division that produces somatic cells daughter cells with identical genetic information in the entire body except testis and ovary Meiosis is the process of cell division that produces gametes sperm or egg in reproductive organs testis and ovary of male and female respectively Meiosis yields nonidentical daughter cells that have only one set of chromosomes half as many as the parent cell Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Somatic cells nonreproductive cells have two sets of chromosomes Gametes reproductive cells sperm and eggs have half as many chromosomes as somatic cells Eukaryotic chromosomes consist of chromatin a complex of DNA and protein that condenses during cell division In preparation for cell division DNA is replicated and the chromosomes condense Each duplicated chromosome has two sister chromatids which separate during cell division The centromere is the narrow waist of the duplicated chromosome where the two chromatids are most closely attached Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 12 4 0 5 m Chromosomes Chromosome arm Centromere DNA molecules Chromosome duplication including DNA synthesis Sister chromatids Separation of sister chromatids Centromere Sister chromatids Eukaryotic cell cycle consists of two phases 1 Mitotic M phase cell division phase a Mitosis division of the nucleus b Cytokinesis division of the cytoplasm 2 Interphase cell growth and copying of chromosomes in preparation for cell division It s about 90 of the cell cycle can be divided into subphases 1 G1 phase first gap 2 S phase synthesis 3 G2 phase second gap Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 12 5 S DNA synthesis G1 M ito si s in k to y C is s e M IT M OTIC PHA SE G2 Eukaryotic cell division Mitosis is conventionally divided into five phases 1 Prophase 2 Prometaphase 3 Metaphase 4 Anaphase 5 Telophase Cytokinesis is well underway by late telophase Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 12 6 G2 of Interphase Centrosomes Chromatin with centriole duplicated pairs Prophase Early mitotic Aster Centromere spindle Nucleolus Nuclear Plasma envelope membrane Chromosome consisting of two sister chromatids Metaphase Prometaphase Fragments Nonkinetochore of nuclear microtubules envelope Kinetochore Kinetochore microtubule Anaphase Cleavage furrow Metaphase plate Spindle Centrosome at one spindle pole Telophase and Cytokinesis Daughter chromosomes Nuclear envelope forming Nucleolus forming Fig 12 8 EXPERIMENT Kinetochore Kinetochores Spindle pole Mark RESULTS CONCLUSION Chromosome movement Motor Microtubule protein Chromosome Kinetochore Tubulin subunits The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis by attaching itself to kinetochores 1 During prophase assembly of spindle microtubules begins in the centrosome the microtubule organizing center The centrosome replicates forming two centrosomes that migrate to opposite ends of the cell as spindle microtubules grow out from them 2 During prometaphase some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings 3 At metaphase the chromosomes are all lined up at the metaphase plate the midway point between the spindle s two poles metaphase is the longest phase 4 In anaphase sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell anaphase is the shortest phase The microtubules shorten by depolymerizing at their kinetochore ends Nonkinetochore microtubules from opposite poles overlap and push against each other elongating the cell 5 In telophase genetically identical daughter nuclei form at opposite ends of the cell Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Metaphase Aster Centrosome Sister chromatids Microtubules Chromosomes Metaphase plate Kinetochores Centrosome 1 m Overlapping nonkinetochore microtubules Kinetochore microtubules 0 5 m Fig 12 9 Cytokinesis after mitosis 100 m Cleavage furrow Contractile ring of microfilaments Vesicles forming cell plate Wall of parent cell Cell plate 1 m New cell wall Daughter cells a Cleavage of an animal cell SEM Daughter cells b Cell plate formation in a plant cell TEM In animal cells cytokinesis occurs by a process known as cleavage forming a cleavage furrow In plant cells a cell plate forms during cytokinesis Cell division in Prokaryotes Binary Fission Prokaryotes bacteria and archaea reproduce by a type of cell division called binary fission In binary fission the chromosome replicates beginning at the origin of replication and the two daughter chromosomes actively move apart Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 12 11 4 Binary Fission Origin of replication E coli cell Two copies of origin Origin Cell wall Plasma membrane Bacterial chromosome Origin Concept 12 3 The eukaryotic cell cycle is regulated by a molecular control system The frequency of cell division varies with the type of cell These cell cycle differences result from regulation at the molecular level The cell cycle appears to be driven by specific chemical signals present in the cytoplasm Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei The sequential events of the cell cycle are directed by a distinct cell cycle control system which is similar to a clock Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin