Biology Sylvia S. Mader Michael Windelspecht Chapter 9 The Cell Cycle and Cellular Reproduction Lecture Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. See separate FlexArt PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes. 1

Outline 9.1 The Cell Cycle 9.2 Mitosis and Cytokinesis 9.3 The Cell Cycle and Cancer 9.4 Prokaryotic Cell Division 2

9.1 The Cell Cycle The cell cycle is an orderly set of stages from the first division to the time the resulting daughter cells divide Just prior to the next division:  The cell grows larger  The number of organelles doubles  The DNA is replicated The two major stages of the cell cycle:  Interphase (includes several stages)  Mitotic Stage (includes mitosis and cytokinesis) 3

The Cell Cycle 4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. G 1 (growth) G0G0 G 2 checkpoint Mitosis checkpoint. Mitosis will occur if DNA has replicated properly. Apoptosis will occur if the DNA is damaged and cannot be repaired. S (growth and DNA replication) M Cytokinesis T elophase Anaphase Metaphase Late prophase Prophase Interphase G1 checkpoint Cell cycle main checkpoint. If DNA is damaged, apoptosis will occur. Otherwise, the cell is committed to divide when growth signals are present and nutrients are available. M checkpoint Spindle assembly checkpoint. Mitosis will not continue if chromosomes are not properly aligned. M G2G2 G1G1 G 2 (growth and final preparations for division)

The Cell Cycle Interphase  Most of the cell cycle is spent in interphase  Cell performs its usual functions  Time spent in interphase varies by cell type  Nerve and muscle cells do not complete the cell cycle (remain in the G 0 stage) 5

The Cell Cycle Interphase consists of: G 1, S, and G 2 phases  G 1 Phase: Recovery from previous division Cell doubles its organelles Cell grows in size Cell accumulates raw materials for DNA synthesis  S Phase: DNA replication Proteins associated with DNA are synthesized Chromosomes enter with 1 chromatid each Chromosomes leave with 2 identical chromatids (sister chromatids) each  G 2 Phase: Between DNA replication and onset of mitosis Cell synthesizes proteins necessary for division 6

The Cell Cycle M (Mitotic) Stage  Includes: Mitosis –Nuclear division –Daughter chromosomes are distributed by the mitotic spindle to two daughter nuclei Cytokinesis –Division of the cytoplasm  Results in two genetically identical daughter cells 7

The Cell Cycle The cell cycle is controlled by internal and external signals A signal is a molecule that either stimulates or inhibits a metabolic event.  Internal signals Family of proteins called cyclins that increase and decrease as the cell cycle continues Without cyclins, the cell cycle stops at G 1, M or G 2 (checkpoints) Allows time for any damage to be repaired 8

The Cell Cycle Apoptosis is programmed cell death It involves a sequence of cellular events:  fragmenting of the nucleus,  blistering of the plasma membrane  engulfing of cell fragments. Apoptosis is caused by enzymes called caspases. Mitosis and apoptosis are opposing forces  Mitosis increases cell number  Apoptosis decreases cell number 9

Apoptosis 10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. apoptotic cell cell fragment DNA fragment Cell rounds up, and nucleus collapses. Chromatin condenses, and nucleus fragments. Plasma membrane blisters, and blebs form. Cell fragments contain DNA fragments. blebs Courtesy Douglas R. Green/LaJolla Institute for Allergy and Immunology

The Cell Cycle Apoptosis  Cells harbor caspases that are kept in check by inhibitors Can be unleashed by internal or external signals  Signal protein p53 Stops the cell cycle at G 1 when DNA is damaged Initiates an attempt at DNA repair –If successful, the cycle continues to mitosis –If not, apoptosis is initiated 11

Regulation at the G1 Checkpoint 12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. P P RB protein RB protein E2F CDK not present E2F not released released E2F E2F binds to DNA. DNA cell cycle proteins phosphorylated RB CDK present b. P P P P breakdown of p53 no DNA damage DNA damage phosphorylated p53 DNA repair proteins apoptosis p53 binds to DNA. DNA p53

9.2 Mitosis and Cytokinesis DNA is in very long threads  Chromosomes  Stretched out and intertwined between divisions  DNA is associated with histones (proteins)  DNA and histone proteins are collectively called chromatin Before mitosis begins:  Chromatin condenses (coils) into distinctly visible chromosomes  Each species has a characteristic chromosome number 13

14 Diploid Chromosome Numbers of Some Eukaryotes

Mitosis and Cytokinesis The diploid (2n) number includes two sets of chromosomes of each type  Humans have 23 different types of chromosomes Each type is represented twice in each body cell (diploid) Only sperm and eggs have one of each type ·termed haploid (n)  The haploid (n) number for humans is 23 Two representatives of each chromosome type Makes a total of 2n = 46 in each nucleus –One set of 23 from individual’s father (paternal) –Other set of 23 from individual’s mother (maternal) 15

Mitosis and Cytokinesis At the end of S phase:  Each chromosome internally duplicated  Consists of two identical DNA chains Sister chromatids (two strands of genetically identical chromosomes) Attached together at a single point (called centromere) During mitosis:  Centromeres holding sister chromatids together separate  Sister chromatids separate  Each becomes a daughter chromosome  Sisters of each type are distributed to opposite daughter nuclei 16

Duplicated Chromosomes 17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centromere sister chromatids one chromatid a. b. kinetochore 9,850 © Andrew Syred/Photo Researchers, Inc.

Mitosis and Cytokinesis Just outside the nucleus is the centrosome  This is the microtubule organizing center in animal cells  Organizes the mitotic spindle Contains many fibers Each fiber is composed of a bundle of microtubules  In animals, the centrosome contains two barrel- shaped centrioles Oriented at right angles to each other within the centrosome Each has 9 triplets of microtubules arranged in a cylinder Centrosome was also replicated in S-phase, so there are two centrosomes before mitosis begins 18

Mitosis and Cytokinesis Phases of Mitosis:  Prophase Chromatin has condensed –Chromosomes are distinguishable with microscope –Each chromosome has two sister chromatids attached at the centromere Nucleolus disappears Nuclear envelope disintegrates Spindle begins to assemble The two centrosomes move away from each other Microtubules form star-like arrays termed asters 19

Mitosis and Cytokinesis Phases of Mitosis  Prometaphase The centromere of each chromosome develops two kinetochores –Specialized protein complex –One attached to each sister chromatid »Physically connect sister chromatids with specialized microtubules (kinetochores) »These connect sister chromatids to opposite poles of the mother cell 20

Mitosis and Cytokinesis Stages of Mitosis  Metaphase Chromosomes are pulled around by kinetochore fibers Forced to align across the equatorial plane of the cell –Metaphase plate - Represents plane through which mother cell will be divided  Anaphase Centromere dissolves, releasing sister chromatids Sister chromatids separate –Now called daughter chromosomes –Pulled to opposite poles along kinetochore fibers 21

Mitosis and Cytokinesis Stages of Mitosis  Telophase Spindle disappears Now two clusters of daughter chromosomes –Still two of each type with all types represented –Clusters are incipient daughter nuclei Nuclear envelopes form around the two incipient daughter nuclei –Each daughter nucleus receives one chromosome of each type 22

Mitosis and Cytokinesis Cytokinesis = division of cytoplasm Allocates the mother cell’s cytoplasm equally to daughter nucleus Encloses each daughter cell in its own plasma membrane Often begins in anaphase Animal cytokinesis:  A cleavage furrow appears between daughter nuclei  Formed by a contractile ring of actin filaments  Like pulling on a drawstring  Eventually pinches the mother cell in two 23

Mitosis and Cytokinesis Cytokinesis in plant cells begins with the formation of a cell plate  Rigid cell walls outside plasma membrane do not permit furrowing  Many small membrane-bounded vesicles  Eventually fuse into one thin vesicle extending across the mother cell  The membranes of the cell plate become the plasma membrane between the daughter cells  The space between the daughter cells becomes filled with the middle lamella  Daughter cells later secrete primary cell walls on opposite sides of the middle lamella 24

Cytokinesis in Animal Cells 25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cleavage furrow Contractile ring X 4,000 (top): © Thomas Deerinck/Visuals Unlimited; (bottom): © SPL/Getty RF

Cytokinesis in Plant Cells 26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. daughter cells nucleoli daughter nucleus cell plate formation daughter nucleus vesicles containing membrane components fusing to form cell plate (left): © B.A. Palevitz and E.H. Newcomb/BPS/Tom Stack & Associates

27 Phases of Mitosis in Animal and Plant Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal cell(Early prophase, Prophase, Metaphase, Anaphase, Telophase): © Ed Reschke; Animal cell(Prometaphase): © Michael Abbey/Photo Researchers, Inc.; Plant cell(Early prophase, Prometaphse): © Ed Reschke; Plant cell(Prophase, Metaphase, Anaphase): © R. Calentine/Visuals Unlimited; Plant cell(Telophase): © Jack M. Bostrack/Visuals Unlimited; Plant Cell at Interphase centromere aster kinetochore polar spindle fiber chromosomescell wall 25µm centrosome lacks centrioles MITOSIS centrosome has centrioles Animal Cell at Interphase nuclear envelope fragments chromatin condenses nucleolus disappears Early Prophase Centrosomes have duplicated. Chromatin is condensing into chromosomes, and the nuclear envelope is fragmenting. Prophase Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming. Prophase Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming. 20 µm duplicated chromosome 20 µm spindle fibers forming spindle pole 9 µm kinetochore spindle fiber cleavage furrow spindle fibers 20µm 16µm kinetochore spindle fiber Anaphase Sister chromatids part and become daughter chromosomes that move toward the spindle poles. In this way, each pole receives the same number and kinds of chromosomes as the parent cell. Metaphase Centromeres of duplicated chromosomes are aligned at the metaphase plate (center of fully formed spindle). Kinetochore spindle fibers attached to the sister chromatids come from opposite spindle poles. chromosomes at metaphase plate 6.2µm 20µm 6.2µm spindle pole lacks centrioles and aster Telophase Daughter cells are forming as nuclear envelopes and nucleoli reappear. Chromosomes will become indistinct chromatin. daughter chromosome 20µm nucleolus cell plate 6.6µm

Phases of Mitosis in Animal and Plant Cells Animal cell(Early prophase): © Ed Reschke; Plant cell(Early prophase): © Ed Reschke Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Early Prophase aster 20 µm 25 µm MITOSIS Early Prophase Centrosomes have duplicated. Chromatin is condensing into chromosomes, and the nuclear envelope is fragmenting. chromatin condenses nucleolus disappears nuclear envelope fragments Animal Cell at Interphase centrosome has centrioles centrosome lacks centrioles Plant Cell at Interphase

Phases of Mitosis in Animal and Plant Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal cell(Early prophase, Prophase): © Ed Reschke; Plant cell(Early prophase): © Ed Reschke; Plant cell(Prophase): © R. Calentine/Visuals Unlimited Prophase spindle fibers forming centromere aster chromosomescell wall 20 µm 6.2 µm25 µm Prophase Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming. MITOSIS centrosome lacks centrioles Animal Cell at Interphase centrosome has centrioles nuclear envelope fragments chromatin condenses nucleolus disappears Early Prophase Centrosomes have duplicated. Chromatin is condensing into chromosomes, and the nuclear envelope is fragmenting. duplicated chromosome Plant Cell at Interphase

Phases of Mitosis in Animal and Plant Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal cell(Early prophase, Prophase, Metaphase): © Ed Reschke; Animal cell(Prometaphase): © Michael Abbey/Photo Researchers, Inc.; Plant cell(Early prophase, Prometaphse): © Ed Reschke; Plant cell(Prophase): © R. Calentine/Visuals Unlimited Prometaphase centromere spindle pole aster kinetochore polar spindle fiber chromosomescell wall 20 µm 6.2 µm25 µm20 µm 9 µm Prometaphase The kinetochore of each chromatid is attached to a kinetochore spindle fiber. Polar spindle fibers stretch from each spindle pole and overlap. centrosome has centrioles Animal Cell at Interphase nuclear envelope fragments chromatin condenses nucleolus disappears spindle fibers forming Early Prophase Centrosomes have duplicated. Chromatin is condensing into chromosomes, and the nuclear envelope is fragmenting. Prophase Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming. kinetochore spindle fiber duplicated chromosome spindle pole lacks centrioles and aster Plant Cell at Interphase centrosome lacks centrioles MITOSIS

Phases of Mitosis in Animal and Plant Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal cell(Early prophase, Prophase, Metaphase): © Ed Reschke; Animal cell(Prometaphase): © Michael Abbey/Photo Researchers, Inc.; Plant cell(Early prophase, Prometaphse): © Ed Reschke; Plant cell(Prophase, Metaphase): © R. Calentine/Visuals Unlimited duplicated chromosome centromere aster kinetochore polar spindle fiber chromosomescell wall 20 µm 6.2 µm25 µm20 µm 9 µm spindle fibers 20 µm 6.2 µm Metaphase Centromeres of duplicated chromosomes are aligned at the metaphase plate (center of fully formed spindle). Kinetochore spindle fibers attached to the sister chromatids come from opposite spindle poles. centrosome has centrioles Animal Cell at Interphase nuclear envelope fragments nucleolus disappears chromatin condenses centrosome lacks centrioles Early Prophase Centrosomes have duplicated. Chromatin is condensing into chromosomes, and the nuclear envelope is fragmenting. Prophase Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming. Prometaphase The kinetochore of each chromatid is attached to a kinetochore spindle fiber. Polar spindle fibers stretch from each spindle pole and overlap. Metaphase kinetochore spindle fiber chromosomes at metaphase plate spindle pole kinetochore spindle fiber spindle fibers forming Plant Cell at Interphase spindle pole lacks centrioles and aster MITOSIS 20 µm

Phases of Mitosis in Animal and Plant Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal cell(Early prophase, Prophase, Metaphase, Anaphase): © Ed Reschke; Animal cell(Prometaphase): © Michael Abbey/Photo Researchers, Inc.; Plant cell(Early prophase, Prometaphse): © Ed Reschke; Plant cell(Prophase, Metaphase, Anaphase): © R. Calentine/Visuals Unlimited centromere spindle pole aster kinetochore spindle fiber polar spindle fiber chromosomescell wall 20 µm 6.2 µm25 µm20 µm 9 µm daughter chromosome spindle fibers 20 µm 6.2 µm centrosome has centrioles Animal Cell at Interphase nuclear envelope fragments nucleolus disappears chromatin condenses centrosome lacks centrioles Early Prophase Centrosomes have duplicated. Chromatin is condensing into chromosomes, and the nuclear envelope is fragmenting. Prophase Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming. Prometaphase The kinetochore of each chromatid is attached to a kinetochore spindle fiber. Polar spindle fibers stretch from each spindle pole and overlap. Metaphase Centromeres of duplicated chromosomes are aligned at the metaphase plate (center of fully formed spindle). Kinetochore spindle fibers attached to the sister chromatids come from opposite spindle poles. kinetochore spindle fiber Anaphase chromosomes at metaphase plate spindle pole lacks centrioles and aster Plant Cell at Interphase duplicated chromosome spindle fibers forming MITOSIS Anaphase Sister chromatids part and become daughter chromosomes that move toward the spindle poles. In this way, each pole receives the same number and kinds of chromosomes as the parent cell.

Phases of Mitosis in Animal and Plant Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centromere aster kinetochore polar spindle fiber chromosomescell wall 20 µm 6.2 µm25 µm20 µm 9 µm daughter chromosome spindle fiberscell plate 20 µm 16 µm 6.2 µm6.6 µm6.2 µm nucleolus cleavage furrow Telophase Daughter cells are forming as nuclear envelopes and nucleoli reappear. Chromosomes will become indistinct chromatin. centrosome has centrioles Animal Cell at Interphase nuclear envelope fragments chromatin condenses nucleolus disappears centrosome lacks centrioles Plant Cell at Interphase Early Prophase Centrosomes have duplicated. Chromatin is condensing into chromosomes, and the nuclear envelope is fragmenting. spindle fibers forming duplicated chromosome kinetochore spindle fiber Prophase Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming. Prometaphase The kinetochore of each chromatid is attached to a kinetochore spindle fiber. Polar spindle fibers stretch from each spindle pole and overlap. Metaphase Centromeres of duplicated chromosomes are aligned at the metaphase plate (center of fully formed spindle). Kinetochore spindle fibers attached to the sister chromatids come from opposite spindle poles. Anaphase Sister chromatids part and become daughter chromosomes that move toward the spindle poles. In this way, each pole receives the same number and kinds of chromosomes as the parent cell. Telophase spindle pole lacks centrioles and aster spindle pole chromosomes at metaphase plate kinetochore spindle fiber Animal cell(Early prophase, Prophase, Metaphase, Anaphase, Telophase): © Ed Reschke; Animal cell(Prometaphase): © Michael Abbey/Photo Researchers, Inc.; Plant cell(Early prophase, Prometaphse): © Ed Reschke; Plant cell(Prophase, Metaphase, Anaphase): © R. Calentine/Visuals Unlimited; Plant cell(Telophase): © Jack M. Bostrack/Visuals Unlimited; MITOSIS

Phases of Mitosis in Animal and Plant Cells Functions of mitosis:  Permits growth and repair.  In flowering plants, meristematic tissue retains the ability to divide throughout the life of the plant  In mammals, mitosis is necessary when: A fertilized egg becomes an embryo An embryo becomes a fetus A cut heals or a broken bone mends 34

Phases of Mitosis in Animal and Plant Cells Stem Cells  Many mammalian organs contain stem cells Retain the ability to divide Red bone marrow stem cells divide to produce various types of blood cells  Therapeutic cloning to produce human tissues can begin with either adult stem cells or embryonic stem cells  Embryonic stem cells can be used for reproductive cloning, the production of a new individual 35

Reproductive and Therapeutic Cloning 36 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. remove and discard egg nucleus egg remove G o nucleus G o cells from animal to be cloned egg fuse egg with G o nucleus Implant embryo into surrogate mother Clone is born culture embryonic stem cells nervous blood muscle embryonic stem cells culture fuse egg with G o nucleus remove and discard egg nucleus G o somatic cells remove G o nucleus b. Therapeutic cloning a. Reproductive cloning

9.3 The Cell Cycle and Cancer Abnormal growth of cells is called a tumor  Benign tumors are not cancerous Encapsulated Do not invade neighboring tissue or spread  Malignant tumors are cancerous Not encapsulated Readily invade neighboring tissues May also detach and lodge in distant places (metastasis) Results from mutation of genes regulating the cell cycle Development of cancer  Tends to be gradual  May take years before a cell is obviously cancerous 37

The Cell Cycle and Cancer Characteristics of Cancer Cells  Lack differentiation Are non-specialized Are immortal (can enter cell cycle repeatedly)  Have abnormal nuclei May be enlarged May have abnormal number of chromosomes Often have extra copies of genes  Do not undergo apoptosis Normally, cells with damaged DNA undergo apoptosis The immune system can also recognize abnormal cells and trigger apoptosis Cancer cells are abnormal but fail to undergo apoptosis 38

The Cell Cycle and Cancer Characteristics of Cancer Cells  Form tumors Mitosis is normally controlled by contact with neighboring cells – contact inhibition Cancer cells have lost contact inhibition  Undergo metastasis Original tumor easily fragments New tumors appear in other organs  Undergo angiogenesis Formation of new blood vessels –Brings nutrients and oxygen to the tumor 39

Progression of Cancer 40 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. primary tumor New mutations arise, and one cell (brown) has the ability to start a tumor. Cancer in situ. The tumor is at its place of origin. One cell (purple) mutates further. Cancer cells now have the ability to invade lymphatic and blood vessels and travel throughout the body. New metastatic tumors are found some distance from the primary tumor. lymphatic vessel blood vessel lymphatic vessel blood vessel

Cancer Cells vs. Normal Cells 41

The Cell Cycle and Cancer Origin of Cancer  Oncogenes Proto-oncogenes promote the cell cycle in various ways If a proto-oncogene is mutated, it may become an oncogene  Tumor suppressor genes inhibit the cell cycle in various ways If a tumor suppressor gene becomes inactive, it may promote cancer development  Both proto-oncogenes and tumor suppressor genes are normally regulated in coordination with organism’s growth plan 42

The Cell Cycle and Cancer Origin of Cancer  Chromosomes normally have special material at each end called telomeres  These get shorter each cell division  When they get very short, the cell will no longer divide  Telomerase is an enzyme that maintains the length of telomeres  Mutations in telomerase gene: Cause telomeres to continue to lengthen, which Allows cancer cells to continually divide 43

Causes of Cancer 44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. d: © Biophoto Associates/Photo Researchers, Inc. activated signaling protein growth factor receptor protein signaling protein phosphate b. Effect of growth factor P P P proto-oncogene Codes for a growth factor, a receptor protein, or a signaling protein in a stimulatory pathway. If a proto-oncogene becomes an oncogene, the end result can be active cell division. tumor suppressor gene Codes for a signaling protein in an inhibitory pathway. If a tumor suppressor gene mutates, the end result can be active cell division. c. Stimulatory pathway and inhibitory pathway 1,100X d. Cancerous skin cell gene product promotes cell cycle Stimulatory pathway Inhibitory pathway gene product inhibits cell cycle growth factor Activates signaling proteins in a stimulatory pathway that extends to the nucleus. a. Influences that cause mutated proto-oncogenes (called oncogenes) and mutated tumor suppressor genes Heredity Radiation sources Pesticides and herbicides Viruses oncogene

9.4 Prokaryotic Cell Division The prokaryotic chromosome is a ring of DNA  Folded up in an area called the nucleoid  1,000 X the length of cell  Replicated into two rings prior to cell division  Replicated rings attach to the plasma membrane Binary fission  Splitting in two  Two replicate chromosomes are distributed to two daughter cells  Produces two daughter cells identical to original cell – asexual reproduction 45

Binary Fission 46 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. chromosome cell wall plasma membrane cytoplasm SEM2,345X 1. Attachment of chromosome to a special plasma membrane site indicates that this bacterium is about to divide. © Dennis Kunkel Microscopy, Inc./Visuals Unlimited

Binary Fission 47 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. chromosome cell wall plasma membrane cytoplasm SEM2,345X 1. Attachment of chromosome to a special plasma membrane site indicates that this bacterium is about to divide. 2. The cell is preparing for binary fission by enlarging its cell wall, plasma membrane, and overall volume. © Dennis Kunkel Microscopy, Inc./Visuals Unlimited

Binary Fission 48 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. chromosome cell wall plasma membrane cytoplasm SEM2,345X 1. Attachment of chromosome to a special plasma membrane site indicates that this bacterium is about to divide. 2. The cell is preparing for binary fission by enlarging its cell wall, plasma membrane, and overall volume. 3. DNA replication has produced two identical chromosomes. Cell wall and plasma membrane begin to grow inward. © Dennis Kunkel Microscopy, Inc./Visuals Unlimited

Binary Fission 49 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. chromosome cell wall plasma membrane cytoplasm SEM2,345X © Dennis Kunkel Microscopy, Inc./Visuals Unlimited 1. Attachment of chromosome to a special plasma membrane site indicates that this bacterium is about to divide. 2. The cell is preparing for binary fission by enlarging its cell wall, plasma membrane, and overall volume. 3. DNA replication has produced two identical chromosomes. Cell wall and plasma membrane begin to grow inward. 4. As the cell elongates, the chromosomes are pulled apart. Cytoplasm is being distributed evenly.

Binary Fission 50 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1. Attachment of chromosome to a special plasma membrane site indicates that this bacterium is about to divide. 2. The cell is preparing for binary fission by enlarging its cell wall, plasma membrane, and overall volume. 3. DNA replication has produced two identical chromosomes. Cell wall and plasma membrane begin to grow inward. 4. As the cell elongates, the chromosomes are pulled apart. Cytoplasm is being distributed evenly. 5. New cell wall and plasma membrane has divided the daughter cells. chromosome cell wall plasma membrane cytoplasm SEM2,345X © Dennis Kunkel Microscopy, Inc./Visuals Unlimited

Functions of Cell Division 51