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Chapter 11: How Cells Divide

Chapter Outline

Chapter 11: How Cells Divide

11.0 Introduction

  1. All Organisms Grow and Reproduce

    1. All Species Pass Their Hereditary Information on to Their Offspring

    2. How Do Cells Reproduce? fig 11.1

11.1 Bacteria divide far more simply than do eukaryotes
  1. Cell Division In Bacteria

    1. Binary Fission Is Bacterial Cell Division

      1. Composition of the bacterial genome

        1. Exists as one double-stranded circle of DNA

        2. Attached to one point on the interior of the cell membrane

      2. Genome replicated early in the life of the cell fig 11.2

        1. Copying of the DNA circle occurs at the replication origin

        2. Requires a battery of enzymes

        3. End result: Two side-by-side circles of DNA on membrane

    2. Division Initiated by Growth of the Cell to a Certain Size

      1. New plasma membrane and cell wall materials laid down

      2. Growing membrane pinches inward, cell constricted in two fig 11.3

      3. Each cell contains a copy of the genome

      4. New cell wall forms around membrane

  2. Cell Division in Eukaryotes

    1. Eukaryotic Genome Is Larger than Bacterial Genome

    2. Eukaryotic DNA Located within Linear Chromosomes

      1. Organization more complex

      2. DNA forms a complex with histone proteins and is tightly coiled

11.2 Chromosomes are highly ordered structures
  1. Discovery of Chromosomes

    1. Eukaryotic Chromosomes

      1. Chromosomes first observed in dividing salamander larvae cells

      2. Division of cells called mitosis, derivative of word for thread

    2. Chromosome Number

      1. The number of chromosomes varies within species tbl 11.1

      2. Humans have 46 chromosomes, 23 identical pairs fig 11.4

        1. All chromosomes are necessary for survival

        2. In monosomy only one chromosome missing, lack of embryonic development

        3. With extra chromosome, trisomy, development proceeds improperly

          1. Most trisomy’s are fatal

          2. Downs syndrome is trisomy in chromosome 21

  2. The Structure of Eukaryotic Chromosomes

    1. Composition of Chromatin

      1. Chromosomes are composed of chromatin

        1. Complex of 40% DNA and 60% protein

        2. Contains some RNA since DNA is the site of RNA synthesis

      2. DNA exists as a long double-stranded fiber

      3. DNA is highly coiled to fit within cell

    2. Chromosome Coiling

      1. DNA resembles a string of beads fig 11.5

        1. DNA is coiled around histone polypeptides every 200 nucleotides

        2. Eight histones form a core called a nucleosome

        3. Basic, positively charged histones attract negatively charged DNA

        4. String of nucleosomes further wrapped into supercoils

      2. Heterochromatin

        1. Highly condensed portions of chromatin

        2. Some portions permanently condensed to prevent DNA expression

      3. Euchromatin

        1. Remainder of chromatin condensed only during cell division

        2. Movement of chromosomes facilitated by packaging

        3. DNA is uncondensed to allow for gene expression

    3. Chromosome Karyotypes

      1. Chromosomes vary widely in appearance

        1. Size and staining properties

        2. Location of the centromere

        3. Relative length of the arms on either side of the centromere

        4. Position of additional constricted regions along arms

      2. Karyotype: Array of an individual’s chromosomes fig 11.6

        1. In humans, blood sample collected, cells induced to divide

        2. Chemicals stop division at metaphase when chromosomes are most condensed

        3. Contents spread out, stained and then photographed

        4. Chromosomes cut out and arranged in order

    4. How Many Chromosomes Are in a Cell?

      1. All human body cells are diploid (2n)

        1. Contain 46 chromosomes composed of 23 pairs

        2. Pairs are nearly identical

      2. Human gametes have haploid (1n) complement with 23 chromosomes

      3. Nearly identical pairs are called homologous chromosomes or homologues

      4. Before division each of the two homologues replicates

        1. Produce two identical copies called sister chromatids

        2. Chromatids remain joined together at the centromere fig 11.7

        3. Cells have 46 replicated chromosomes each with two chromatids

          1. Possess 46 centromeres. 92 chromatids

          2. Number of chromosomes indicated by number of centromeres

11.3 Mitosis is a key phase of the cell cycle
  1. Phases of the Cell Cycle

    1. Division Process Diagramed as the Cell Cycle fig 11.8

    2. The Five Phases

      1. G1 phase: Primary growth phase, most of cell’s life span

      2. S phase: Genome replica synthesized

      3. G2 phase: Preparations made for genomic separation

        1. Replication of mitochondria and other organelles

        2. Chromosome condensation

        3. Restructuring of microtubules and assembly at spindle

      4. M phase: Mitosis

        1. Microtubular apparatus assembled

        2. Sister chromatids moved apart from one another

        3. Continuous process, traditionally divided into four stages

        4. Mitosis subdivided into four continuous stages fig 11.9

          1. Prophase

          2. Metaphase

          3. Anaphase

          4. Telophase

      5. C phase: Cytokinesis

        1. Physical division of the cell, creates two daughter cells

        2. Animal spindle helps position contracting cleavage furrow actin ring

        3. Plants and other cells with cell wall form plate between dividing cells

    3. Duration of the Cell Cycle

      1. Greatly variable among organisms

      2. Embryos exhibit shortest cycles

        1. Divide as quickly as DNA can be replicated

        2. Half of cycle is S, half is M, virtually no G1 or G2

      3. Mature cells have longer cycles

        1. Mammalian cell cycle averages 24 hours

        2. Growth occurs during G1 and G2

          1. G phases may be referred to as gap phases

          2. They separate the S phase from the M phase

        3. M phase takes only small portion of cycle

      4. Length of cycle variability is usually in G1

        1. Many cells pause in a G0 resting stage

        2. May remain there for days to years, some remain permanently

        3. Most body cells are in G0 at any one time

        4. Injury may stimulate some cells to enter G2 from G0

  2. Interphase: Preparing for Mitosis

    1. Events of Interphase Necessary for Successful Completion of Mitosis

      1. G1 phase: Cells undergo major portion of growth

      2. S phase: Chromosome replicates to produce sister chromatids

        1. Remain attached at the centromere fig 11.10

        2. Specific DNA sequence bound to a protein kinetochore fig 11.11

        3. Location specific to each chromosome

    2. Cell Grows Throughout Interphase

      1. G1 and G2 are periods of active growth, protein synthesis, organelle replication

      2. Cell’s DNA replicates only during S phase

      3. After S replication, chromosomes remain extended and uncoiled

      4. G2 phase: Chromosomes begin process of condensation

        1. Motor proteins involved in rapid, final condensation

        2. In G2 cells assemble machinery used to move chromosomes apart

          1. Animals replicate centriole, nuclear microtubule-organizing centers

          2. Eukaryotic cells synthesize tubulin, microtubule protein component

  3. Prophase: Formation of the Mitotic Apparatus

    1. Characteristics that Indicate Beginning of Prophase fig 11.12

      1. Individual condensed chromosomes become visible

      2. Condensation continues through prophase

      3. Ribosomal RNA synthesis ceases, nucleolus disappears

    2. Assembling the Spindle Apparatus

      1. Microtubule apparatus made of spindle fibers continues to assemble

        1. In animal cells the two centrioles move apart

        2. Spindle apparatus, a bridge of microtubules, forms between them

        3. In plant cells, spindle apparatus forms without visible centrioles

        4. Nuclear envelope breaks down, materials absorbed by ER

        5. Position of spindle microtubules determines plane of cell division

        6. Division occurs at right angles through the spindle

      2. Animal cells form an arrangement called an aster

        1. Centrioles at opposite poles extend radial array of microtubules

        2. Function to stiffen point of microtubular attachment

        3. Rigid plant cells do not form asters

    3. Linking Sister Chromatids to Opposite Poles

      1. Second group of microtubules grow out from centromeres to poles

        1. Each chromosome possesses two kinetochores fig 11.11a

        2. Two sets of microtubules extend from each chromosome

        3. Kinetochore of each sister chromatid connected to one pole

        4. Microtubules grow until they make contact with poles

      2. Sister chromatids won’t separate if both connected to same pole

  4. Metaphase: Division of the Centromeres

    1. Key Characteristics of Metaphase

      1. Stage begins when chromosomes align in center of the cell

      2. Alignment occurs along the metaphase plate fig 11.13

        1. Not a physical structure

        2. Indicates where future axis of cell division occurs

      3. Centromeres are equidistant from each pole

    2. Centromeres Divide at the End of Metaphase

      1. Centromere splits in two, freeing sister chromatids from one another

      2. All centromeres divide in synchrony

  5. Anaphase and telophase: Separation of the Chromatids and Reformation of Nuclei

    1. Anaphase

      1. Shortest phase, during which sister chromatids separate

      2. Chromatid drawn to pole to which it’s kinetochore is attached

      3. Separation achieved by two simultaneous microtubular actions

        1. Poles move apart fig 11.14

          1. Microtubular spindle fibers slide past one another

          2. Microtubules are anchored at poles which are pushed apart

          3. Chromatids attached to poles move apart as well

        2. Centromeres move toward poles

          1. Shortening process is not a contraction

          2. Microtubules shorten as tubulin subunits are removed

          3. Chromatids are therefore pulled toward poles

    2. Telophase

      1. Separation of chromatids completes partitioning of replicated genome

      2. Spindle apparatus is disassembled

      3. Tubulin units of microtubules are used to build new cytoskeleton

      4. Nuclear envelope re-forms around each new set of chromosomes

      5. Chromosomes begin to uncoil to allow gene expression

      6. rRNA genes begin transcription, nucleolus reappears

    3. Photographic Highlights of Mitosis fig 11.15

  6. Cytokinesis

    1. Mitosis Complete at End of Telophase

      1. Replicated genome divided into two new nuclei at opposite ends of cell

      2. Cytoplasmic organelles assort to regions that will become separated

      3. Cleavage of the cell into two halves constitutes cytokinesis

    2. Cytokinesis in Animal Cells

      1. Cell is pinched in two by a constricting belt of microfilaments

        1. Actin filaments slide past one another

        2. Produces distinct cleavage furrow around circumference of cell fig 11.16a

      2. Furrow deepens until the cell pinched in two fig 11.16b

    3. Cytokinesis in Plant Cells

      1. Rigid cell wall, cannot be deformed by microfilament contraction

      2. Membrane components assembled in the cell interior fig 11.17

        1. Occurs at right angles to the spindle apparatus

        2. Expanding partition called the cell plate

        3. Grows outward to the interior surface of the cell membrane

        4. Cellulose then added on the membrane making two new cells

      3. Middle lamellae: Space between cells impregnated with pectins

    4. Cytokinesis in Fungi and Protists

      1. In fungi and some protists mitosis is confined to the nucleus

      2. After completion of mitosis nucleus divides into daughter nuclei

    5. Distribution of Other Cell Organelles

      1. No discrete process similar to mitosis

      2. Organelles replicate increasing number of individuals

      3. Cell division distributes at least one organelle in each cell, can later replicate

11.4 The cell cycle is carefully controlled
  1. General Strategy of Cell Cycle Control

    1. Events of Cell Cycle Coordinated Similarly in All Eukaryotes

    2. Goal of Cell Cycle Control Is to Optimize Duration of Cycle

      1. Internal clock control cannot provide sufficient flexibility

      2. Eukaryotes use a centralized controller based on cellular feedback

        1. Analogy: Furnace heating a house

        2. At points in cycle feedback determines if cycle continues or is delayed

    3. Architecture of the Control System

      1. Three principal check points fig 11.18

        1. Cell growth assessed at G1 check point

        2. Determines if cell divides, delays division or enters resting stage fig 11.19

          1. Check point called START in yeasts

          2. If conditions favorable cell starts copying DNA, starting S phase

        3. DNA replication assessed at G2 check point

        4. Mitosis assessed at M check point

  2. Molecular Mechanisms of Cell Cycle Control

    1. Basic Mechanism of Cell Cycle Control Is Simple

      1. Associated with interactions of proteins sensitive to cell conditions

      2. Two key types of proteins fig 11.20

        1. Cyclin-dependent protein kinases

        2. Cyclins

    2. The Cyclin Control System

      1. Cyclin-dependent protein kinases (Cdk’s)

        1. These enzymes phosphorylate serine and threonine of certain proteins

        2. Phosphorylate histones, nuclear membrane filaments, microtubule proteins at G2

        3. Phosphorylation initiates activities that carry cycle past checkpoint to mitosis

      2. Cyclins

        1. Bind to Cdk’s, enable them to act as enzymes

        2. Are destroyed and resynthesized at each turn of cell cycle

        3. Different kinds of cyclins regulate at the two check points fig 11.21

      3. The G2 check point

        1. Cell accumulates G2 cyclin (mitotic cyclin) during G2

          1. Binds to Cdk forming mitosis promoting factor, MPF

          2. MPF are phosphorylated and activated by cellular enzymes

          3. Positive feedback increases this activity, more MPF activated

          4. G2 ends when sufficient activated MPF

        2. Duration of M phase determined by MPF activity

          1. MPF also activates proteins that destroy cyclin

          2. Degradation of G2 cyclin decreases activity of MPF, ending mitosis

      4. The G1 check point

        1. Similar to G2 control

        2. Main factor triggering DNA replication in yeast is cell size

        3. Yeasts compare volume of cytoplasm to size of genome

        4. In growth size increases, amount of DNA constant

        5. Threshold ration reached promoting cyclin production

    3. Controlling the Cell Cycle in Multicellular Eukaryotes

      1. Cells of multicellular organisms can’t make individual decisions

      2. Organization dependent limiting cell proliferation

      3. In cell culture cells stop dividing when sufficient numbers

        1. Growing cells take up growth factors like MPF fig 11.22

        2. Example of positive regulatory signal

        3. If other cells take up factor, none left to trigger division in any cell

    4. Growth Factors and the Cell Cycle

      1. Growth factors trigger intracellular signalling systems

      2. Example: Fibroblasts

        1. Possess membrane receptors for platelet-derived growth factor, PDGF

        2. Binding PDGF and receptor initiates amplifying chain of events

        3. Tissue injury causes release of PDGF to promote healing

      3. Characteristics of growth factors

        1. Isolation of fifty growth factor proteins tbl 11.2

        2. Each factor specifically recognized by specific cell surface receptor fig 11.23

        3. Some affect broad range of cell types, PDGF and E (epidermal) GF

        4. Some affect only certain cell types, N (nerve) GF and erythropoietin

      4. The G0 phase

        1. Cells deprived of growth factors stop at G1, stay in G0

        2. G0 is a non-growing state distinctly different from G1, S and G2

        3. Accounts for great diversity in length of cell cycle among different organisms

          1. Gut lining cells divide twice per day

          2. Liver cells divide once per year or two

          3. Mature neurons and muscle cells usually stay in G0

  3. Cancer and the Control of Cell Proliferation

    1. Cancer Is A Disease of Altered Control of the Cell Cycle

      1. Discovery of p53 gene ("Guardian Angel gene")

      2. Involved in G1 check point of cell division

        1. Gene’s product is p53 protein, monitors integrity of DNA

        2. If p53 detects damaged DNA cell division halted, enzyme repair initiated

        3. DNA replication continued after DNA is repaired

        4. If DNA irreparable p53 directs cell to kill itself (apoptosis)

      3. Halting division prevents development of numerous mutated cells

        1. p53 considered a tumor-suppressor gene

        2. Activities not limited to cancer prevention

      4. p53 absent or grossly damaged in all examined cancerous cells

        1. Cancer cells undergo repeated division without halt at G1 check point fig 11.24

        2. If p53 protein added to dividing cancer cells their division stops

        3. Cigarette smoking causes mutations in p53 gene

    2. Growth Factors and Cancer

      1. Proto-oncogenes

        1. Normally stimulate cell division, positive approach

        2. Trigger passage through G1 check point

          1. Aid formation of cyclins

          2. Activate genes that promote cell division

        3. Mutations causing them to overact change them into oncogenes

          1. Mutations are dominant

          2. Leads to excessive cell proliferation characteristic of cancer fig 11.25

        4. 30 different proto-oncogenes exist

        5. myc, fos, jun cause unrestrained cell growth and division

          1. myc in normal cell helps to regulate G1 check point fig 11.26

          2. Genes also stimulate delayed response genes that produce cyclins, Cdk

      2. Tumor-suppressor genes

        1. Normally inhibit cell division, negative approach

          1. Prevents binding of cyclins to Cdk, block passage through G1

          2. Mutations are recessive, unrestrained division if both copies mutated

        2. Retinoblastoma (Rb) gene is tumor-suppressor gene, causes rare eye cancer

          1. Normal gene is a cancer suppressor

          2. Rb gene encodes protein present in nucleus

          3. Rb protein is dephosphorylated in G0 phase fig 11.23

          4. Binds regulatory proteins needed for cell proliferation

          5. Inhibit cell division

        3. When Rb is phosphorylated it releases regulatory proteins

          1. Cell division thus promoted

          2. Cells produce cyclins and Cdk, pass G1 check point

      3. Summary of genes that cause cancer when mutated fig 11.27

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