ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Eleven Outline INTRODUCTION All Organisms Grow and Reproduce All Species Pass Their Hereditary Information on to Their Offspring fig 11.1 CELL DIVISION IN BACTERIA Binary Fission Is Bacterial Cell Division Genome replicated early in the life of the cell fig 11.2 Copying of the DNA circle occurs at the replication origin Requires a battery of enzymes End result: two side-by-side circles of DNA on membrane Composition of the bacterial genome Exists as one double-stranded circle of DNA Attached to one point on the interior of the cell membrane Division Initiated by Growth of the Cell to a Certain Size New plasma membrane and cell wall materials laid down Growing membrane pinches inward, cell constricted in two fig 11.3 Each cell contains a copy of the genome CELL DIVISION IN EUKARYOTES Eukaryotic Genome Is Larger and More Complex than Bacterial Genome DNA located within linear chromosomes DNA forms a complex with histone proteins and is tightly coiled fig 11.4 The Structure of Eukaryotic Chromosomes Chromosomes first observed in dividing salamander larvae cells The number of chromosomes varies within species tbl 11.1 Chromosomes are composed of chromatin Complex of 40% DNA and 60% protein Contains some RNA since DNA is the site of RNA synthesis DNA exists as a long double-stranded fiber DNA coiled to fit into a smaller space than otherwise possible DNA resembles a string of beads fig 11.5 DNA is coiled around histone polypeptides every 200 nucleotides Eight histones form a core called a nucleosome Basic, positively charged histones attract negatively charged DNA String of nucleosomes further wrapped into supercoils Heterochromatin Highly condensed portions of chromatin Some portions permanently condensed to prevent DNA expression Euchromatin Remainder of chromatin condensed only during cell replication Movement of chromosomes facilitated by packaging DNA is uncondensed to allow for gene expression Chromosomes vary widely in appearance Position of the centromere Relative length of the arms on either side of the centromere Size and staining properties Position of additional constricted regions along arms fig 11.6 Karyotype: array of an individual's chromosomes fig 11.7 In humans, blood sample collected, cells induced to divide Chemicals stop division at metaphase when chromosomes are most condensed Contents spread out, stained and then photographed Chromosomes cut out and arranged in order Karyotypes may reflect genetic abnormalities How Many Chromosomes Are in a Cell? All human body cells are diploid Contain 46 chromosomes composed of 23 pairs Pairs are nearly identical and are called homologues Human gametes have haploid complement with 23 chromosomes Before division each of the two homologues replicates fig 11.8 Produce two identical copies called sister chromatids Chromatids remain joined together at the centromere Cells have 46 replicated chromosomes each with two chromatids Possess 46 centromeres Four sets of genetic material: 23 pairs x 2 chromatids each Number of chromosomes indicated by number of centromeres THE CELL CYCLE Cycle of Cell Growth and Division Has Five Stages G1 phase: primary growth phase S phase: genome replica synthesized G2 phase: preparations made for genomic separation Replication of mitochondria and other organelles Chromosome condensation Restructuring of microtubules and assembly at spindle M phase: mitosis Microtubular apparatus assembled Sister chromatids move apart from one another C phase: cytokinesis Physical division of the cell, creates two daughter cells Animal spindle helps position contracting cleavage furrow actin ring Duration of Cell Cycle Is Variable Embryos exhibit shortest cycles Divide as quickly as DNA can be replicated Half of cycle is S, half is M, virtually no G1 or G2 Mature cells have longer cycles fig 11.9 Mammalian cell cycle averages 24 hours Growth occurs during G1 and G2 G phases may be referred to as gap phases They separate the S phase from the M phase M phase takes only small portion of cycle Length of cycle variability is in G1 Many cells pause in a G0 resting stage May remain there for days to years, some remain permanently Most body cells are in G0 at any one time Injury may stimulate some cells to enter G1 from G0 MITOSIS M and C Phases Are Readily Observed Constitute only small part of cell cycle fig 11.10 Mitosis subdivided into four continuous stages fig 11.11 Prophase Metaphase Anaphase Telophase Preparing for Mitosis: Interphase G1 phase: cells undergo major portion of growth S phase: chromosome replicates to produce sister chromatids Remain attached at the centromere fig 11.12 Specific DNA sequence bound to a protein kinetochore fig 11.13 Location specific to each chromosome G2 phase: chromosomes begin process of condensation Motor proteins involved in rapid, final condensation In G2 cells assemble machinery used to move chromosomes apart Animals replicate centriole, nuclear microtubule-organizing centers Eukaryotic cells synthesize tubulin, microtubule protein component Formation of the Mitotic Apparatus: Prophase Individual condensed chromosomes become visible Condensation continues through prophase Ribosomal RNA synthesis ceases, nucleolus disappears Microtubule apparatus made of spindle fibers begins to assemble In animal cells the two centrioles move apart Spindle apparatus, a bridge of microtubules, forms between them In plant cells, spindle apparatus forms without visible centrioles Position of spindle microtubules determines plane of cell division Division occurs at right angles through the spindle Nuclear envelope breaks down, materials absorbed by ER Animal cells form an arrangement called an aster fig 11.14 Centrioles at opposite poles extend radial array of microtubules Function to stiffen point of microtubular attachment Rigid plant cells do not form asters Second group of microtubules grow out from centromeres to poles Each chromosome possesses two kinetochores Two sets of microtubules extend from each chromosome Kinetochore of each sister chromatid connected to one pole Microtubules grow until they make contact with poles Sister chromatids won't separate if both connected to same pole Division of the Centromeres: Metaphase Begins when pairs of sister chromatids align in center of the cell Chromosomes align along the metaphase plate fig 11.15 Not a physical structure Indicates where future axis of cell division occurs Centromeres are equidistant from each pole Centromeres divide at the end of metaphase Centromere splits in two All centromeres divide in synchrony Separation of the Chromatids: Anaphase Shortest phase, during which sister chromatids separate Chromatid drawn to pole to which it is attached Separation achieved by two simultaneous microtubular actions Poles move apart fig 11.16 Microtubular spindle fibers slide past one another Microtubules are anchored at poles which are pushed apart Chromatids attached to poles move apart as well Centromeres move toward poles Shortening process is not a contraction Microtubules shorten as tubulin subunits are removed Chromatids are therefore pulled toward poles Reformation of Nuclei: Telophase Separation of chromatids completes partitioning of replicated genome Spindle apparatus is disassembled Tubulin units of microtubules are used to build new cytoskeleton Nuclear envelope re-forms around each new set of chromosomes Chromosomes begin to uncoil to allow gene expression rRNA genes begin transcription, nucleolus reappears CYTOKINESIS Mitosis Complete at End of Telophase Replicated genome divided into two new nuclei at opposite ends of cell Cytoplasmic organelles assort to regions that will become separated Cleavage of the cell into two halves constitutes cytokinesis Cytokinesis in Plants and Animals Progresses Differently Animal cytokinesis Cell is pinched in two by a constricting belt of microfilaments Actin filaments slide past one another Produces distinct cleavage furrow around circumference of cell fig 11.17 Furrow deepens until the cell is literally pinched in two fig 11.18 Plant cytokinesis Rigid cell wall, cannot be deformed by microfilament contraction Membrane components assembled in the cell interior fig 11.19 Occurs at right angles to the spindle apparatus Expanding partition called the cell plate Grows outward to the interior surface of the cell membrane Cellulose then added on the membrane making two new cells Middle lamellae: space between cells impregnated with pectins In fungi and some protists mitosis is confined to the nucleus CONTROL OF THE CELL CYCLE Events of Cell Cycle Coordinated Similarly in All Eukaryotes fig 11.20 Little change in processes over billions of years Human proteins can function when transferred to yeast cell General Strategy of Cell Cycle Control Goal of control is to optimize duration of cycle Internal clock control cannot provide sufficient flexibility Eukaryotes use a centralized controller based on cellular feedback Analogy: furnace heating a house At points in cycle feedback determines if cycle continues or is delayed Three principle check points fig 11.21 Cell growth assessed at G1 check point Called START in yeasts If conditions favorable cell starts copying DNA, starting S phase DNA replication assessed at G2 check point Mitosis assessed at M check point Molecular Mechanisms of Cell Cycle Control Associated with interactions of proteins sensitive to cell conditions fig 11.22 Cyclin-dependent protein kinases (Cdk's) These enzymes phosphorylate serine and threonine of certain proteins Histones, nuclear membrane filaments, microtubule proteins at G2 Cyclins Bind to Cdk's, enable them to act as enzymes Are destroyed and resynthesized at each turn of cell cycle The G2 check point fig 11.23 Cell accumulates G2 cyclin (mitotic cyclin) during G2 Binds to Cdk forming mitosis promoting factor, MPF MPF are phosphorylated and activated by cellular enzymes Positive feedback increases this activity, more MPF activated G2 ends when sufficient activated MPF Duration of M phase determined by MPF activity fig 11.23 MPF also activates proteins that destroy cyclin Degradation of G2 cyclin decreases activity of MPF, ending mitosis The G1 check point Similar to G2 control Yeasts compare volume of cytoplasm to size of genome In growth size increases, amount od DNA constant Threshold ration reached promoting cyclin production Controlling the Cell Cycle in Multicellular Eukaryotes Cells of multicellular organisms can't make individual decisions Organization dependent limiting cell proliferation In cell culture cells stop dividing when sufficient numbers Growing cells take up growth factors like MPF fig 11.24 Example of positive regulatory signal If other cells take up factor, none left to trigger division in any cell Growth Factors and the Cell Cycle Growth factors trigger intracellular signalling systems Example: fibroblasts Possess membrane receptors for platelet-derived growth factor, PDGF Binding PDGF and receptor initiates amplifying chain of events Tissue injury causes release of PDGF to promote healing Isolation of fifty growth factor proteins tbl 11.2 Each factor specifically recognized by specific cell surface receptor fig 11.25 Some affect broad range of cell types, PDGF and E (epidermal) GF Some affect only certain cell types, N (nerve) GF and erythropoietin Cells deprived of growth factors stop at G1, stay in G0 Cancer and the Control of Cell Proliferation Proto-oncogenes normally stimulate cell division, positive approach Mutations causing them to overact change them into oncogenes Mutations are dominant Leads to excessive cell proliferation characteristic of cancer fig 11.26 30 different proto-oncogenes exist myc, fos, jun cause unrestrained cell growth and division myc in normal cell helps to regulate G1 check point fig 11.27 Genes also stimulate delayed response genes that produce cyclins, Cdk Tumor-suppressor genes normally inhibit cell division, negative approach Prevents binding of cyclins to Cdk, block passage through G1 Mutations are recessive, unrestrained division if both copies mutated Retinoblastoma (Rb) gene is tumor-suppressor gene, causes rare eye cancer Normal gene is a cancer suppressor Rb gene encodes protein present in nucleus Rb protein is dephosphorylated in G0 phase fig 11.24 Binds regulatory proteins needed for cell proliferation Inhibit cell division When Rb is phosphorylated it releases regulatory proteins Cell division thus promoted Cells produce cyclins and Cdk, pass G1 check point COMPARING CELL DIVISION IN EUKARYOTES AND PROKARYOTES Cell Division in Eukaryotes Is More Sophisticated than Division in Bacteria Chromosome movement is rapid and accurately partitions genome Bacterial replication depends on slow, uninterrupted membrane growth Different Processes Related to Size of Genome