Sinh học - Chapter 012: The cell cycle

The Cell CycleChapter 12Overview: The Key Roles of Cell DivisionThe ability of organisms to produce more of their own kind best distinguishes living things from nonliving matterThe continuity of life is based on the reproduction of cells, or cell division© 2011 Pearson Education, Inc.In unicellular organisms, division of one cell reproduces the entire organismMulticellular organisms depend on cell division forDevelopment from a fertilized cellGrowthRepairCell division is an integral part of the cell cycle, the life of a cell from formation to its own division© 2011 Pearson Education, Inc.Figure 12.2(a) Reproduction(b) Growth and development(c) Tissue renewal20 m100 m200 mConcept 12.1: Most cell division results in genetically identical daughter cellsMost cell division results in daughter cells with identical genetic information, DNAThe exception is meiosis, a special type of division that can produce sperm and egg cells© 2011 Pearson Education, Inc.Cellular Organization of the Genetic MaterialAll the DNA in a cell constitutes the cell’s genomeA genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells)DNA molecules in a cell are packaged into chromosomes© 2011 Pearson Education, Inc.Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell divisionEvery eukaryotic species has a characteristic number of chromosomes in each cell nucleusSomatic cells (nonreproductive cells) have two sets of chromosomesGametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells© 2011 Pearson Education, Inc.Distribution of Chromosomes During Eukaryotic Cell DivisionIn preparation for cell division, DNA is replicated and the chromosomes condenseEach duplicated chromosome has two sister chromatids (joined copies of the original chromosome), which separate during cell divisionThe centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached© 2011 Pearson Education, Inc.Figure 12.40.5 mCentromereSisterchromatidsDuring cell division, the two sister chromatids of each duplicated chromosome separate and move into two nucleiOnce separate, the chromatids are called chromosomes© 2011 Pearson Education, Inc.Figure 12.5-3ChromosomesChromosomalDNA moleculesCentromereChromosomearmChromosome duplication(including DNA replication)and condensationSisterchromatidsSeparation of sisterchromatids intotwo chromosomes123Eukaryotic cell division consists ofMitosis, the division of the genetic material in the nucleusCytokinesis, the division of the cytoplasmGametes are produced by a variation of cell division called meiosisMeiosis yields nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell© 2011 Pearson Education, Inc.Phases of the Cell CycleThe cell cycle consists ofMitotic (M) phase (mitosis and cytokinesis)Interphase (cell growth and copying of chromosomes in preparation for cell division)© 2011 Pearson Education, Inc.Interphase (about 90% of the cell cycle) can be divided into subphases G1 phase (“first gap”)S phase (“synthesis”)G2 phase (“second gap”)The cell grows during all three phases, but chromosomes are duplicated only during the S phase © 2011 Pearson Education, Inc.Figure 12.6INTERPHASEG1G2S(DNA synthesis)MITOTIC(M) PHASECytokinesisMitosisMitosis is conventionally divided into five phasesProphasePrometaphaseMetaphaseAnaphaseTelophaseCytokinesis overlaps the latter stages of mitosis© 2011 Pearson Education, Inc.Figure 12.7aG2 of InterphaseProphasePrometaphaseCentrosomes(with centriole pairs)Chromatin(duplicated)NucleolusNuclearenvelopePlasmamembraneEarly mitoticspindleAsterCentromereChromosome, consistingof two sister chromatidsFragments of nuclearenvelopeNonkinetochoremicrotubulesKinetochoreKinetochoremicrotubuleFigure 12.7bMetaphaseMetaphase plateAnaphaseTelophase and CytokinesisSpindleCentrosome atone spindle poleDaughterchromosomesCleavagefurrowNucleolusformingNuclearenvelopeformingThe Mitotic Spindle: A Closer LookThe mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosisIn animal cells, assembly of spindle microtubules begins in the centrosome, the microtubule organizing centerThe centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase© 2011 Pearson Education, Inc.An aster (a radial array of short microtubules) extends from each centrosomeThe spindle includes the centrosomes, the spindle microtubules, and the asters© 2011 Pearson Education, Inc.During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes Kinetochores are protein complexes associated with centromeresAt metaphase, the chromosomes are all lined up at the metaphase plate, an imaginary structure at the midway point between the spindle’s two poles© 2011 Pearson Education, Inc.Figure 12.8SisterchromatidsAsterCentrosomeMetaphaseplate(imaginary)Kineto-choresOverlappingnonkinetochoremicrotubulesKinetochoremicrotubulesMicrotubulesChromosomesCentrosome0.5 m1 mIn anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cellThe microtubules shorten by depolymerizing at their kinetochore ends© 2011 Pearson Education, Inc.Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cellIn telophase, genetically identical daughter nuclei form at opposite ends of the cellCytokinesis begins during anaphase or telophase and the spindle eventually disassembles© 2011 Pearson Education, Inc.Cytokinesis: A Closer LookIn animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrowIn plant cells, a cell plate forms during cytokinesis© 2011 Pearson Education, Inc.Figure 12.10a(a) Cleavage of an animal cell (SEM)Cleavage furrowContractile ring ofmicrofilamentsDaughter cells100 mFigure 12.10b(b) Cell plate formation in a plant cell (TEM)Vesiclesformingcell plateWall of parent cellCell plateNew cell wallDaughter cells1 mFigure 12.11ChromatincondensingNucleusNucleolusChromosomesCell plate10 mProphasePrometaphaseMetaphaseAnaphaseTelophase12345Binary Fission in BacteriaProkaryotes (bacteria and archaea) reproduce by a type of cell division called binary fissionIn binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apartThe plasma membrane pinches inward, dividing the cell into two© 2011 Pearson Education, Inc.1Origin ofreplicationE. coli cellTwo copies of originCell wallPlasma membraneBacterial chromosomeOriginOriginChromosomereplicationbegins.Replicationcontinues.Replicationfinishes.Two daughtercells result.234Figure 12.12-4Concept 12.3: The eukaryotic cell cycle is regulated by a molecular control systemThe frequency of cell division varies with the type of cellThese differences result from regulation at the molecular levelCancer cells manage to escape the usual controls on the cell cycle © 2011 Pearson Education, Inc.Evidence for Cytoplasmic SignalsThe cell cycle appears to be driven by specific chemical signals present in the cytoplasmSome 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© 2011 Pearson Education, Inc.Figure 12.14Experiment 1Experiment 2SSSG1G1MMMEXPERIMENTRESULTSWhen a cell in the Sphase was fusedwith a cell in G1,the G1 nucleusimmediately enteredthe S phase—DNAwas synthesized.When a cell in the M phase was fused witha cell in G1, the G1nucleus immediatelybegan mitosis—a spindleformed and chromatincondensed, even thoughthe chromosome had notbeen duplicated.The Cell Cycle Control SystemThe sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clockThe cell cycle control system is regulated by both internal and external controlsThe clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received© 2011 Pearson Education, Inc.G1 checkpointG1G2G2 checkpointM checkpointMSControlsystemFigure 12.15For many cells, the G1 checkpoint seems to be the most important If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divideIf the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase© 2011 Pearson Education, Inc.Figure 12.16G1 checkpointG1G1G0(a) Cell receives a go-ahead signal.(b) Cell does not receive a go-ahead signal.The Cell Cycle Clock: Cyclins and Cyclin-Dependent KinasesTwo types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)Cdks activity fluctuates during the cell cycle because it is controled by cyclins, so named because their concentrations vary with the cell cycleMPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase© 2011 Pearson Education, Inc.Figure 12.17Cyclin accumulation(a) Fluctuation of MPF activity and cyclin concentration  during the cell cycle(b) Molecular mechanisms that help regulate the cell cycleMPF activityCyclinconcentrationTimeMMMSSG1G2G1G2G1CdkDegradedcyclinCyclin isdegradedMPFG2checkpointCdkCyclinMSG1G2Stop and Go Signs: Internal and External Signals at the CheckpointsAn example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphaseSome external signals are growth factors, proteins released by certain cells that stimulate other cells to divideFor example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture© 2011 Pearson Education, Inc.Figure 12.18A sample of humanconnective tissue iscut up into smallpieces.Enzymes digestthe extracellularmatrix, resulting ina suspension offree fibroblasts.Cells are transferred toculture vessels.ScalpelsPetridishPDGF is addedto half thevessels.Without PDGFWith PDGF10 m1234A clear example of external signals is density-dependent inhibition, in which crowded cells stop dividingMost animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divideCancer cells exhibit neither density-dependent inhibition nor anchorage dependence© 2011 Pearson Education, Inc.Figure 12.19Anchorage dependenceDensity-dependent inhibitionDensity-dependent inhibition(a) Normal mammalian cells(b) Cancer cells20 m20 mLoss of Cell Cycle Controls in Cancer CellsCancer cells do not respond normally to the body’s control mechanismsCancer cells may not need growth factors to grow and divideThey may make their own growth factorThey may convey a growth factor’s signal without the presence of the growth factorThey may have an abnormal cell cycle control system© 2011 Pearson Education, Inc.A normal cell is converted to a cancerous cell by a process called transformationCancer cells that are not eliminated by the immune system form tumors, masses of abnormal cells within otherwise normal tissueIf abnormal cells remain only at the original site, the lump is called a benign tumorMalignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form additional tumors© 2011 Pearson Education, Inc.MitosisCytokinesisMITOTIC (M) PHASEG1G2STelophase andCytokinesisAnaphaseMetaphasePrometaphaseProphaseITRHASEEPNFigure 12.UN01Figure 12.UN02Figure 12.UN03Figure 12.UN04Figure 12.UN05Figure 12.UN06