Chapter 12: The Cell CycleChapter 12: The Cell CycleCell cycleCell cycle: : life of a celllife of a cell from its from its formation from a dividing parent formation from a dividing parent cell until its own divisioncell until its own division into 2 cells into 2 cellsCell division-Cell division- results in results in genetically genetically identical daughter cellsidentical daughter cells

Mitosis: division of the nucleusMitosis: division of the nucleus Cytokinesis: division of the cytoplasmCytokinesis: division of the cytoplasmMeiosis: non-identical daughter cellsMeiosis: non-identical daughter cells produced with only one set of produced with only one set of chromosomeschromosomes

Concept CheckConcept Check Starting with a zygote, how many cell Starting with a zygote, how many cell

would result following five cell would result following five cell divisions?divisions?

A chicken has 78 chromosomes in its A chicken has 78 chromosomes in its somatic cells.somatic cells. How many chromosomes are in the How many chromosomes are in the

chicken’s egg?chicken’s egg? How many cells in the chicken zygote?How many cells in the chicken zygote? How many chromosome pairs are there How many chromosome pairs are there

in a chicken?in a chicken?

Phases of the cell cyclePhases of the cell cycle

InterphaseInterphase G1 phaseG1 phase (first gap)- (5-6 hours) (first gap)- (5-6 hours) S phase (DNA synthesis)-S phase (DNA synthesis)- (10-12 hours) (10-12 hours) G2 phaseG2 phase (second gap)- (4-6 hours) (second gap)- (4-6 hours)

M phaseM phase (<1 hour) (<1 hour) MitosisMitosis CytokinesisCytokinesis

MitosisMitosis

PhasesPhases:: ProphaseProphase PrometaphasePrometaphase MetaphaseMetaphase AnaphaseAnaphase Telophase & CytokinesisTelophase & Cytokinesis

Mitotic Mitotic SpindleSpindle: fibers of : fibers of microtubules & associated proteinsmicrotubules & associated proteins; ; assembly starts at the centrosomeassembly starts at the centrosome

GG22 of Interphase of Interphase 2 centrosomes2 centrosomes have formed by have formed by

replication replication In animal cells centrosomes have 2 In animal cells centrosomes have 2

centriolescentrioles

ProphaseProphase Chromatin condensesChromatin condenses into into

chromosomeschromosomes Nucleoli disappearNucleoli disappear Duplicated chromosomes appear as Duplicated chromosomes appear as

2 2 identical sister chromatidsidentical sister chromatids Mitotic Mitotic spindle begins to formspindle begins to form Centrosomes Centrosomes move away from each move away from each

other other toward opposite polestoward opposite poles as as spindle lengthensspindle lengthens

PrometaphasePrometaphase Nuclear envelope fragmentsNuclear envelope fragments Spindle invades nuclear areaSpindle invades nuclear area Kinetochores of Kinetochores of sister chromatids sister chromatids

attach to spindleattach to spindle at the centromere at the centromere

MetaphaseMetaphase

Longest Longest phase (~20 phase (~20 minutes)minutes)

ChromosomChromosomes arrange es arrange on on metaphase metaphase plateplate

AnaphaseAnaphase Shortest Shortest

phasephase Sister Sister

chromatids chromatids separate & separate & move move opposite opposite polespoles as as microtubulemicrotubules shortens shorten

TelophaseTelophase

Daughter cell nuclei begin to formDaughter cell nuclei begin to form Nuclear envelope arisesNuclear envelope arises Chromosomes de-condenseChromosomes de-condense

CytokinesisCytokinesis

Occurs Occurs simultaneously with simultaneously with telophasetelophase

Animals- cleavage furrow pinches the Animals- cleavage furrow pinches the cell in 2cell in 2

Plants- cell plate forms to separate Plants- cell plate forms to separate cells in 2 (cell plate will become cell cells in 2 (cell plate will become cell wall)wall)

Cell division in Prokaryotes: Cell division in Prokaryotes: Binary FissionBinary Fission

Circular DNA replicatesCircular DNA replicates beginning at beginning at the origin of replicationthe origin of replication

As DNA replicates As DNA replicates cell elongatescell elongates When replication is complete When replication is complete plasma plasma

membrane grows inward separating membrane grows inward separating the parent cell into 2 daughter cellsthe parent cell into 2 daughter cells

Evolution of the mitotic Evolution of the mitotic processprocess

Concept CheckConcept Check

In which stages of the cell cycle would a In which stages of the cell cycle would a chromosome consist of two identical chromosome consist of two identical chromatids?chromatids?

What are the similarities & differences in What are the similarities & differences in the cytokinesis of plant & animal cells?the cytokinesis of plant & animal cells?

Why would it be a disadvantage for a cell Why would it be a disadvantage for a cell to divide first then replicate its DNA?to divide first then replicate its DNA?

Cell Cycle ControlCell Cycle Control Each stage is triggered by specific Each stage is triggered by specific

molecular signals present in the molecular signals present in the cytoplasmcytoplasm

CheckpointsCheckpoints

3 major checkpoints at G3 major checkpoints at G11, G, G22, & M , & M phasesphases

GG11 checkpoint (restriction point)- checkpoint (restriction point)- appears to be most importantappears to be most important If a cell is given If a cell is given go-ahead it completes go-ahead it completes

S, GS, G22, & M phases, & M phases If If not given go-ahead, cell enters Gnot given go-ahead, cell enters G00

(non-dividing state)(non-dividing state)

Regulating proteins for cell Regulating proteins for cell cycle control: kinases & cyclinscycle control: kinases & cyclinsKinasesKinases enzymes that activate/inactivate enzymes that activate/inactivate

other proteins through other proteins through phosphorylationphosphorylation

Cause go-ahead signals at GCause go-ahead signals at G11 & G & G22 checkpointscheckpoints

CyclinsCyclins Attach to & activate kinases (cyclin Attach to & activate kinases (cyclin

dependant kinases (Cdks))dependant kinases (Cdks))

Regulating proteins for cell Regulating proteins for cell cycle control: kinases & cyclinscycle control: kinases & cyclins

Cdk Example: Maturation promoting Cdk Example: Maturation promoting factor (MPF)factor (MPF)

Rises during GRises during G22 to initiate mitosis to initiate mitosis Actions include- nuclear envelope Actions include- nuclear envelope

fragmentation, chromosome fragmentation, chromosome condensation, spindle formationcondensation, spindle formation

Destroys own cyclin during anaphase Destroys own cyclin during anaphase to initiate end of mitosisto initiate end of mitosis

Cell Cycle Control: internal Cell Cycle Control: internal signals at checkpointssignals at checkpoints

Growth factorsGrowth factors Proteins that cells release to cause other Proteins that cells release to cause other

cells to divide-cells to divide- (promote mitosis= mitogen) (promote mitosis= mitogen) Over 50Over 50 different growth factors that trigger different growth factors that trigger

cell division have been cell division have been identifiedidentified

Example: platelet-derived growth factor (PDGF)Example: platelet-derived growth factor (PDGF) Release by platelets to cause fibroblast Release by platelets to cause fibroblast

(connective tissue cell) division to repair (connective tissue cell) division to repair woundwound

Cell Cycle Control: external Cell Cycle Control: external signals at checkpointssignals at checkpoints

Density-dependant inhibitionDensity-dependant inhibition Crowded cells stop dividingCrowded cells stop dividing Determined by the amount of Determined by the amount of

nutrients & growth factors availablenutrients & growth factors available to cellsto cells

Anchorage dependanceAnchorage dependance Cells must be attached to a Cells must be attached to a

substratumsubstratum (culture dish or tissue (culture dish or tissue matrix) in order matrix) in order to divideto divide

Loss of cell cycle control: Loss of cell cycle control: CancerCancer Cancer cells do not respond to regulating Cancer cells do not respond to regulating

signals of cell cyclesignals of cell cycle Divide as long as nutrients availableDivide as long as nutrients available while while

normal cells divide only 20-50 times before normal cells divide only 20-50 times before deathdeath Ex. “immortal cells” – HeLa cellsEx. “immortal cells” – HeLa cells from Henrietta from Henrietta

Lacks tumor removed in 1951 still dividing in Lacks tumor removed in 1951 still dividing in cultureculture

Transformation: normal cells convert to Transformation: normal cells convert to cancer cellscancer cells

Tumor: transformed cell mass that evaded Tumor: transformed cell mass that evaded destruction by immune systemdestruction by immune system Benign tumor: remain at original site; easier to Benign tumor: remain at original site; easier to

removeremove Malignant tumor: invades nearby tissues & organsMalignant tumor: invades nearby tissues & organs

Metastasis: spread of cancer cells to Metastasis: spread of cancer cells to locations in the body distant from original locations in the body distant from original sitesite

Henrietta LacksHenrietta Lacks Source of Source of

HeLa cellsHeLa cells

Cancer TreatmentsCancer Treatments

Radiation Radiation cause cause more damage to cancer cells more damage to cancer cells

than normal cells than normal cells Cancer cells lack repair mechanismsCancer cells lack repair mechanisms

ChemotherapyChemotherapy Chemicals interrupt steps in cell Chemicals interrupt steps in cell

divisiondivision process of cancer cells process of cancer cells

Concept CheckConcept Check If a drug is administered that prevents DNA If a drug is administered that prevents DNA

synthesis of a cancer cell which phase of the synthesis of a cancer cell which phase of the cell cycle is the cell trapped in?cell cycle is the cell trapped in?

What is the purpose of Cdks?What is the purpose of Cdks? What features distinguish cancer cells & What features distinguish cancer cells &

normal cells?normal cells? Distinguish between a benign & malignant Distinguish between a benign & malignant

tumor.tumor. Why do you believe more money is invested Why do you believe more money is invested

in researching cancer treatments rather than in researching cancer treatments rather than researching cancer prevention?researching cancer prevention?