Chpt. 12 ~ The Cell CycleChpt. 12 ~ The Cell Cycle

Cell division results in genetically identical daughter cells.Cell division results in genetically identical daughter cells. The mitotic phase alternates with interphase in the cell cycle.The mitotic phase alternates with interphase in the cell cycle. The cell cycle is regulated by a molecular control system.The cell cycle is regulated by a molecular control system.

Key Roles of Cell DivisionKey Roles of Cell Division

When a unicellular organism divides and forms When a unicellular organism divides and forms duplicate offspring, the division of one cell duplicate offspring, the division of one cell reproduces an entire organismreproduces an entire organism

Cell division on a larger scale can produce Cell division on a larger scale can produce progeny from some multicellular organisms progeny from some multicellular organisms such as plants that grow from cuttingssuch as plants that grow from cuttings

Cell division also enables sexually reproducing Cell division also enables sexually reproducing organisms to develop from a single cell~ the organisms to develop from a single cell~ the fertilized egg or zygotefertilized egg or zygote

Cell division continues after an organism is Cell division continues after an organism is fully grown to repair and replace cells that die fully grown to repair and replace cells that die from normal wear and tear or accidentsfrom normal wear and tear or accidents

Cellular Organization of Genetic Cellular Organization of Genetic MaterialMaterial

Genome: cell’s genetic informationGenome: cell’s genetic information• Prokaryotic single circular or Prokaryotic single circular or

elongated DNA elongated DNA • Eukaryotic genomes made of DNA Eukaryotic genomes made of DNA

moleculesmolecules DNA molecules are packaged into DNA molecules are packaged into

chromosomes making the replication chromosomes making the replication and distribution more manageableand distribution more manageable• Eukaryotic species chromosome # Eukaryotic species chromosome #

unique by speciesunique by species• Somatic cells - body cellsSomatic cells - body cells• Gametes – sperm & egg cellsGametes – sperm & egg cells

Eukaryotic chromosomes are made Eukaryotic chromosomes are made of chromatin a DNA-protein complexof chromatin a DNA-protein complex

Chromosome duplication & Chromosome duplication & distribution during cell distribution during cell

divisiondivision

When a cell is not dividing each When a cell is not dividing each chromosome is in the form of a long, chromosome is in the form of a long, thin chromatin fiberthin chromatin fiber

After DNA duplication the After DNA duplication the chromosomes condense chromosomes condense

Each duplicated chromosome has:Each duplicated chromosome has:• Two sister chromatidsTwo sister chromatids

– Each contain identical DNAEach contain identical DNA– Attached at centromere a narrowing Attached at centromere a narrowing

“waist” of sister chromatids“waist” of sister chromatids– Each sister chromatids have a Each sister chromatids have a

kinetochore, a structure of proteins kinetochore, a structure of proteins associated with specific sections of associated with specific sections of chromosomal DNA at the centromere chromosomal DNA at the centromere

– Later in cell division the sister chromatids Later in cell division the sister chromatids separate and move into two new nucleiseparate and move into two new nuclei

– Once sister chromatids separate they are Once sister chromatids separate they are called chromosomescalled chromosomes

Cell DivisionCell Division

MitosisMitosis• Division of the nucleusDivision of the nucleus• Each new nucleus receives a group of Each new nucleus receives a group of

chromosomes identical to the original groupchromosomes identical to the original group MeiosisMeiosis

• Division of gametes (egg & sperm cells)Division of gametes (egg & sperm cells)• Yields non-identical daughter cells that have Yields non-identical daughter cells that have

only one set of chromosomesonly one set of chromosomes• Daughter cells have half as many Daughter cells have half as many

chromosomes as the parent cellchromosomes as the parent cell

The Cell CycleThe Cell Cycle

Interphase Interphase (90% of cycle)(90% of cycle) • GG11 phase - growth phase - growth

• S phase - synthesis of DNA S phase - synthesis of DNA

• GG22 phase - grows and phase - grows and prepares for cell divisionprepares for cell division

Mitotic phaseMitotic phase • • Mitosis - nuclear division • Mitosis - nuclear division • Cytokinesis - cytoplasm Cytokinesis - cytoplasm

divisiondivision

A typical human cell might undergo one

division in 24 hours!Cells Alive Animation!http://www.cellsalive.com/cell_cycle.htm

MitosisMitosis

ProphaseProphase PrometaphasePrometaphase MetaphaseMetaphase AnaphaseAnaphase TelophaseTelophase

InteractiveInteractivehttp://www.sci.sdsu.eduhttp://www.sci.sdsu.edu/multimedia/mitosis//multimedia/mitosis/

Mitotic SpindleMitotic Spindle

Constructed of fibers Constructed of fibers made of microtubules and made of microtubules and associated proteinsassociated proteins Kinetichore - Kinetichore -

chromosome attachedchromosome attached Nonkinetichore or Polar – Nonkinetichore or Polar –

no chromosome attachedno chromosome attached Begins to form in the Begins to form in the

cytoplasm during cytoplasm during prophase at the prophase at the centrosomecentrosome

The spindle includes the The spindle includes the centrosomes, the spindle centrosomes, the spindle microtubules, and the microtubules, and the asters.asters.

http://www.meta-library.net/media/mito2-sm.jpg

CentrosomeCentrosome

In animal cells, a pair of centrioles is located at the In animal cells, a pair of centrioles is located at the center of the centrosome but are not essential for center of the centrosome but are not essential for cell divisioncell division

Most plant cells centrosomes lack centriolesMost plant cells centrosomes lack centrioles Centrosome replicates during interphase and Centrosome replicates during interphase and remain together near the nucleusremain together near the nucleus The two centrosomes move apart from each other The two centrosomes move apart from each other

during prophase and prometaphase of mitosis as during prophase and prometaphase of mitosis as spindle microtubules grow out from themspindle microtubules grow out from them

By the end of prometaphase, the two centrosomes, at are opposite poles of the cell with an aster radiating from each centrosome

http://www.biology.ucsc.edu/mcd/images/centrosome.jpg

http://www.fotosearch.com/comp/LIF/LIF113/SA202005.jpg

http://www.bio.unc.edu/faculty/peifer/documents/people's%20pages/lab%20website/RusanColor2small.jpg

ProphaseProphase

Chromatin fibers condense Chromatin fibers condense into visible Chromosomesinto visible Chromosomes

Nucleoli disappearNucleoli disappear Each duplicated Each duplicated

chromosome appears as chromosome appears as two identical Sister two identical Sister chromatids joined togetherchromatids joined together

Mitotic spindle begins to Mitotic spindle begins to formform

Centrosomes move away Centrosomes move away from each otherfrom each other

PrometaphasePrometaphase

Nuclear membrane Nuclear membrane fragmentsfragments

Spindle interaction with Spindle interaction with chromosomeschromosomes

Kinetochore develops at Kinetochore develops at centromere of each centromere of each chromatidchromatid

Microtubules making up Microtubules making up spindle spindle Kinetochore microtubuleKinetochore microtubule

connect to kinetochore connect to kinetochore

Nonkinetochore microtubulesNonkinetochore microtubules communicate only with polescommunicate only with poles

MetaphaseMetaphase

Centrosomes at Centrosomes at opposite polesopposite poles

Centromeres are Centromeres are alignedaligned

Kinetochores of Kinetochores of sister chromatids sister chromatids attached to attached to microtubules microtubules (spindle)(spindle)

AnaphaseAnaphase

Paired centromeres Paired centromeres separate; sister separate; sister chromatids chromatids liberatedliberated

Chromosomes move Chromosomes move to opposite polesto opposite poles

Each pole now has a Each pole now has a complete set of complete set of chromosomeschromosomes

TelophaseTelophase

Daughter nuclei Daughter nuclei formform

Nuclear Nuclear envelopes ariseenvelopes arise

Chromatin Chromatin becomes less becomes less coiledcoiled

Two new nuclei Two new nuclei complete mitosiscomplete mitosis

CytokinesisCytokinesis

Cytoplasmic Cytoplasmic divisiondivision

Animals: Animals: cleavage cleavage

furrowfurrow Plants: Plants:

cell platecell plate

Mitosis animationMitosis animation http://www.johnkyhttp://www.johnkyrk.com/mitosis.htrk.com/mitosis.htmlml

Binary FissionBinary Fission

Cell division of prokaryotesCell division of prokaryotes DNA replication begins at a specific sites DNA replication begins at a specific sites

on chromosome called origin of on chromosome called origin of replicationreplication

As DNA replicates one origin moves As DNA replicates one origin moves rapidly toward the opposite end of the rapidly toward the opposite end of the cell while the cell elongatescell while the cell elongates

When replication is complete its plasma When replication is complete its plasma membrane grows inward producing two membrane grows inward producing two daugther cellsdaugther cells

Cell Cycle Control SystemCell Cycle Control System

Molecules present in the cytoplasm regulate Molecules present in the cytoplasm regulate progress through the cell cycleprogress through the cell cycle

Cyclic changes in regulatory proteins works as Cyclic changes in regulatory proteins works as a cell cycle clock through signal transduction a cell cycle clock through signal transduction pathwayspathways• Cyclin-Dependent Kinases, CdksCyclin-Dependent Kinases, Cdks• CyclinsCyclins

External factorsExternal factors• Density-dependent inhibition of cell divisionDensity-dependent inhibition of cell division

• Crowded cells stop dividingCrowded cells stop dividing

• Anchorage DependenceAnchorage Dependence• Cell must be attached to a substrate in order to divideCell must be attached to a substrate in order to divide

CheckpointsCheckpoints

GG11

• Most important restriction point Most important restriction point • If cell does not receive a go-ahead signal If cell does not receive a go-ahead signal

at this checkpoint, the cell exits the cell at this checkpoint, the cell exits the cell cycle and goes into Gcycle and goes into G00

GG22

MM GG00

• Non-dividing stateNon-dividing state

Cancer ~ Loss of Cell Cycle Cancer ~ Loss of Cell Cycle ControlsControls

TransformationTransformation• Process that converts a normal cell into a Process that converts a normal cell into a

cancer cellcancer cell• If left unchecked by the immune system a If left unchecked by the immune system a

tumor may developtumor may develop• Benign Benign • MalignantMalignant

MetastasisMetastasis• Spread of cancer cells to new locationsSpread of cancer cells to new locations