Chapter 9Chapter 9

The Cell The Cell CycleCycle

Cell Division: Key TermsCell Division: Key Terms Genome: cell’s genetic Genome: cell’s genetic

informationinformation Somatic (body cells) cellsSomatic (body cells) cells Gametes (germ cells): Gametes (germ cells):

sperm and egg cellssperm and egg cells Chromosomes: DNA Chromosomes: DNA

moleculesmolecules Diploid (2n): 2 sets of Diploid (2n): 2 sets of

chromosomes chromosomes Haploid (1n): 1 set of Haploid (1n): 1 set of

chromosomeschromosomes

Cell Division: Key TermsCell Division: Key Terms

Chromatin: DNA-protein Chromatin: DNA-protein complexcomplex

Chromatids: replicated Chromatids: replicated strands of a chromosomestrands of a chromosome

Centromere: narrowing Centromere: narrowing “waist” of sister “waist” of sister chromatidschromatids

Mitosis: nuclear divisionMitosis: nuclear division Cytokinesis: cytoplasm Cytokinesis: cytoplasm

divisiondivision Meiosis: gamete cell Meiosis: gamete cell

divisiondivision

Purpose of cell divisionPurpose of cell division

Multicellular organisms depend on Multicellular organisms depend on cell division forcell division for• Development from a fertilized cellDevelopment from a fertilized cell• GrowthGrowth• RepairRepair

20 µm200 µm

(b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).

(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

The Cell CycleThe Cell Cycle

Interphase (90% Interphase (90% of cycle)of cycle)

G1 phase~ G1 phase~ growthgrowth

S phase~ S phase~ synthesis of DNAsynthesis of DNA

G2 phase~ G2 phase~ preparation for preparation for cell divisioncell division

The Cell CycleThe Cell Cycle

Mitotic phaseMitotic phase Mitosis~ Mitosis~

nuclear nuclear divisiondivision

Cytokinesis~ Cytokinesis~ cytoplasm cytoplasm

divisiondivision

MitosisMitosis

ProphaseProphase PrometaphPrometaph

asease MetaphaseMetaphase AnaphaseAnaphase TelophaseTelophase

Mitosis AnimationMitosis Animation

Spindle microtubulesSpindle microtubules

CentrosomeAster

Sisterchromatids

MetaphasePlate

Kinetochores

Overlappingnonkinetochoremicrotubules

Kinetochores microtubules

Centrosome

ChromosomesMicrotubules

1 µm

ProphaseProphase

Chromosomes Chromosomes visiblevisible

Nucleoli disappearNucleoli disappear Sister chromatidsSister chromatids Mitotic spindle Mitotic spindle

formsforms Centrosomes Centrosomes

movemove

PrometaphasePrometaphase

Nuclear Nuclear membrane membrane fragmentsfragments

Spindle Spindle interaction with interaction with chromosomeschromosomes

Kinetochore Kinetochore developsdevelops

QuickTime™ and aCinepak decompressor

are needed to see this picture.

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)

QuickTime™ and aCinepak decompressor

are needed to see this picture.

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

QuickTime™ and aCinepak decompressor

are needed to see this picture.

TelophaseTelophase

Daughter nuclei Daughter nuclei formform

Nuclear envelopes Nuclear envelopes reappearreappear

Chromatin Chromatin becomes less becomes less coiledcoiled

Two new nuclei Two new nuclei complete mitosiscomplete mitosis

QuickTime™ and aCinepak decompressor

are needed to see this picture.

CytokinesisCytokinesis

Cytoplasmic Cytoplasmic divisiondivision

Animals~ Animals~ cleavage cleavage furrowfurrow

Cleavage furrow

Contractile ring of microfilaments

Daughter cells

100 µm

(a) Cleavage of an animal cell (SEM)

CytokinesisCytokinesis

In plant cells, In plant cells, during cytokinesisduring cytokinesis• A cell plate formsA cell plate forms

Daughter cells

Vesiclesforming cell plate

Wall of parent cell Cell plateNew cell wall

(b) Cell plate formation in a plant cell (SEM)

Binary FissionBinary Fission

Prokaryotes (bacteria)Prokaryotes (bacteria)• Reproduce by a type of cell division Reproduce by a type of cell division

called binary fissioncalled binary fission

• The bacterial chromosome replicatesThe bacterial chromosome replicates

• The two daughter chromosomes The two daughter chromosomes actively move apartactively move apart

Binary FissionBinary Fission

Origin ofreplication

E. coli cell BacterialChromosome

Cell wall

Plasma Membrane

Two copiesof origin

OriginOrigin

Chromosome replication begins.Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.

1

Replication continues. One copy ofthe origin is now at each end of the cell.

2

Replication finishes. The plasma membrane grows inward, andnew cell wall is deposited.

3

Two daughter cells result.4

The Evolution of MitosisThe Evolution of Mitosis

Since prokaryotes preceded Since prokaryotes preceded eukaryotes by billions of yearseukaryotes by billions of years• It is likely that mitosis evolved from It is likely that mitosis evolved from

bacterial cell divisionbacterial cell division Certain protistsCertain protists

• Exhibit types of cell division that Exhibit types of cell division that seem intermediate between binary seem intermediate between binary fission and mitosis carried out by fission and mitosis carried out by most eukaryotic cellsmost eukaryotic cells

Cell Cycle RegulationCell Cycle Regulation

Growth factorsGrowth factors Density-Density-

dependent dependent inhibitioninhibition

Anchorage Anchorage dependencedependence

The Cell Cycle Control The Cell Cycle Control SystemSystem

The sequential events of the The sequential events of the cell cyclecell cycle

Control system

G2 checkpoint

M checkpoint

G1 checkpoint

G1

S

G2M

The clock has specific checkpointsThe clock has specific checkpointsWhere the cell cycle stops until a go-Where the cell cycle stops until a go-ahead signal is receivedahead signal is received

G1 checkpoint

G1G1

G0

(a) If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues      on in the cell cycle.

(b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.

The Cell Cycle Clock: Cyclins and The Cell Cycle Clock: Cyclins and

Cyclin-Dependent KinasesCyclin-Dependent Kinases

Two types of regulatory proteins are Two types of regulatory proteins are involved in cell cycle controlinvolved in cell cycle control

Cyclins and cyclin-dependent kinases Cyclins and cyclin-dependent kinases (Cdks)(Cdks)

Loss of Cell Cycle Controls Loss of Cell Cycle Controls in Cancer Cellsin Cancer Cells

Cancer cellsCancer cells• Do not respond normally to the Do not respond normally to the

body’s control mechanismsbody’s control mechanisms• Form tumorsForm tumors

CancerCancer

TransformationTransformation Tumor: benign or malignantTumor: benign or malignant MetastasisMetastasis