The Cell Cycle

Key Concepts

• Most division results in genetically identical cells

• Cell cycle consists of alternating periods of mitosis and interphase

• Eukaryotic cell cycle is highly regulated

Purpose

• Ultimate purpose of cell cycle is to propagate genetic information

• Also important for– Tissue repair – Growth – Reproduction

Figure 12.5-1

ChromosomesChromosomal

DNA molecules

Centromere

Chromosomearm

1

Figure 12.5-2

ChromosomesChromosomal

DNA molecules

Centromere

Chromosomearm

Chromosome duplication(including DNA replication)and condensation

Sisterchromatids

1

2

Figure 12.5-3

ChromosomesChromosomal

DNA molecules

Centromere

Chromosomearm

Chromosome duplication(including DNA replication)and condensation

Sisterchromatids

Separation of sisterchromatids intotwo chromosomes

1

2

3

Phases of cell cycle

• Interphase– G1– G2– S

• Mitosis– Prophase– Metaphase– Anaphase– Telophase

Figure 12.6

INTERPHASE

G1

G2

S(DNA synthesis)

MITOTIC(M) PHASE

CytokinesisM

itosis

Figure 12.7

G2 of Interphase Prophase Prometaphase

Centrosomes(with centriole pairs)

Chromatin(duplicated)

Nucleolus Nuclearenvelope

Plasmamembrane

Early mitoticspindle

AsterCentromere

Chromosome, consistingof two sister chromatids

Fragments of nuclearenvelope

Nonkinetochoremicrotubules

Kinetochore Kinetochoremicrotubule

Metaphase

Metaphase plate

Anaphase Telophase and Cytokinesis

Spindle Centrosome atone spindle pole

Daughterchromosomes

Cleavagefurrow

Nucleolusforming

Nuclearenvelopeforming

10

m

Figure 12.7a

G2 of Interphase Prophase Prometaphase

Centrosomes(with centriole pairs)

Chromatin(duplicated)

NucleolusNuclearenvelope

Plasmamembrane

Early mitoticspindle

AsterCentromere

Chromosome, consistingof two sister chromatids

Fragments of nuclearenvelope

Nonkinetochoremicrotubules

Kinetochore Kinetochoremicrotubule

Figure 12.7b

Metaphase

Metaphase plate

Anaphase Telophase and Cytokinesis

Spindle Centrosome atone spindle pole

Daughterchromosomes

Cleavagefurrow

Nucleolusforming

Nuclearenvelopeforming

Mitotic Spindle

Sisterchromatids

AsterCentrosome

Metaphaseplate(imaginary)

Kineto-chores

Overlappingnonkinetochoremicrotubules Kinetochore

microtubules

Microtubules

Chromosomes

Centrosome

0.5 m

1 m

Figure 12.9a

Kinetochore

Mark

Spindlepole

EXPERIMENT

RESULTS

Figure 12.9b

Chromosomemovement

Microtubule

Motor protein

Chromosome

Kinetochore

Tubulinsubunits

CONCLUSION

Cytokenisis

Figure 12.10a(a) Cleavage of an animal cell (SEM)

Cleavage furrow

Contractile ring ofmicrofilaments

Daughter cells

100 m

Figure 12.10b (b) Cell plate formation in a plant cell (TEM)

Vesiclesformingcell plate

Wall of parent cell

Cell plate New cell wall

Daughter cells

1 m

Binary Fission

• Prokaryotes replicate via binary fission• Genome replication starts at location called

origin of replication• Cell membrane pinches around midline of cell

Figure 12.12-1

1

Origin ofreplication

E. coli cell

Two copies of origin

Cell wallPlasma membrane

Bacterial chromosomeChromosomereplicationbegins.

1

Origin ofreplication

E. coli cell

Two copies of origin

Cell wallPlasma membrane

Bacterial chromosome

Origin Origin

Chromosomereplicationbegins.

Replicationcontinues.

2

Figure 12.12-2

1

Origin ofreplication

E. coli cell

Two copies of origin

Cell wallPlasma membrane

Bacterial chromosome

Origin Origin

Chromosomereplicationbegins.

Replicationcontinues.

Replicationfinishes.

2

3

Figure 12.12-3

1

Origin ofreplication

E. coli cell

Two copies of origin

Cell wallPlasma membrane

Bacterial chromosome

Origin Origin

Chromosomereplicationbegins.

Replicationcontinues.

Replicationfinishes.

Two daughtercells result.

2

3

4

Figure 12.12-4

Evolution of mitosis

• Eukaryotes most likely evolved from prokaryotic organisms

• Some organisms display intermediate “levels” of mitotic behavior.

Cell Cycle RegulationG1 checkpoint

G1

G2

G2 checkpoint

M checkpoint

M

SControlsystem

Figure 12.15

Cyclins and CDKs

• Two types of regulatory proteins in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)

• Cdks activity fluctuates during cell cycle because it is controled by cyclins, so named because their concentrations vary with the cell cycle

• MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase

Figure 12.17a

(a) Fluctuation of MPF activity and cyclin concentration during the cell cycle

MPF activity

Cyclinconcentration

Time

M M MS SG1 G2 G1 G2 G1

(b) Molecular mechanisms that help regulate the cell cycle

Cdk

Degradedcyclin

Cyclin isdegraded

MPF

G2checkpoint

Cdk

Cyclin

M

S

G 1

G 2

Figure 12.17b

Stop and Go Signals

• An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase

• Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide

• For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture

Figure 12.18

A sample of humanconnective tissue iscut up into smallpieces.

Enzymes digestthe extracellularmatrix, resulting ina suspension offree fibroblasts.

Cells are transferred toculture vessels.

Scalpels

Petridish

PDGF is addedto half thevessels.

Without PDGF With PDGF

10 m

1

2

34

External Signals

• A clear example of external signals is density-dependent inhibition, in which crowded cells stop dividing

• Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide

• Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence

Cancer

• Normal cells undergo a transformation • Typically result of genetic mutation• Loss of ability to govern cell cycle

– Too much growth – Not enough death

• Cancers typically pick up more and more mutations as they progress.