Purpose of cell division› Unicellular organisms

Reproduction › Multicellular organisms

Development from a fertilized cell Growth Repair

Reproduction

100 µm

Tissue renewalGrowth and development

20 µm200 µm

Genome› Complete set of genetic information

Chromosomes› DNA molecules packaged within cell

Chromatin› complex of DNA and protein that condenses during

cell division (in eukaryotes) Somatic cells

› nonreproductive cells› have two sets of chromosomes

Gametes › reproductive cells

sperm and eggs have half as many chromosomes as somatic cells

25 µm

Chromosomeduplication(including DNAsynthesis)

0.5 µm

Centromere

Sisterchromatids

Separationof sister

chromatids

Centromeres Sister chromatids

Eukaryotic cell division consists of:› Mitosis, the division of the nucleus› Cytokinesis, the division of the cytoplasm

Meiosis › yields nonidentical daughter cells › have only one set of chromosomes

half as many as the parent cell Gametes

The cell cycle consists of› Mitotic (M) phase

mitosis and cytokinesis› Interphase

cell growth and copying of chromosomes preparation for cell division

• Interphase • about 90% of the cell cycle• divided into subphases:

G1 phase (“first gap”) G0 (resting phase) S phase (“synthesis”) G2 phase (“second gap”)

G1

G2

S(DNA synthesis)

INTERPHASE

Cytokinesis

MITOTIC(M) PHASE

Mitosis

Mitosis is conventionally divided into five phases:› Prophase› Prometaphase› Metaphase› Anaphase› Telophase

Cytokinesis is well underway by late telophase

G2 OF INTERPHASE PROPHASE PROMETAPHASE

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

10 µ

m

Video: Animal MitosisVideo: Animal Mitosis

Video: Sea Urchin (time lapse)Video: Sea Urchin (time lapse)

Animation: Mitosis (All Phases)Animation: Mitosis (All Phases)

Animation: Mitosis OverviewAnimation: Mitosis Overview

Animation: Late InterphaseAnimation: Late Interphase

Animation: ProphaseAnimation: Prophase

Animation: PrometaphaseAnimation: Prometaphase

Animation: MetaphaseAnimation: Metaphase

Animation: AnaphaseAnimation: Anaphase

Animation: TelophaseAnimation: Telophase

• Includes microtubules, centrosomes, the spindle microtubules, and the asters• Some microtubules attach to the kinetochores of

chromosomes and move the chromosomes to the metaphase plate

Assembly of begins in the centrosome› microtubule organizing center

centrosome replicates› migrate to opposite ends of the cell, as spindle

microtubules grow out from them An aster (a radial array of short

microtubules) extends from each centrosome

Microtubules Chromosomes

Sisterchromatids

AsterCentrosome

Metaphaseplate

Kineto-chores

Kinetochoremicrotubules

0.5 µm

Overlappingnonkinetochoremicrotubules

1 µmCentrosome

Anaphase› sister chromatids separate› move along the kinetochore microtubules

toward opposite ends of the cell

microtubules shorten› depolymerizing at their kinetochore ends

Chromosomemovement

Microtubule Motorprotein

Chromosome

Kinetochore

Tubulinsubunits

Nonkinetochore microtubules › overlap and push against each other,

elongating the cell

Telophase› genetically identical daughter nuclei form at

opposite ends of the cell

In animal cells› Process of cleavage

forms a cleavage furrow

In plant cells› a cell plate forms

Cleavage furrow100 µm

Contractile ring ofmicrofilaments

Daughter cells

Cleavage of an animal cell (SEM)

1 µm

Daughter cellsCell plate formation in a plant cell (TEM)

New cell wallCell plate

Wall ofparent cell

Vesiclesformingcell plate

NucleusCell plateChromosomesNucleolus

Chromatincondensing 10 µm

Prophase. The chromatin is condensing.The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is starting to form.

Prometaphase. Wenow see discrete chromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelope will fragment.

Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate.

Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore micro- tubules shorten.

Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.

Reproduction by cell division Prokaryotes

› bacteria and archaea

In binary fission:› the chromosome replicates

beginning at the origin of replication› two daughter chromosomes actively move

apart

Origin ofreplication

Cell wall

Plasmamembrane

Bacterialchromosome

E. coli cell

Two copiesof origin

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

Origin ofreplication

Cell wall

Plasmamembrane

Bacterialchromosome

E. coli cell

Two copiesof origin

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

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

Origin Origin

Origin ofreplication

Cell wall

Plasmamembrane

Bacterialchromosome

E. coli cell

Two copiesof origin

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

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

Origin Origin

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

Two daughtercells result.

G1 checkpoint

G1

S

M

M checkpointG2 checkpoint

G2

Controlsystem

G1

G1 checkpoint

G1

G0

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

If a cell does not receive a go-ahead signal at the G1 checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.

regulatory proteins involved in cell cycle control: › cyclins › cyclin-dependent kinases (Cdks)

Cancer cells do not exhibitanchorage dependenceor density-dependent inhibition.

Cancer cells25 µm

Cancer

Tumors

Cancer cell

Bloodvessel

LymphvesselTumor

Glandulartissue

Metastatictumor

A tumor grows from asingle cancer cell.

Cancer cells invadeneighboring tissue.

Cancer cells spreadthrough lymph andblood vessels toother parts of thebody.

A small percentageof cancer cells maysurvive and establisha new tumor in anotherpart of the body.