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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.