The Cell Cycle. In unicellular organisms, division of one cell reproduces the entire organism In unicellular organisms, division of one cell reproduces

The Cell CycleThe Cell Cycle

In unicellular organisms, division of one cell In unicellular organisms, division of one cell reproduces the entire organismreproduces the entire organism

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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cell division results in Cell division results in genetically identical genetically identical

daughter cellsdaughter cells• Most cell division Most cell division (mitosis)(mitosis) results in results in

daughter cells with identical genetic daughter cells with identical genetic information, DNAinformation, DNA

• A special type of division A special type of division (meiosis)(meiosis) produces nonidentical daughter cells produces nonidentical daughter cells (gametes, or sperm and egg cells)(gametes, or sperm and egg cells)


Cellular Organization of Cellular Organization of the Genetic Materialthe Genetic Material

All the DNA in a cell constitutes the All the DNA in a cell constitutes the cell’s cell’s genomegenome

A genome can consist of A genome can consist of a single DNA molecule (prokaryotic cells)a single DNA molecule (prokaryotic cells) a number of DNA molecules (eukaryotic a number of DNA molecules (eukaryotic

cells)cells)

DNA molecules in a cell are packaged DNA molecules in a cell are packaged into into chromosomeschromosomes


Every eukaryotic species has a characteristic Every eukaryotic species has a characteristic number of chromosomes in each cell nucleusnumber of chromosomes in each cell nucleus

Somatic cells Somatic cells (nonreproductive cells) have (nonreproductive cells) have two sets of chromosomestwo sets of chromosomes

Gametes Gametes (reproductive cells: sperm and (reproductive cells: sperm and eggs) have half as many chromosomes as eggs) have half as many chromosomes as somatic cellssomatic cells

Eukaryotic chromosomes consist of Eukaryotic chromosomes consist of chromatinchromatin, a complex of DNA and protein , a complex of DNA and protein (histones).(histones).


Distribution of Distribution of Chromosomes During Chromosomes During

Eukaryotic Cell DivisionEukaryotic Cell Division Each duplicated chromosome has two Each duplicated chromosome has two

sister chromatidssister chromatids, which separate , which separate during cell divisionduring cell division

The The centromere centromere is where the two is where the two chromatids are most closely attachedchromatids are most closely attached


Fig. 12-4Fig. 12-4 0.5 µm Chromosomes

Chromosomeduplication(including DNAsynthesis)

Chromo-some arm

Centromere

Sisterchromatids

DNA molecules

Separation ofsister chromatids

Centromere

Sister chromatids

Eukaryotic cell division consists of:Eukaryotic cell division consists of: MitosisMitosis, the division of the nucleus, the division of the nucleus CytokinesisCytokinesis, the division of the cytoplasm, the division of the cytoplasm

Gametes are produced by a variation of Gametes are produced by a variation of cell division called cell division called meiosismeiosis

Meiosis yields nonidentical daughter Meiosis yields nonidentical daughter cells that have only one set of cells that have only one set of chromosomes, half as many as the chromosomes, half as many as the parent cellparent cell


Phases of the Cell CyclePhases of the Cell Cycle The cell cycle consists ofThe cell cycle consists of

Interphase Interphase (cell growth and copying of (cell growth and copying of chromosomes in preparation for cell chromosomes in preparation for cell division)division) GG11 phasephase- cell growscell grows S phase- S phase- chromosomes duplicatechromosomes duplicate GG22 phasephase- cell grows/prepare for divisioncell grows/prepare for division

Mitotic (M) phase Mitotic (M) phase mitosis mitosis cytokinesiscytokinesis


Video: MITOSIS

Fig. 12-UN1Fig. 12-UN1

Telophase andCytokinesis

Anaphase

Metaphase

Prometaphase

Prophase

MITOTIC (M) PHASE

Cytokinesis

Mitosis

SG1

G2

Mitosis is conventionally divided into Mitosis is conventionally divided into five phases:five phases: ProphaseProphase PrometaphasePrometaphase MetaphaseMetaphase AnaphaseAnaphase TelophaseTelophase

Cytokinesis: (division of cytoplasm)Cytokinesis: (division of cytoplasm)


PROPHASEPROPHASE

Chromatids condense Chromatids condense and become visibleand become visible

Nucleoli dissapearsNucleoli dissapears Centrosomes (pair of Centrosomes (pair of

centrioles) move to the centrioles) move to the polespoles

Spindle begins to formSpindle begins to form

PROMETAPHASEPROMETAPHASE

Nuclear envelope breaksNuclear envelope breaks Microtubules form from Microtubules form from

centrosome and move centrosome and move toward the middle of celltoward the middle of cell

Sister chromatids have a Sister chromatids have a kinetochore (specialized kinetochore (specialized protein located on protein located on centromere)centromere)

Spindle attach to Spindle attach to kinetochorekinetochore

METAPHASEMETAPHASE Longest stage of Longest stage of

mitosismitosis Spindle is completely Spindle is completely

formedformed Spindle attaches to Spindle attaches to

kinetochore of kinetochore of chromatidschromatids

Spindle moves Spindle moves chromatids to middle of chromatids to middle of cell (metaphase plate)cell (metaphase plate)

ANAPHASEANAPHASE Shortest stage of MitosisShortest stage of Mitosis Spindle pulls sister chromatids apart Spindle pulls sister chromatids apart

(become chromosomes)(become chromosomes) Chromosomes are pulled toward the Chromosomes are pulled toward the

polespoles Spindle shortens and cell elongatesSpindle shortens and cell elongates Chromosomes reach polesChromosomes reach poles

TELOPHASETELOPHASE

Nuclear envelopes Nuclear envelopes reappearreappear

Spindle dissappearsSpindle dissappears 2 identical nuclei and 2 identical nuclei and

1 cytoplasm1 cytoplasm

Video: MITOSIS

Cleavage furrow

Contractile ring ofmicrofilaments Daughter cells

(a) Cleavage of an animal cell (SEM)(b) Cell plate formation in a plant cell (TEM)

Vesiclesformingcell plate

Wall ofparent cell

Cell plate

Daughter cells

New cell wall

Cytokinesis: Cytokinesis: A Closer A Closer LookLook

Division of cytoplasmDivision of cytoplasm In animal cells, cytokinesis occurs by In animal cells, cytokinesis occurs by

a process known as a process known as cleavagecleavage, , forming a forming a cleavage furrowcleavage furrow

In plant cells, a In plant cells, a cell plate cell plate forms forms during cytokinesisduring cytokinesis

The Cell Cycle Control The Cell Cycle Control SystemSystem

The sequential events of the cell cycle are The sequential events of the cell cycle are directed by a distinct directed by a distinct cell cycle control cell cycle control systemsystem, which is similar to a clock, which is similar to a clock

The clock has specific The clock has specific checkpoints checkpoints where the cell cycle stops until a go-where the cell cycle stops until a go-ahead signal is receivedahead signal is received


Video: CONTROL SYSTEM CELL CYCLE

Video: CELL CYCLE

SG1

M checkpoint

G2M

Controlsystem

G1 checkpoint

G2 checkpoint

• Cell receives a go-ahead signal at the GCell receives a go-ahead signal at the G11 checkpoint, it will checkpoint, it will usually complete the S, Gusually complete the S, G22, and M phases and divide, and M phases and divide

• Cell does not receive the go-ahead signal, it will exit the Cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the cycle, switching into a nondividing state called the GG00 phasephase


G0

Cell does not receive ago-ahead signal

Cell receives a go-aheadsignal

G1 checkpoint

G1G1

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The Cell Cycle. In unicellular organisms, division of one cell reproduces the entire organism In unicellular organisms, division of one cell reproduces