Why do cells divide?

4. For the reproduction of unicellular organisms (like bacteria)

1. To heal/repair tissue2. For multicellular organisms to grow in size

3. To keep cell sizes small for increased efficiency

The Cell CycleThe Cell CycleThe Cell Cycle is the The Cell Cycle is the preparation and division of preparation and division of the nucleusthe nucleus of a cell to of a cell to form two new cellsform two new cells

InterphaseInterphase and and MitosisMitosis make up the Cell Cyclemake up the Cell Cycle

Stages of the cell cycleStages of the cell cycle• INTERPHASEINTERPHASE

– G1G1 stage = "GAP 1” stage = "GAP 1”

– SS stage = "Synthesis”stage = "Synthesis”• This is the stage when DNA This is the stage when DNA replication occursreplication occurs

– G2 G2 stage = "GAP 2”stage = "GAP 2”

• MITOSISMITOSIS

– M M stage = mitosis & cytokinesisstage = mitosis & cytokinesis

Cell CycleCell Cycle

What is Mitosis?What is Mitosis?

• Nuclear divisionNuclear division plus division of plus division of the cell itself (the cell itself (cytokinesiscytokinesis))

• Produces Produces two identical daughter two identical daughter cellscells. .

Regulation of the Cell CycleRegulation of the Cell Cycle• Very complexVery complex

• Errors can lead to cancer.Errors can lead to cancer.– In cancer, regulation of the cell cycle In cancer, regulation of the cell cycle

goes awrygoes awry

–Normal cell growth & behavior is lost. Normal cell growth & behavior is lost.

Regulatory EnzymesRegulatory Enzymes• CdkCdk (cyclin dependent kinase) (cyclin dependent kinase)• Major control switch, along with other Major control switch, along with other

cyclinscyclins–causes cell to move from G1 to S or G2 causes cell to move from G1 to S or G2

to M. to M. • MPFMPF (Maturation Promoting Factor) (Maturation Promoting Factor)

–triggers progression through cell cycle. triggers progression through cell cycle.

Cancer PromotorsCancer Promotors• p53p53 = protein that blocks the cell cycle if = protein that blocks the cell cycle if

DNA is damaged. Cell can die. DNA is damaged. Cell can die. • p53 mutationp53 mutation is the most frequent mutation is the most frequent mutation

leading to cancer. leading to cancer. • p27p27 = protein that blocks entry into S phase = protein that blocks entry into S phase

–Low levels may mean eventual Low levels may mean eventual breast breast cancercancer

Chromosome Characteristics - Chromosome Characteristics -

PloidyPloidy• Number of sets of chromosomes in Number of sets of chromosomes in

a cella cell

• Haploid (Haploid (nn)-- one set chromosomes )-- one set chromosomes

• Diploid (Diploid (2n2n)-- two sets )-- two sets chromosomes chromosomes

• Most plant and animal adults are Most plant and animal adults are diploid (2n) diploid (2n)

• Eggs and spermEggs and sperm are haploid ( are haploid (nn) )

Chromosome CharacteristicsChromosome Characteristics• Diploid set for humans; 2n = Diploid set for humans; 2n = 4646

(n=23)sperm + (n=23)egg = (2n=46)zygote = (2n=46)zygote

• Autosomes Autosomes = body chromosomes= body chromosomes

• Female-sex chromosomes = homologous Female-sex chromosomes = homologous (XX)(XX)

• Male-sex chromosomes = non-Male-sex chromosomes = non-homologous homologous (XY)(XY)

Interphase - GInterphase - G1 1 S GS G22• The cell is metabolizing, growing, The cell is metabolizing, growing,

replicating DNA and preparing for replicating DNA and preparing for mitosismitosis

Interphase – GInterphase – G1 1 S GS G22• Chromosomes are Chromosomes are notnot clearly seen clearly seen

• NucleolusNucleolus may be visible may be visible

• In animal cells ONLY, a In animal cells ONLY, a pair of pair of centriolescentrioles may start moving into may start moving into position.position.

Interphase summaryInterphase summaryThree Phases:Three Phases:

• The cell grows & The cell grows & develops (Gap 1)develops (Gap 1)

• DNA is replicated (S)DNA is replicated (S)

• Necessary materials Necessary materials are made (Gap 2)are made (Gap 2)

ANIMAL

PLANT

Most of these cells are in Interphase

How do you know these cells are in How do you know these cells are in interphase?interphase?

Intact Nucleus

Mitosis (M Phase)Mitosis (M Phase)The M Phase consists of:The M Phase consists of:• Mitosis and CytokinesisMitosis and Cytokinesis

Mitosis: the division of the nucleusMitosis: the division of the nucleusCytokinesis: the division of the cytoplasmCytokinesis: the division of the cytoplasm

Four Phases of Mitosis:Four Phases of Mitosis:

• PProphase rophase

• MMetaphaseetaphase

• AAnaphasenaphase

• TTelophaseelophase

Late Interphase: Late Interphase: GG22

Preparing Preparing for for

DivisionDivision

Chromosome consisting of two sister chromatids Chromosome consisting of two sister chromatids becomes visiblebecomes visible

CentromereCentromereEarly Spindle ApparatusEarly Spindle ApparatusProphaseProphase

Nuclear Nuclear membrane membrane disappearsdisappears

ProphaseProphase• Chromatin begins to Chromatin begins to condensecondense and and

becomes visible. becomes visible.

• Nucleolus Nucleolus disappearsdisappears. .

• Centrioles begin Centrioles begin moving moving to opposite to opposite ends of the cell ends of the cell

How is DNA packaged into How is DNA packaged into chromosomes?chromosomes?

DNA coils around set of 4 histone proteins, creating a “bead on a string” called a nucleosome

The nucleosome coils into structures called chromatin

Chromatin supercoils = chromosome!

sisterchromatids

centromere

• The chromosome is The chromosome is duplicated (during S)duplicated (during S)

• Each duplicated Each duplicated chromosome consists chromosome consists of two of two

..• These are connected These are connected

by a by a ..

Late ProphaseLate Prophase• Nuclear membrane dissolvesNuclear membrane dissolves• Proteins attach to Proteins attach to centromerescentromeres creating creating

the the kinetochoreskinetochores..• MicrotubulesMicrotubules attach at attach at

centromeres/kinetochores centromeres/kinetochores and chromosomes and chromosomes begin moving. begin moving.

MetaphaseMetaphase

Chromosomes line upChromosomes line up at an at an imaginary line between the imaginary line between the centrosomes at opposite ends centrosomes at opposite ends of the cell.of the cell.

The spindle attaches to The spindle attaches to the centromeres.the centromeres.

MetaphaseMetaphase• Spindle fibers align the Spindle fibers align the

chromosomes along chromosomes along metaphase metaphase plate (middle)plate (middle). .

• This helps to ensure This helps to ensure each new nucleus will each new nucleus will receive one copy receive one copy of each chromosome. of each chromosome.

AnaphaseAnaphase

Spindles pull the Spindles pull the “daughter” “daughter” chromosomes apart chromosomes apart

AnaphaseAnaphase• Paired Paired chromosomes separatechromosomes separate at at

kinetochores and move to opposite kinetochores and move to opposite sidessides

AnaphaseAnaphase

Remember: the Remember: the DNA in these DNA in these chromosomes chromosomes is identical on is identical on each end!each end!

IDENTICAL

DNA

TelophaseTelophase• Chromatids arriveChromatids arrive at opposite poles at opposite poles• Membranes Membranes re-formre-form around daughter around daughter

nuclei. nuclei. • Chromosomes & spindles Chromosomes & spindles

dispersedisperse and are no and are no longer visible.longer visible.

• CytokinesisCytokinesis may begin may begin

Telophase & Telophase & CytokinesisCytokinesis

TelophaseTwo nuclei become visible

In animal cells, a Cleavage Furrow is visible

In plant cells, a cell plate is now visible

CytokinesisThe cytoplasm and organelles divide.

Cytokinesis: AnimalsCytokinesis: Animals• Fiber ring of protein called Fiber ring of protein called actinactin around around

center of cell contracts and center of cell contracts and pinchespinches cell into cell into two identical daughter cellstwo identical daughter cells

PlantsPlants• In plant cells, In plant cells,

the rigid wall the rigid wall requires that a requires that a cell platecell plate be be synthesized synthesized betweenbetween the the two daughter two daughter cells. cells.

The END RESULTThe END RESULTof the CELL CYCLEof the CELL CYCLE

Two identical cellsTwo identical cells as the as the original, ready to go through original, ready to go through

interphase and mitosis, interphase and mitosis, repeating the processrepeating the process

Mitosis & Cytokinesis

animal cell division

What is the purpose of What is the purpose of the Cell Cycle?the Cell Cycle?

To replace somatic cellsTo replace somatic cells

(skin, bone, lining, etc.)(skin, bone, lining, etc.)

Somatic cells are body cellsSomatic cells are body cellsSome cells like your nerve, muscle and red blood Some cells like your nerve, muscle and red blood

cells do not undergo division. (One reason cells do not undergo division. (One reason stem cell research is so important.)stem cell research is so important.)

Asexual ReproductionAsexual Reproduction• Uses only Uses only mitosismitosis• Cells duplicate to form 2 Cells duplicate to form 2

exact copies (exact copies (clonesclones) of each ) of each otherother

• Most primitive form of reproductionMost primitive form of reproduction–NO GENETIC VARIATIONNO GENETIC VARIATION

• Used by higher organisms when conditions Used by higher organisms when conditions are are not favorablenot favorable for sexual reproduction for sexual reproduction

Chromosome # during MitosisChromosome # during Mitosis

Diploid cellDiploid cell

=2n (46 in =2n (46 in

humans)humans)

2(2n) = 92 due to DNA replication2(2n) = 92 due to DNA replication

chromosomeschromosomes

in humansin humans

one cell division occursone cell division occurs

Diploid cellDiploid cell Diploid cell Diploid cell

=2n (46 in =2n (46 in =2n (46 in =2n (46 in

humans)humans) TWO TWO humans) humans)

IDENTICALIDENTICAL

CELLSCELLS