Mitosis

CELL DIVISION EUKARYOTIC CELL DIVISION

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Unicellular organisms

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Multicellular organisms

– Creates duplicate offspring – Growth – Development – Repair

EUKARYOTIC GENOME (human)

EUKARYOTIC GENOME

about 3.3 billion nucleotide pairs in a “haploid” genome about 100,000 genes

PROBLEM: NEED TO USE, DUPLICATE AND THEN SEPARATE A LOT MORE INFORMATION PRECISELY

LOTS MORE THAN A PROKARYOTE

EUKARYOTIC CHROMOSOME ●

LINEAR – one long molecule of DNA with two ends • contains genetic information in a linear sequence – each chromosome contain 1,000s of genes

EUKARYOTIC CHROMOSOME ●

MULTIPLE - many chromosomes – human = 46 ; goldfish = 94; fruitfly = 8 • each species has a characteristic number

– may come in sets • diploid = pairs of chromosomes

• genes contained are in specific location and are specific for the chromosome

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EUKARYOTIC CHROMOSOME ●

COMPLEXED – associated with many types of proteins

EUKARYOTIC CHROMOSOME TELOMERE

CENTROMERE kinetochore

• give structure to chromosome

– chromatin = DNA-protein complex – highly folded and coiled • coiling increases when cell enters mitosis • degree of coiling related to gene activity during interphase

LONG ARM

SHORT ARM

before DNA replication

EUKARYOTIC CHROMOSOME

CELL CYCLE ●

centromere

ORDERED SEQUENCE OF EVENTS BETWEEN: – The time a cell divides to form two daughter cells, and – The time those daughter cells DIVIDE

sister chromatids

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includes doubling of cytoplasm, cellular organelles and DNA

CELL CYCLE STARTING POINT

G1

M

S G2

G1 + S + G2 = INTERPHASE

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INTERPHASE G1 first growth phase

CELL-CYCLE CONTROL ●

SYSTEM OF CHECKPOINTS – BASED ON A CYCLIC SET OF MOLECULES

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general growth and metabolism – organelles replicate

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increase proteins and RNA • but not DNA

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centrioles replicate (in animals)

INTERPHASE G1 ●

must make decision to divide – restriction point • related to cytoplasm/genome volume • determined by environmental and developmental conditions

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commits cell to rest of cell cycle

INTERPHASE G1

CELL CYCLE G1

M ●

DIFFERENTIATION MAY OCCUR S

–(go to G0) • MUSCLE • NERVE

G0 G2

G1 + S + G2 = INTERPHASE

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G1 the decision to divide ●

decision may require a specific GROWTH FACTOR – example: wound healing • wound: platelets in blood fragment to release PDGF (platelet derived growth factor ) • stimulates fibroblasts to divide

G1 the decision to divide ●

decision may be inhibited by cell contact or cell density

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decision inhibited by lack of adhesion

– density-dependent inhibition – most unanchored cells do not divide ●

– cell size and cell density

G1 the decision to divide

INHIBITIONS TO DIVISION ●

cell contact or cell density

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– density-dependent inhibition ●

anchorage dependence (adherence)

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nutrient levels must be acceptable

nutrient levels must be acceptable

signalled by activation of several cyclin-dependent protein kinases (Cdk)

– most unanchored cells do not divide – cell size and cell density

INTERPHASE S phase

CELL CYCLE MAKE DNA?

G1

M

Exact doubling of DNA amount in cell ● All chromosomes replicated ● THEREFORE: information content of cell DNA accurately copied ●

S G2

DNA SYNTHESIS

restriction point

G1 + S + G2 = INTERPHASE

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G2

CELL CYCLE

SECOND GROWTH PHASE

G1

M

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S G2

DNA DOUBLED

Some limited continued growth synthesis of proteins required for mitosis

G1 + S + G2 = INTERPHASE

HOW DOES THE CELL KNOW WHEN TO ENTER MITOSIS?

CYCLICAL CHANGES IN MOLECULES DURING CELL CYCLE G1

S

G2

M

G1

S

G2

M

availability of proteins required for mitosis ● TRIGGER = MPF PROTEIN ●

– maturation promoting factor DNA

PROT RNA

MPF cyclin-Cdk complex ●

protein kinase – phosphorylates other proteins • Phosphorylation usually turns “on” protein activity by changing conformation

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active form composed of two proteins:

MPF cyclin-Cdk complex ●

CYCLIN + Cdk = MPF – TRIGGERS MITOSIS BY CASCADE OF PHOSPHORYLATION – ACTIVATES CYCLIN-DEGRADING ENZYME

– CYCLIN – Cdk

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Cdk = Cyclin dependent kinase cell-division control ● concentration constant throughout cell cycle ● recycled at end of mitosis ●

CHANGES IN CYCLIN AND MPF DURING CELL CYCLE

CYCLIN destroyed at end of mitosis ● produced throughout cell cycle ●

– therefore concentration increase ●

as concentration increases, binds with Cdk to form active MPF (cyclin-Cdk complex)

CELL JUST BEFORE MITOSIS

WHAT ABOUT THE CELL AT END OF G2 nucleus present with nuclear envelope nucleolus present in nucleus ● chromosomes duplicated (not condensed) ● two pairs of centrioles (animals) ● microtubule “asters” may be seen around centrioles ● high levels of MPF starting kinase cascade ● ●

UNCONDENSED CHROMATIN IN NUCLEUS

SPECIFIC CHROMOSOMES NOT VISIBLE

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MITOSIS

CELL CYCLE

M PHASE

G1

M BEGIN DIVISION

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PROCESS OF EQUAL DISTRIBUTION OF CHROMOSOMES – karyokinesis

S G2 ●

PROCESS OF DISTRIBUTION OF CELL COMPONENTS – cytokinesis

G1 + S + G2 = INTERPHASE

MITOSIS

PROPHASE

SEVERAL STAGES Prophase ●

Prometaphase Metaphase

CONTINUOUS DYNAMIC PROCESS

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Anaphase ●

Telophase two equal daughter cells

CHROMATIN BEGINS TO CONDENSE – chromosomes become visible – nucleoli disappear CENTRIOLES MIGRATE TOWARDS POLES (in animals) – pairs = centrosome SPINDLE BEGINS TO APPEAR – formation occurs at centrosome • MTOC = microtubule organizing center

CHROMOSOMES IN PROPHASE

centrioles

nucleus spindle

aster

chromosomes

CONSIST OF TWO IDENTICAL SISTER CHROMATIDS JOINED AT CENTROMERE

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centrioles and aster

PROMETAPHASE NUCLEAR MEMBRANE BEGINS TO DISAPPEAR ● CENTRIOLES AT POLES ● SPINDLE FORMED ●

spindle

– microtubules interact with chromosomes ●

CHROMOSOMES BEGIN TO MOVE

SPINDLE COMPOSED OF MICROTUBULES ● SPINDLE FIBERS = bundles of microtubules ●

equator

– kinetochore microtubules • attached to centromere region

– non-kinetochore microtubules • act to elongate the cell

kinetochore kinetochore fibers

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kinetochore

centromere

non-kinetochore fibers

astral microtubules

centrioles

CHROMOSOMES

DO LOOK LIKE THIS BY METAPHASE

CONDENSED

TWO SISTER CHROMATIDS

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METAPHASE ●

CHROMOSOMES MOVED TO EQUATOR – Metaphase plate – Centromeres aligned at equator

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SPINDLE AT GREATEST LEVEL – centrosomes at POLES

ANAPHASE ●

DOES NOT START UNTIL ALL CHROMOSOMES ARE ATTACHED TO SPINDLE – APC (ANAPHASE PROMOTING COMPLEX)

ANAPHASE ● ●

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CYCLIN BEGINS TO DEGRADE PROTEOLTIC ENZYMES CAUSE CENTROMERES SPLIT SISTER CHROMATIDS SEPARATE KINETOCHORE MICROTUBULES SHORTEN CELL ELONGATES

ANAPHASE ● ●

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CENTROMERES SPLIT SISTER CHROMATIDS SEPARATE – move towards opposite poles – V-shape KINETOCHORE MICROTUBULES SHORTEN – at point of kinetochore attachment CELL ELONGATES – sliding of non-kinetochore microtubules

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CHROMOSOME (one chromatid)

MOVEMENT OF CHROMOSOMES attachment of microtubules at kinetochore ● loss of tubulin subunits ●

– at (+) microtubule end • end of kinetochore attachment CHROMOSOME (one chromatid)

kinetochore microtubule

kinetochore microtubule

tubulin subunits

Figure 11.8

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TELOPHASE FURTHER ELONGATION OF CELL ● DAUGHTER NUCLEI FORM ● NUCLEOLI REAPPEAR ● CHROMATIN UNCOILS AND CHROMOSOMES BECOME DIFFUSE ● CYTOKINESIS OCCURRING ●

CYTOKINESIS PROCESS OF CYTOPLASMIC DIVISION ● NOT EXACT ●

– SOMETIMES DESIGNED TO BE VERY UNEQUAL • YEAST BUDDING

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CYTOKINESIS

CYTOKINESIS

PROCESS OF CELL SEPARATION

PROCESS OF CELL SEPARATION

ANIMALS (OUT --> IN) [elastic] – cleavage furrow / contractile ring

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ANIMALS (OUT --> IN) [elastic] – cleavage furrow / contractile ring • contractile ring = actin microfilaments • ring breaks remains of spindle

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CYTOKINESIS PROCESS OF CELL SEPARATION ●

TELOPHASE IN PLANT CELLS

PLANTS (IN --> OUT) [rigid] – cell plate - fused vesicles – cell wall forms between two membranes of cell plate

RIGID CELL WALL

CELL PLATE

MITOSIS WITHOUT CYTOKINESIS? ●

AFTER CYTOKINESIS

SOMETIMES; result: multinucleated cell – some slime molds --> plasmodium – some embryos --> fruit flies --> syncitium – some algae and fungi --> coenocytic plant body – seed of flowering plants – muscle cells

RETURN TO INTERPHASE

G1 CONTINUE THE CELL CYCLE

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LOSS OF CONTROL OF CELL CYCLE

RESULTS OF MITOSIS ● ●

two daughter cells with:

– unresponsive to normal control signals

– IDENTICAL genetic information – SIMILAR cytoplasmic contents

• TRANSFORMATION = conversion from normal regulation to uncontrolled growth ●

ASEXUAL PROCESS OF REPRODUCTION NO VARIATION IN INDIVIDUALS

CANCER = excessive division

changes in cell cycle control almost always “genetic” – multiple changes usually required

LOSS OF CONTROL OF CELL CYCLE ●

uncontrolled production of growth factors – oncogenes – tumor suppressor genes

growth factor receptors in membranes which are always turned on ● loss of regulation of DNA synthesis ●

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