Cell Division

Part 1

A Generalized Cell

Golgibody

Nuclearenvelope

ChromosomalDNA NucleusNucleolus

Polyribosomes

Ribosome

Rough ER

Cytoplasm

Membrane protein

Plasma membrane

Smooth ER

MitochondrionCentrioles

Microtubules

Microfilaments

Lysosome

(b) Animal cell

The Cell Cycle

G1 G2

S

Twodaughtercells

M

Cytokinesis

Telo

phas

eA

nap

has

e Metaph

aseProm

etaphaseProphase

Mitosis

Interphase

Gap 1 Gap 2

Synthesis

GrowthGene expressionDifferentiation

DNA Synthesis

Gene expressionQuality control

Actual division process

Three Little Words Geneticist Need to Hear… Homolog, Loci, Allele

Homologouspair ofchromo-somes

Gene loci (location)

A b c

A B c

AA Bb ccGenotype:Homozygousfor thedominantallele

Heterozygous Homozygousfor therecessiveallele

Unreplicated chromosome pair

Replicated ChromosomePair of sister chromatids

Kinetochoreproteins

Centromere(DNA that ishidden beneaththe kinetochoreproteins)

Onechromatid(dark blue)

Onechromatid(light blue)

(b)(a)

• At the end of S phase, a cell has twice as many chromatids as there were chromosomes in G1 phase– i.e. - human cell

• 46 chromosomes in G1 phase

• 46 pairs of sister chromatids in G2 phase

• chromosome is therefore a relative term– In G1, anaphase, & telophase it refers to the

equivalent of one chromatid

– In G2, prophase, & metaphase, it refers to a pair of sister chromatids

Chromatids, Chromosomes… What the…

Interphase

• Chromosomes are decondensed

• chromosomes replicate

• The centrosome divides

Nuclear membrane

Chromosomes

Two centrosomes,each with centriole pairs

Prophase

• Nuclear envelope dissociates

• Centrosomes move to opposite poles

• mitotic spindle apparatus forms

Microtubulesforming mitotic spindle Sister

chromatids

Centromere

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Polar microtubule

Kinetochoreproteins attachedto centromere Kinetochore

microtubule

Astral microtubule

Metaphaseplate

(d) METAPHASE

Spindle Apparatus

• Composed of microtubules originated from centrioles• Microtubules are formed polymerization of tubulin

proteins

• 3 types of spindle microtubules– Aster microtubules

• Important for positioning of the spindle apparatus

– Polar microtubules• Help to “push” the poles away from each other

– Kinetochore microtubules• Attach to kinetochore , at the centromere

Figure 3.8

Kinetochore Spindle Fibers

Prometaphase

• Spindle fibers bind kinetochores

• The two kinetochores on a pair of sister chromatids are attached to kinetochore MTs from opposite poles

Nuclear membranefragmenting

Spindle pole

Mitoticspindle

Metaphase

• Pairs of sister chromatids align themselves at the metaphase plate Polar

microtubuleKinetochoreproteins attachedto centromere Kinetochore

microtubule

Astral microtubule

Metaphaseplate

Anaphase

• Centromeres separate• Each chromatid, is

linked to only one pole• As anaphase proceeds

– Kinetochore MTs shorten• Chromosomes move to

opposite poles– Polar MTs lengthen

• Poles themselves move further away from each other

Chromosomes

Telophase & Cytokinesis

• Chromosomes reach poles & decondense

• Nuclear membrane reforms • Quickly followed by

cytokinesis– In animals

• Formation of a cleavage furrow

– In plants• Formation of a cell plate

• Mitosis ultimately produces two daughter cells genetically identical to the mother cell– Barring rare mutations

• Processes requireing mitotic cell division– Development of multicellularity– Organismal growth– Wound repair– Tissue regeneration

Some Key Points