Copyright © 2009 Pearson Education Inc. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Mitosis is divided into four

Copyright © 2009 Pearson Education Inc.

8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Mitosis is divided into four phases.

• Prophase• Metaphase• Anaphase• Telophase


nuclearenvelope chromatin

nucleolus

centriolepairs

beginning ofspindle formation

kinetochore

spindle pole

spindle polecondensingchromosomes

spindlemicrotubules

Late InterphaseDuplicated chromosomesare in the relaxeduncondensed state;duplicated centriolesremain clustered.

Early ProphaseChromosomes condenseand shorten; spindlemicrotubules begin toform between separatingcentriole pairs.

Late Prophase Thenucleolus disappears; thenuclear envelope breaksdown; spindle microtubulesattach to the kinetochoreof each sister chromatid.

MetaphaseKinetochores interact;spindle microtubulesline up thechromosomesat the cell’s equator.

(a) (b) (c) (d)

8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Interphase, prophase, and metaphase

Fig. 8-9a–d


chromosomesextending

nuclear envelopere-forming

Anaphase Sisterchromatids separateand move to oppositepoles of the cell; spindlemicrotubules that arenot attached to thechromosomes push thepoles apart.

Telophase One set ofchromosomes reacheseach pole and relaxesinto the extended state;nuclear envelopes startto form around each set;spindle microtublesbegin to disappear.

CytokinesisThe cell divides intwo; each daughtercell receives onenucleus and abouthalf of the cytoplasm.

Interphase ofdaughter cells Spindlesdisappear, intact nuclearenvelopes form,chromosomes extendcompletely, and thenucleolus reappears.

unattached spindlemicrotubules

(e) (f) (g) (h)

8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Anaphase, telophase, cytokinesis, and

interphase

Fig. 8-9e–h


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During prophase, the chromosomes

condense and are captured by the spindle microtubules.

Three major events happen in prophase:• The duplicated chromosomes condense.• The spindle microtubules form.• The chromosomes are captured by the

spindle.


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? The centriole pairs migrate with the spindle

poles to opposite sides of the nucleus.• When the cell divides, each daughter cell

receives a centriole.

Every sister chromatid has a structure called a kinetochore located at the centromere, which attaches to a spindle apparatus.


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Prophase

Fig. 8-9b–c


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During metaphase, the chromosomes line

up along the equator of the cell.• At this phase, the spindle apparatus lines up

the sister chromatids at the equator, with one kinetochore facing each cell pole.


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Metaphase

Fig. 8-9d


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During anaphase, sister chromatids

separate and move to opposite poles of the cell.• Sister chromatids separate, becoming

independent daughter chromosomes.• The kinetochores pull the chromosomes

poleward along the spindle microtubules.


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Anaphase

Fig. 8-9e


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During telophase, nuclear envelopes form

around both groups of chromosomes.• Telophase begins when the chromosomes

reach the poles.• The spindle microtubules disintegrate and the

nuclear envelop forms around each group of chromosomes.


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Telophase

Fig. 8-9f


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Cytokinesis occurs during telophase,

separating each daughter nucleus into a separate cell that then begins interphase.


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Cytokinesis

Fig. 8-9g


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During cytokinesis, the cytoplasm is divided

between two daughter cells.• Microfilaments attached to the plasma

membrane form a ring around the equator of the cell.

• During cytokinesis, the ring contracts and constricts the cell’s equator.

• Eventually, the constriction divides the cytoplasm into two new daughter cells.


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? During cytokinesis, the cytoplasm is divided

between two daughter cells.

Fig. 8-10

Scanning electron micrographof cytokinesis.

Microfilaments contract, pinching the cell in two

The microfilamentring contracts, pinchingin the cell’s “waist.”

The waistcompletelypinches off,forming twodaughter cells

Microfilaments forma ring around the cell’sequator.

(b)(a)


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?

PLAYPLAY Animation—Mitosis


Mitosis

Suggested Media Enhancement:

MitosisTo access this animation go to folder C_Animations_and_Video_Filesand open the BioFlix folder.


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Cytokinesis in plant cells is different than in

animal cells.• In plants, carbohydrate-filled vesicles bud off

the Golgi apparatus and line up along the cell’s equator between the two nuclei.

• The vesicles fuse, forming a cell plate.• The carbohydrate in the vesicles become the

cell wall between the two daughter cells.


8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Cytokinesis in a plant cell

Fig. 8-11


8.6 How Does Meiotic Cell Division Produce Haploid Cells? Meiosis is the production of haploid cells

with unpaired chromosomes derived from diploid parent cells with paired chromosomes.

Meiosis includes two nuclear divisions, known as meiosis I and meiosis II.• In meiosis I, homologous chromosomes pair

up, but sister chromatids remain connected to each other.

• In meiosis II, chromosomes behave as they do in mitosis—sister chromatids separate and are pulled to opposite poles of the cell.


8.6 How Does Meiotic Cell Division Produce Haploid Cells?

Fig. 8-12a–d

paired homologouschromosomes

recombinedchromatids

spindlemicrotubule kinetochoreschiasma

(a) (b) (c) (d) Prophase IDuplicated chromosomescondense. Homologouschromosomes pair upand chiasmata occur aschromatids of homologuesexchange parts by crossingover. The nuclear envelopedisintegrates, and spindlemicrotubules form.

Metaphase IPaired homologouschromosomes line up alongthe equator of the cell. Onehomologue of each pairfaces each pole of the celland attaches to the spindlemicrotubules via thekinetochore (blue).

Anaphase IHomologues separate,one member of eachpair going to eachpole of the cell. Sisterchromatids do notseparate.

Telophase ISpindle microtubules disappear.Two clusters of chromosomeshave formed, each containingone member of each pair ofhomologues. The daughternuclei are therefore haploid.Cytokinesis commonly occursat this stage. There is littleor no interphase betweenmeiosis I and meiosis II.


(e) (f) (g) (h) (i) Prophase IIIf the chromosomeshave relaxed aftertelophase I, theyrecondense. Spindlemicrotubules re-formand attach to thesister chromatids.

Metaphase IIThe chromosomes lineup along the equator,with sister chromatidsof each chromosomeattached to spindlemicrotubules that leadto opposite poles.

Anaphase IIThe chromatids separateinto independentdaughter chromosomes,one former chromatidmoving toward eachpole.

Telophase IIThe chromosomesfinish moving toopposite poles.Nuclear envelopesre-form, and thechromosomesbecome extendedagain (not shownhere).

Four haploidcellsCytokinesis resultsin four haploid cells,each containing onemember of eachpair of homologouschromosomes(shown here in thecondensed state).


Fig. 8-12e–i


8.6 How Does Meiotic Cell Division Produce Haploid Cells? Meiosis I separates homologous

chromosomes into two haploid daughter nuclei.• During prophase I, homologues pair up.

• The two homologues in a pair intertwine, forming chiasmata (singular, chiasma).

• At some chiasmata, the homologues exchange parts in a process known as crossing over.


8.6 How Does Meiotic Cell Division Produce Haploid Cells? Prophase I

Fig. 8-12a


8.6 How Does Meiotic Cell Division Produce Haploid Cells? During metaphase I, paired homologues line

up at the equator of the cell.• Interactions between the kinetochores and the

spindle microtubules move the paired homologues to the equator of the cell.


8.6 How Does Meiotic Cell Division Produce Haploid Cells? Metaphase I

Fig. 8-12b


8.6 How Does Meiotic Cell Division Produce Haploid Cells? During anaphase I, homologous

chromosomes separate.• One duplicated chromosome (consisting of

two sister chromatids) from each homologous pair moves to each pole of the dividing cell.

• At the end of anaphase I, the cluster of chromosomes at each pole contains one member of each pair of homologous chromosomes.


8.6 How Does Meiotic Cell Division Produce Haploid Cells? Anaphase I

Fig. 8-12c


8.6 How Does Meiotic Cell Division Produce Haploid Cells? After telophase I and cytokinesis, there are

two haploid daughter cells.• The spindle microtubules disappear and the

nuclear envelope may reappear.• Cytokinesis takes place and divides the cell

into two daugher cells; each cell has only one of each pair of homologous chromosomes and is haploid.

• Each chromosome still has two sister chromatids.


8.6 How Does Meiotic Cell Division Produce Haploid Cells? Telophase I

Fig. 8-12d


8.6 How Does Meiotic Cell Division Produce Haploid Cells? Meiosis II separates sister chromatids into

four haploid daughter cells. Meiosis II is virtually identical to mitosis,

although it occurs in haploid cells.


8.6 How Does Meiotic Cell Division Produce Haploid Cells? Prophase II: the spindle microtubules re-

form

Fig. 8-12e


8.6 How Does Meiotic Cell Division Produce Haploid Cells? Metaphase II: duplicated chromosomes line

up at the cell’s equator

Fig. 8-12f


8.6 How Does Meiotic Cell Division Produce Haploid Cells? Anaphase II: sister chromatids move to

opposite poles

Fig. 8-12g


8.6 How Does Meiotic Cell Division Produce Haploid Cells? Telophase II and cytokinesis: four haploid

cells are formed

Fig. 8-12h–i


Meiosis

Suggested Media Enhancement:

MeiosisTo access this animation go to folder C_Animations_and_Video_Filesand open the BioFlix folder.




8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Ways to produce genetic variability from

meiotic cell division and sexual reproduction• Shuffling of homologues• Crossing over• Fusion of gametes


8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Shuffling of homologues creates novel

combinations of chromosomes.• There is a random assortment of homologues

to daughter cells at meiosis I.• At metaphase I, paired homologues line up at

the cell’s equator.• Which chromosome faces which pole is

random, so it is random as to which daughter cell will receive each chromosome.


The four possible chromosome arrangements at metaphaseof meiosis I

The eight possible sets of chromosomes after meiosis I

(a)

(b)

8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Random separation of homologues during

meiosis produces genetic variability.

Fig. 8-13


8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Crossing over creates chromosomes with

novel combinations of genetic material.• Exchange of genetic material during prophase

I, through crossing over, is a unique event each time.

• Genetic recombination through crossing over results in the formation of new combinations of genes on a given chromosome .

• As a result of genetic recombination, each sperm and each egg is genetically unique.


8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Crossing over

Fig. 8-14

pair ofhomologousduplicatedchromosomes

sisterchromatids ofone duplicatedhomologue

chiasmata(sites ofcrossing over)

parts of chromosomesthat have beenexchanged betweenhomologues


8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability?

PLAYPLAY Animation—Crossing Over and Random Alignment

PLAYPLAY Animation—Crossing Over and Random Alignment


8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? Fusion of gametes creates genetically

variable offspring.• Because every egg and sperm are genetically

unique, and it is random as to which sperm fertilizes which egg, every fertilized egg is also genetically unique.

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Copyright © 2009 Pearson Education Inc. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? Mitosis is divided into four