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

1. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? • Mitosis is divided into four phases. • Prophase • Metaphase • Anaphase • Telophase

2. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? • Interphase, prophase, and metaphase nuclearenvelope spindlemicrotubules chromatin spindle pole condensingchromosomes nucleolus kinetochore beginning ofspindle formation centriolepairs spindle pole (a) Late Interphase Duplicated chromosomesare in the relaxeduncondensed state;duplicated centriolesremain clustered. Early Prophase Chromosomes condenseand shorten; spindlemicrotubules begin toform between separatingcentriole pairs. (b) (c) Late Prophase The nucleolus disappears; thenuclear envelope breaksdown; spindle microtubulesattach to the kinetochoreof each sister chromatid. Metaphase Kinetochores interact;spindle microtubulesline up thechromosomesat the cell’s equator. (d) Fig. 8-9a–d

3. unattached spindlemicrotubules chromosomesextending nuclear envelopere-forming Anaphase Sister chromatids separateand move to oppositepoles of the cell; spindlemicrotubules that arenot attached to thechromosomes push thepoles apart. (e) Telophase One set of chromosomes reacheseach pole and relaxesinto the extended state;nuclear envelopes startto form around each set;spindle microtublesbegin to disappear. (f) Cytokinesis The cell divides intwo; each daughtercell receives onenucleus and abouthalf of the cytoplasm. (g) (h) Interphase ofdaughter cells Spindles disappear, intact nuclearenvelopes form,chromosomes extendcompletely, and thenucleolus reappears. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? • Anaphase, telophase, cytokinesis, and interphase Fig. 8-9e–h

4. 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.

5. 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.

6. 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. Fig. 8-9d

7. 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. 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.

9. 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. • The cytoplasm is divided between two daughter cells. • Microfilaments attached to the plasma membrane form a ring around the equator of the cell. • The ring contracts and constricts the cell’s equator. • The constriction divides the cytoplasm into two new daughter cells.

10. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? • CYTOKINESIS Microfilaments forma ring around the cell’sequator. The microfilamentring contracts, pinchingin the cell’s “waist.” The waistcompletelypinches off,forming twodaughter cells (a) (b) Microfilaments contract, pinching the cell in two Scanning electron micrographof cytokinesis. Fig. 8-10

11. 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.

12. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? • Cytokinesis in a plant cell Fig. 8-11

13. 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.

14. 8.6 How Does Meiotic Cell Division Produce Haploid Cells? paired homologouschromosomes recombinedchromatids spindlemicrotubule chiasma kinetochores (a) Prophase IDuplicated chromosomescondense. Homologouschromosomes pair upand chiasmata occur aschromatids of homologuesexchange parts by crossingover. The nuclear envelopedisintegrates, and spindlemicrotubules form. (b) Metaphase I Paired homologouschromosomes line up alongthe equator of the cell. Onehomologue of each pairfaces each pole of the celland attaches to the spindlemicrotubules via thekinetochore (blue). (c) Anaphase I Homologues separate,one member of eachpair going to eachpole of the cell. Sisterchromatids do notseparate. (d) Telophase I Spindle 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. Fig. 8-12a–d

15. 8.6 How Does Meiotic Cell Division Produce Haploid Cells? (e) Prophase II If the chromosomeshave relaxed aftertelophase I, theyrecondense. Spindlemicrotubules re-formand attach to thesister chromatids. (f) Metaphase II The chromosomes lineup along the equator,with sister chromatidsof each chromosomeattached to spindlemicrotubules that leadto opposite poles. (g) Anaphase II The chromatids separateinto independentdaughter chromosomes,one former chromatidmoving toward eachpole. (h) Telophase II The chromosomesfinish moving toopposite poles.Nuclear envelopesre-form, and thechromosomesbecome extendedagain (not shownhere). (i) Four haploidcells Cytokinesis resultsin four haploid cells,each containing onemember of eachpair of homologouschromosomes(shown here in thecondensed state). Fig. 8-12e–i

16. 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.

17. 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.

18. 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.

19. 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.

20. 8.6 How Does Meiotic Cell Division Produce Haploid Cells? • Meiosis II separates sister chromatids into four haploid daughter cells. • It is virtually identical to mitosis, although it occurs in haploid cells.

21. 8.6 How Does Meiotic Cell Division Produce Haploid Cells? • Prophase II: the spindle microtubules re-form Fig. 8-12e

22. 8.6 How Does Meiotic Cell Division Produce Haploid Cells? • Metaphase II: duplicated chromosomes line up at the cell’s equator Fig. 8-12f

23. 8.6 How Does Meiotic Cell Division Produce Haploid Cells? • Anaphase II: sister chromatids move to opposite poles Fig. 8-12g

24. 8.6 How Does Meiotic Cell Division Produce Haploid Cells? • Telophase II and cytokinesis: four haploid cells are formed Fig. 8-12h–i

25. 8.6 How Does Meiotic Cell Division Produce Haploid Cells?

26. 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

27. 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.

28. 8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? • Random separation of homologues during meiosis produces genetic variability. (a) The four possible chromosome arrangements at metaphaseof meiosis I (b) The eight possible sets of chromosomes after meiosis I Fig. 8-13

29. 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.

30. 8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? • Crossing over sisterchromatids ofone duplicatedhomologue pair ofhomologousduplicatedchromosomes chiasmata(sites ofcrossing over) parts of chromosomesthat have beenexchanged betweenhomologues Fig. 8-14

31. 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.