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

Copyright © 2009 Pearson Education Inc. nuclear envelope chromatin nucleolus centriole pairs beginning of spindle formation kinetochore spindle pole condensing chromosomes spindle microtubules Late Interphase Duplicated chromosomes are in the relaxed uncondensed state; duplicated centrioles remain clustered. Early Prophase Chromosomes condense and shorten; spindle microtubules begin to form between separating centriole pairs. Late Prophase The nucleolus disappears; the nuclear envelope breaks down; spindle microtubules attach to the kinetochore of each sister chromatid. Metaphase Kinetochores interact; spindle microtubules line up the chromosomes at 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

Copyright © 2009 Pearson Education Inc. chromosomes extending nuclear envelope re-forming Anaphase Sister chromatids separate and move to opposite poles of the cell; spindle microtubules that are not attached to the chromosomes push the poles apart. Telophase One set of chromosomes reaches each pole and relaxes into the extended state; nuclear envelopes start to form around each set; spindle microtubles begin to disappear. Cytokinesis The cell divides in two; each daughter cell receives one nucleus and about half of the cytoplasm. Interphase of daughter cells Spindles disappear, intact nuclear envelopes form, chromosomes extend completely, and the nucleolus reappears. unattached spindle microtubules (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

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Prophase Fig. 8-9b–c

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Metaphase Fig. 8-9d

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Anaphase Fig. 8-9e

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Telophase Fig. 8-9f

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Cytokinesis Fig. 8-9g

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  During cytokinesis, the cytoplasm is divided between two daughter cells. Fig Scanning electron micrograph of cytokinesis. Microfilaments contract, pinching the cell in two The microfilament ring contracts, pinching in the cell’s “waist.” The waist completely pinches off, forming two daughter cells Microfilaments form a ring around the cell’s equator. (b)(a)

Copyright © 2009 Pearson Education Inc. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? PLAY Animation—Mitosis

Copyright © 2009 Pearson Education Inc. Mitosis Suggested Media Enhancement: Mitosis To access this animation go to folder C_Animations_and_Video_Files and open the BioFlix folder.

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Cytokinesis in a plant cell Fig. 8-11

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.6 How Does Meiotic Cell Division Produce Haploid Cells? Fig. 8-12a–d paired homologous chromosomes recombined chromatids spindle microtubul e kinetochores chiasma (a)(b)(c)(d) Prophase I Duplicated chromosomes condense. Homologous chromosomes pair up and chiasmata occur as chromatids of homologues exchange parts by crossing over. The nuclear envelope disintegrates, and spindle microtubules form. Metaphase I Paired homologous chromosomes line up along the equator of the cell. One homologue of each pair faces each pole of the cell and attaches to the spindle microtubules via the kinetochore (blue). Anaphase I Homologues separate, one member of each pair going to each pole of the cell. Sister chromatids do not separate. Telophase I Spindle microtubules disappear. Two clusters of chromosomes have formed, each containing one member of each pair of homologues. The daughter nuclei are therefore haploid. Cytokinesis commonly occurs at this stage. There is little or no interphase between meiosis I and meiosis II.

Copyright © 2009 Pearson Education Inc. (e)(f)(g)(h)(i) Prophase II If the chromosomes have relaxed after telophase I, they recondense. Spindle microtubules re-form and attach to the sister chromatids. Metaphase II The chromosomes line up along the equator, with sister chromatids of each chromosome attached to spindle microtubules that lead to opposite poles. Anaphase II The chromatids separate into independent daughter chromosomes, one former chromatid moving toward each pole. Telophase II The chromosomes finish moving to opposite poles. Nuclear envelopes re-form, and the chromosomes become extended again (not shown here). Four haploid cells Cytokinesis results in four haploid cells, each containing one member of each pair of homologous chromosomes (shown here in the condensed state). 8.6 How Does Meiotic Cell Division Produce Haploid Cells? Fig. 8-12e–i

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.6 How Does Meiotic Cell Division Produce Haploid Cells?  Prophase I Fig. 8-12a

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.6 How Does Meiotic Cell Division Produce Haploid Cells?  Metaphase I Fig. 8-12b

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.6 How Does Meiotic Cell Division Produce Haploid Cells?  Anaphase I Fig. 8-12c

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.6 How Does Meiotic Cell Division Produce Haploid Cells?  Telophase I Fig. 8-12d

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.6 How Does Meiotic Cell Division Produce Haploid Cells?  Prophase II: the spindle microtubules re- form Fig. 8-12e

Copyright © 2009 Pearson Education Inc. 8.6 How Does Meiotic Cell Division Produce Haploid Cells?  Metaphase II: duplicated chromosomes line up at the cell’s equator Fig. 8-12f

Copyright © 2009 Pearson Education Inc. 8.6 How Does Meiotic Cell Division Produce Haploid Cells?  Anaphase II: sister chromatids move to opposite poles Fig. 8-12g

Copyright © 2009 Pearson Education Inc. 8.6 How Does Meiotic Cell Division Produce Haploid Cells?  Telophase II and cytokinesis: four haploid cells are formed Fig. 8-12h–i

Copyright © 2009 Pearson Education Inc. Meiosis Suggested Media Enhancement: Meiosis To access this animation go to folder C_Animations_and_Video_Files and open the BioFlix folder.

Copyright © 2009 Pearson Education Inc. 8.6 How Does Meiotic Cell Division Produce Haploid Cells?

Copyright © 2009 Pearson Education Inc. 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

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. The four possible chromosome arrangements at metaphase of 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

Copyright © 2009 Pearson Education Inc. 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.

Copyright © 2009 Pearson Education Inc. 8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability?  Crossing over Fig pair of homologous duplicated chromosomes sister chromatids of one duplicated homologue chiasmata (sites of crossing over) parts of chromosomes that have been exchanged between homologues

Copyright © 2009 Pearson Education Inc. 8.7 How Do Meiotic Cell Division And Sexual Reproduction Produce Genetic Variability? PLAY Animation—Crossing Over and Random Alignment PLAY Animation—Crossing Over and Random Alignment

Copyright © 2009 Pearson Education Inc. 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.