Site of Crossover Complex Chromosome Duplication Homologues Centromere A

Sperm (haploid) n

Haploid sperm Paternal Fertilization homologue Zygote Maternal (diploid) 2n

Diploid zygote Somatic Germ-line cells Haploid egg cells Germ-line cells

MITOSIS Adult male (diploid) 2n Adult female (diploid) 2n 1 2

Kinetochore Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sister chromatids Diploid cell Synaptonemal Site of crossover complex Chromosome duplication Homologues Centromere a. Synaptonemal Homologous complex chromosomes

Meiosis I

Meiosis II

Metaphase I Chiasmata hold Metaphase Homologues do homologues not pair; together. The kinetochores of kinetochores of sister chromatids sister chromatids remain separate; fuse and function as microtubules one. Microtubules attach to both can attach to only kinetochores on one side of each opposite sides of centromere. the centromere. Anaphase I Anaphase

Microtubules pull Microtubules the homologous pull sister chromosomes chromatids apart, but sister apart. chromatids are held together.

MEIOSIS I MEIOSIS II Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II

40 µm 40 µm 40 µm 40 µm 40 µm 40 µm 40 µm 40 µm Chromosome (replicated) Kinetochore microtubule Sister chromatids Nonidentical sister chromatids Spindle Sister chromatids Kinetochore Nuclear microtubule membrane re-forming Spindle Sister Chromosome chromatids

Paired homologous Chiasmata Homologue pair chromosomes on metaphase plate Homologous Homologous chromosomes chromosomes

In prophase I of meiosis I, the In metaphase I, the pairs of In anaphase I, kinetochore In telophase I, the separated chromosomes begin to homologous chromosomes microtubules shorten, and homologues form a cluster at condense, and the spindle of align along the metaphase plate. homologous pairs are pulled each pole of the cell, and the microtubules begins to form. Chiasmata help keep the pairs apart. One duplicated homologue nuclear envelope re-forms The DN A has been replicated, together and produce tension goes to one pole of the cell, while around each daughter cell Nuclear membrane breaking down Chromosome Sister chromatids and each chromosome consists when microtubules from the other duplicated homologue nucleus. Cytokinesis may occur . of two sister chromatids opposite poles attach to sister goes to the other pole. Sister The resulting two cells have half attached at the centromere. In kinetochores of each chromatids do not separate.This the number of chromosomes as Following a typically brief In metaphase II, a completed When microtubules shorten in In telophase II, the nuclear the cell illustrated here, there are homologue. A kinetochore is in contrast to mitosis, where the original cell: In this example, interphase, with no S phase, spindle apparatus is in place anaphase II, the centromeres membranes re-form around four chromosomes, or two pairs microtubule from one pole of the duplicated homologues line up each nucleus contains two meiosis II begins. During in each cell. Chromosomes split, and sister chromatids four di f ferent clusters of of homologues. Homologous cell attaches to one homologue individually on the metaphase chromosomes (versus four in prophase II, a new spindle consisting of sister chromatids are pulled to opposite poles chromosomes. After chromosomes pair up and of a chromosome, while a plate, kinetochore microtubules the original cell). Each apparatus forms in each cell, joined at the centromere align of the cells. cytokinesis, four haploid cells become closely associated kinetochore microtubule from from opposite poles of the cell chromosome is still in the and the nuclear envelope along the metaphase plate in result. No two cells are alike during synapsis. Crossing over the other cell pole attaches to attach to opposite sides of one duplicated state and consists of breaks down. In some species each cell. No w , kinetochore due to the random alignment of occurs, forming chiasmata, the other homologue of a pair. homologue's centromere, and two sister chromatids, but sister the nuclear envelope does not microtubules from opposite homologous pairs at which hold homologous sister chromatids are pulled apart chromatids are not identical re-form in telophase I removing poles attach to kinetochores of metaphase I and crossing over chromosomes together. in anaphase. because crossing over has the need for nuclear envelope sister chromatids. during prophase I. occurred. breakdown. © Clare A. Hasenkampf/Biological Photo Service 7 © Clare A. Hasenkampf/Biological Photo Service 8

MEIOSIS I

Prophase I Metaphase I Anaphase I Telophase I Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS II Parent cell (2n) Prophase II Metaphase II Anaphase II Telophase II Paternal Chromosome homologue replication

Homologous chromosomes pair; Paired homologous chromosomes Homologous chromosomes separate; synapsis and crossing over occur. align on metaphase plate. sister chromatids remain together.

MITOSIS

Prophase Metaphase Anaphase Telophase Four daughter Homologous Chromosome chromosomes replication cells Maternal (each n) homologue Two daughter cells (each 2n) Chromosomes align, sister chromatids separate, and four haploid cells result, each containing half the original number of homologues. Homologous chromosomes Individual homologues align Sister chromatids separate, cytokinesis occurs, and two do not pair. on metaphase plate. cellsresult, each containing theoriginal number of homologues.

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