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Module 3B – and Sexual Life Cycles Objective # 12  In this module, we will examine a second type of division used by eukaryotic cells called meiosis..meiosis Explain the role of ,  In addition, we will see how the 2 meiosis, and fertilization in types of eukaryotic , the life cycle of and mitosis and meiosis, are involved in . transmitting genetic information from one generation to the next during eukaryotic life cycles. 1 2

Objective 12 Objective 12

 In eukaryotic life cycles:

 Unlike , which are almost  Mitosis keeps the number of always haploid (each cell has one set of in each cell the same. genetic instructions) , most  Meiosis reduces the number of have a life cycle that alternates chromosomes from 2 sets to 1 set. between haploid and diploid stages.  Fertilization doubles the number of chromosomes from 1 set to 2 sets.

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Objective 12 Typical Life Cycle of and Fungi

Haploid cells (n)  Alternation of meiosis and fertilization

makes possible the development of life MITOSIS cycles that alternate between haploid and diploid stages. MITOSIS (n)  These life cycles involve sexual where two parents each n contribute half the genetic material 2n needed to form a new individual.

5 (2n) 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1 Typical Life Cycle Typical Life Cycle

Gametes (n)

MITOSIS Gametes (n)

Haploid n Haploid cells (n) individual (n) 2n n MEIOSIS FERTILIZATION 2n

Diploid individual (2n) Zygote (2n) Diploid MITOSIS individual (2n) MITOSIS Zygote (2n) 7 8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Objective # 13 Objective 13

 Only diploid cells can divide by meiosis. List, describe, diagram, and  Meiosis involves 2 consecutive cell identify the stages of meiosis. divisions. Since the DNA is duplicated prior to the first division only , the final result is 4 haploid cells.  We will examine the stages of meiosis in a diploid cell where 2N = 6:

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Objective 13 I: Overview of meiosis in a cell where 2N = 6  Chromosomes condense. Because of replication during , each DNA duplication consists of 2 identical during interphase joined by .  – the 2 members of each pair Meiosis II of homologous chromosomes line up sideside--byby--sideside and are closcloselyely associated Meiosis I by a to form a (2 associated chromosomes), After meiosis I the cells are haploid. 11 also called a tetrad (4 chromatids): 12

2 Synapsis Produces Bivalents or Tetrads: Prophase I:

Kinetochore Homologues  During synapsis, sometimes chromatids break and there is an exchange of homologous parts between nonnon--sistersister chromatids. This exchange is called crossing over..  Evidence of crossing over can be seen under the microscope as XX--shapedshaped Synaptonemal structures called chiasmata. complex 13 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Crossing Over between Non-sister Chromatids

Site of crossover

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Prophase I: Prophase I

 Chiasmata hold homologous chromosomes together as they move within the cell.  The cytoskeleton is disassembled and a spindle of begins to form.  The Golgi and ER are dispersed.  The disintegrates.

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3 I: Prometaphase I

 The of sister chromatids fuse and as one. A from one pole of the cell attaches to one member of each hlhomologous pifhpair of chromosomes w hile a kinetochore microtubule from the other pole attaches to the other member.  Homologous chromosomes remain paired as they move toward equator of the cell. 19 20

Metaphase I: Possible Alignment of Chromosomes during I :  Homologous chromosomes line up along *We will the equatorial plate in pairs. follow this  Alignment of each pair of chromosomes alignment. There are relative to the poles of the cell is random actually 8 and independent of the other pairs. This possibilities, * is referred to as independent assortment..assortment the other 4 being the reverse of those shown. 21 22

Anaphase I: I

 Kinetochore microtubules contract, pulling the 2 members of each homologous pair of chromosomes to opposite poles of the cell.  do NOT split and sister chidihromatids remain attac hdhed.  When the kinetochore microtubules have fully contracted, each pole has onecomplete set of duplicated chromosomes.

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4 I: Telophase I  A nuclear membrane reforms around each set of duplicated chromosomes  divides the original cell into 2 haploidhaploid,, non-non-identicalidentical,, daughter cells.  Each chromosome is still duplicated and consists of 2 chromatids, but the sister chromatids are not identical due to crossing over during prophase I.

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Prophase II: Prophase II

 Following a typically brief interphase, with no , meiosis II begins.  In each daughter cell, the nuclear envelope breaks down and a new spindle of microtubules starts to form.

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Prometaphase II: Prometaphase II  For each chromosome, a kinetochore microtubule from one pole of the cell attaches to the kinetochore on one while a kinetochore microtubule from the other pole attaches to the kinetochore on the other chromatid.  Duplicated chromosomes begin to move towards the equatorial plate.

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5 Metaphase II: Metaphase II

 Duplicated chromosomes line up along the equatorial plate in single file.

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Anaphase II: Anaphase II

 Centromeres split and each former chromosome becomes 2 separate chromosomes.  Kinetochore microtubules contract pulling the 2 members of each pair of chromosomes (that were formerly sister chromatids) to opposite poles of the cell.

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Telophase II: Telophase II  Nuclear envelope reforms around each group of daughter chromosomes.  Chromosomes uncoil.  Cytokinesis divides the 2 cells from meiiIi4hlidlliosis I into 4 haploid cells.  No two cells are alike due to crossing over during prophase I and independent assortment of homologous pairs during metaphase I. 35 36

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Objective # 14

Describe the processes of independent assortment and cro ssing over a nd ex pla in the ir biological significance.

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Crossing Over between Non-sister Chromatids

Site of crossover

41 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

7 Objective 14

 Independent assortment and crossing over increase among the daughter cells produced during meiosis.  If there are n pairs of chromosomes in the original cell, independent assortment produces 2n possible chromosome combinations in the daughter cells. 43 44

Objective 14 Objective # 15

 In addition, almost every cross over produces chromosomes with a new combination of maternal and paternal Compare mitosis with . meiosis and explain the  Therefore, as a result of independent importance of each. assortment and crossing over, an almost unlimited number of combinations are possible in the daughter cells produced by meiosis. 45 46

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8 Objective 15 Objective 15 Mitosis Meiosis Mitosis Meiosis Homologous Synapsis One diploid cell produces One diploid cell chromosomes do not 2 diploid cells OR one produces 4 pair up haploid cell produces 2 haploid cells No genetic exchange Crossing over haploid cells between homologous New cells are genetically Each new cell has chromosomes identical to original cell a unique DNA duplication DNA duplication (except for ) combination of followed by 1 cell followed by 2 cell genes division divisions 49 50

Objective 15 Importance:  Mitosis allows a cell to produce more identical copies of itself. This is the basis for .  Meiosis reduces the number of chromosomes from 2 sets to 1 set. This is the basis for life cycles where each parent contributes half the genes

needed to produce a new individual. 51

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