MEIOSIS 2 consecutive divisions: meiosis I and meiosis II No DNA synthesis (S phase) between the two divisions

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CHAPTER 13 Offspring acquire genes from parents by inheriting chromosomes Fertilization and meiosis alternate in sexual life cycles Meiosis reduces the number of chromosome sets from diploid to

MEIOSIS 2 consecutive divisions: meiosis I and meiosis II No DNA synthesis (S phase) between the two divisions If these 2n cells united through fertilization, the result would be a 4n zygote.

1 CHAPTER 13 Offspring acquire genes from parents by inheriting chromosomes Fertilization and meiosis alternate in sexual life cycles Meiosis reduces the number of chromosome sets from diploid to haploid Genetic variation produced in sexual life cycles contributes to evolution In mitosis, 2 diploid (2n) daughter cells are produced exactly like the parent cell. MEIOSIS 2 consecutive divisions: meiosis I and meiosis II No DNA synthesis (S phase) between the two divisions If these 2n cells united through fertilization, the result would be a 4n zygote. The result: 4 cells, with ½ the amount of chromosomes as the starting cell. 2n ---- n. Thus, fertilization will be n + n = 2n zygote In animals: used to produce gametes egg and sperm In plants: used to produce spores, which start gametophyte generation Quick Overview of Meiosis Homologous chromosomes pair in process of synapsis Proteins called synaptonemal complex attaches the homologous chromosomes along their lengths this disappears at end of prophase I Pair of homologous chromosomes called a tetrad 1

Chromatids may criss cross called chiasmata which hold homologous pairs together until anaphase I Centrioles migrate Spindle fibers form

Sister chromatids are held together by their centromeres that provide an attachment site for the spindle fibers.

This allows the precise separation of the homologues during Anaphase I.

2 Chromatids may criss cross called chiasmata which hold homologous pairs together until anaphase I Centrioles migrate Spindle fibers form Spindle fibers connect to kinetchores on chromosomes Independent assortment The tetrads are moved to the center of the cell. Sister chromatids are held together by their centromeres that provide an attachment site for the spindle fibers. Separate spindle fibers, one from each pole attach to one member of each homologous pair. This allows the precise separation of the homologues during Anaphase I. Homologous pairs of chromosomes separate & move to poles of cell Chromosomes continue moving until they reach the poles Each pole has a haploid chromosome (one member of pair) Cell divides into 2 daughter chromosomes Chromosomes may unwind slightly & nuclear envelope and nucleolus may reform 2

NO DNA REPLICATION BETWEEN I & II Prophase II - Chromosomes recoil and spindle apparatus forms

toward poles Telophase II nuclei form Oogenesis egg divides cytoplasm unequally Polar bodies

receives most cytoplasm Ovum mature egg cell Spermatogenesis equal division of cytoplasm ; 4

3 NO DNA REPLICATION BETWEEN I & II Prophase II - Chromosomes recoil and spindle apparatus forms Metaphase II chromosomes line at equatorial plate Anaphase II sister chromatids separate & move toward poles Telophase II nuclei form Oogenesis egg divides cytoplasm unequally Polar bodies receive little or no cytoplasm and are nonfunctional ; will be reabsorbed Ootid immature egg cell; receives most cytoplasm Ovum mature egg cell Spermatogenesis equal division of cytoplasm ; 4 functional spermatids Spermatids (immature) will mature into spermatozoa (mature) Comparison of Mitosis and Meiosis 3

Parent cell Metaphase Metaphase I Anaphase/telophase Non-Disjunction: Chromosomes don t pull apart so one cell gets both sister chromatids, while the other cell doesn't get that chromosome.

2n 2n Meiosis II ½ # chromosomes as parent n n n n It is estimated that from 10-20% of all human fertilized eggs contain chromosome abnormalities, and these are the most common cause of pregnancy

4 Parent cell Metaphase Metaphase I Anaphase/telophase Non-Disjunction: Chromosomes don t pull apart so one cell gets both sister chromatids, while the other cell doesn't get that chromosome. Non-disjunction can occur during either meiosis I or meiosis II as illustrated in the two following graphics. 2n 2n Meiosis II ½ # chromosomes as parent n n n n It is estimated that from 10-20% of all human fertilized eggs contain chromosome abnormalities, and these are the most common cause of pregnancy failure. These chromosome abnormalities arise from errors in meiosis, usually meiosis I; occur more often (90%) during egg formation than during sperm formation; become more frequent as a woman ages. Aneuploidy - the gain or loss of whole chromosomes - is the most common chromosome abnormality. Aneuploidy is caused by nondisjunction, the failure of chromosomes to separate correctly Zygotes missing one chromosome ("monsomy") cannot develop to birth (except for females with a single X chromosome). Three of the same chromosome ("trisomy") is also lethal except for chromosomes 13, 18, and 21 Three or more X chromosomes are viable because all but one of them are inactivated 4

MANY PROTISTS AND SOME FUNGI The only diploid stage is the zygote, it then proceeds through

5 In most animals, meiosis is the formation of gametes. Through syngamy (fertilization), the diploid stage is observed. MANY PROTISTS AND SOME FUNGI The only diploid stage is the zygote, it then proceeds through meiosis immediately after syngamy Thus creating a multi-cellular haploid organism. So, how are gametes produced? mitosis In plants: INCLUDES both a diploid stage and a haploid stage 5

6 The diploid sporophyte stage produces haploid spores by meiosis. The spores undergo mitosis, and produce a haploid plant called GAMETOPHYTE. Gametes are produced by mitosis which are fertilized to become a zygote (which is a sporophyte) 6

MEIOSIS & SEXUAL LIFE CYCLES CHAPTER 13

MEIOSIS & SEXUAL LIFE CYCLES CHAPTER 13 THE PROBLEM In mitosis, 2 diploid (2n) daughter cells are produced exactly like the parent cell. If these 2n cells united through fertilization, the result would