MEIOSIS makes gametes Meiosis produces gametes Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in years before we knew that DNA contained the genetic material Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells The fusion of gametes is called fertilization It creates the zygote, which contains two copies of each chromosome Meiosis only occurs in the gonads (ovaries and testes). All other cell divisions are mitotic The Sexual Life Cycle The life cycles of all sexually-reproducing organisms follows the same basic pattern Haploid cells or organisms alternate with diploid cells or organisms Figure 9.4 The Stages of Meiosis Meiosis consists of two successive divisions, but only one DNA replication Meiosis I Separates the two homologs Meiosis II Separates the two sister chromatids Meiosis halves the number of chromosomes see Figure 9.5 Prophase I Chromosomes condense (coil) Nuclear membrane breaks down Homologous chromosomes undergo synapsis (Pair up ) Cross over occurs (Recombination) and chromosomes exchange segments newly forming microtubules Metaphase I Homologous chromosome pairs line up at random at the equatorial midline spindle equator one pair of homologous chromosomes Anaphase I Centromeres do not divide Homologs separate and move to opposite poles Telophase I Nuclear membrane reforms Chromosomes uncoil After Cytokinesis Two haploid cells are produced Meiosis II After meiosis I there is a brief interphase No DNA synthesis occurs Meiosis II is similar to mitosis, but with two main differences 1. Haploid set of chromosomes 2. Sister chromatids are not identical because of cross over Prophase II begins with haploid cells Chromosomes coil Spindle forms Nuclear membrane breaks down Each chromosome is composed of two sister chromatids attached at the centromere Metaphase II Chromosomes line up on the midline and attach to spindle fibers Anaphase II Centromeres divide Sister chromatids move to opposite poles Telophase II Nuclear membrane reforms Chromosomes uncoil After Cytokinesis Four unique haploid cells are produced see Figure 9.6 Meiosis I has two unique features 1. Synapsis Homologous chromosomes pair all along their lengths in prophase I While paired homologs cross over 2. Reduction division Homologs separate in Anaphase I reducing the chromosome number in 1/2 (2n to 1n) This produces haploid gametes Evolutionary Consequences of Sex Sexual reproduction increases genetic diversity through three key mechanisms 1. Crossing over 2. Independent assortment 3. Random fertilization DNA exchanges between maternal and paternal chromatid pairs Crossing over see Figure 9.7 Independent Assortment Label these SEE process! In humans, a gamete receives one homolog of each of the 23 chromosomes Humans have 23 pairs of chromosomes Independent assortment gives 2 23 combinations in an egg or sperm 8,388,608 possible kinds of gametes Random fertilization of two independently-produced gametes Therefore, the possible combinations in an offspring 8,388,608 X 8,388,608 = 70,368,744,177,664 More than 70 trillion! And this number does not count crossing-over Independent assortment Three chromosome pairs 2 3 combinations Practice this with colored pens so that you can SEE the difference Compare and Contrast Mitosis and Meiosis* *guaranteed to be on quiz and exam