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1. Meiosis and chromosome number 2. Steps in meiosis 3. Source of genetic variation a. Independent alignment of homologues b. Recombination • Somatic cells are diploid. • Gametes are haploid, with only one set of chromosomes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis reduces the number of genomes from diploid to haploid Haploid gametes (n = 23) • human life cycle • Meiosis creates gametes Egg cell Sperm cell MEIOSIS • Mitosis of the zygote produces adult bodies Figure 8.13 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings FERTILIZATION Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development Steps in meiosis I MEIOSIS I: Homologous chromosomes separate INTERPHASE Centrosomes (with centriole pairs) Nuclear envelope PROPHASE I METAPHASE I Microtubules attached to Spindle kinetochore Sites of crossing over Chromatin Sister chromatids Tetrad Figure 8.14, part 1 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Metaphase plate Centromere (with kinetochore) ANAPHASE I Sister chromatids remain attached Homologous chromosomes separate • In meiosis I, homologous chromosomes are paired – While paired, they cross over and exchange genetic information (DNA) – homologous pairs are then separated, and two daughter cells are produced Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Cleavage furrow Sister chromatids separate Figure 8.14, part 2 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Haploid daughter cells forming • Meiosis II is essentially the same as mitosis – sister chromatids of each chromosome separate – result is four haploid daughter cells Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Diploid MITOSIS MEIOSIS 1 somatic cell gamete precursor 2n Diploid 2n duplication 2 2n 2n 3 2n 2n 2n 4 2n division diploid 2n 2n haploid 5 1n 1n 6 division 7 1n 1n 1n 1n Homologous chromosomes carry different versions of genes (alleles) at corresponding loci • Each chromosome of a homologous pair comes from a different parent – Each chromosome thus differs at many points from the other member of the pair Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Independent alignment of homologous chromosomes POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Figure 8.16 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Combination 3 Combination 4 Crossing over further increases genetic variability • the exchange of corresponding segments between two homologous chromosomes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Tetrad Chaisma Centromere Figure 8.18A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSIS I END OF INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Genetic recombination results from crossing over during prophase I of meiosis MEIOSIS TELOPHASE I PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II INDEPENDENT ASSORTMENT TELOPHASE II METAPHASE II METAPHASE I METAPHASE I a SPERMATOGENESIS b OOGENESIS spermatogonium oogonium primary spermatocyte primary oocyte meiosis l secondary spermatocyte secondary oocyte meiosis ll polar body spermatids polar bodies (will be degraded) egg Accidents during meiosis can alter chromosome number • Abnormal chromosome count is a result of nondisjunction – Either homologous pairs fail to separate during meiosis I Nondisjunction in meiosis I Normal meiosis II Gametes n+1 n+1 n–1 n–1 Number of chromosomes Figure 8.21A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings – Or sister chromatids fail to separate during meiosis II Normal meiosis I Nondisjunction in meiosis II Gametes n–1 n+1 n Number of chromosomes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings n Figure 8.21B • Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Egg cell n+1 Zygote 2n + 1 Sperm cell n (normal) Figure 8.21C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Connection: An extra copy of chromosome 21 causes Down syndrome • This karyotype shows three number 21 chromosomes • An extra copy of chromosome 21 causes Down syndrome Figure 8.20A, B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The chance of having a Down syndrome child goes up with maternal age Figure 8.20C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Connection: Abnormal numbers of sex chromosomes do not usually affect survival • Nondisjunction can also produce gametes with extra or missing sex chromosomes – Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Table 8.22 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings