There is a Need for Cell Division During development For the fertilized egg to divide into a multicellular organism In adults To repair damage, and replace worn out cells And is needed in reproduction © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e Mitosis & Cytokinesis Interphase Mitosis Prophase (late Prophase) Metaphase Anaphase Telophase and cytokinesis © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

Term: Homologous Chromosomes 46 chromosomes are arranged in 23 pairs One came from each parent Talking only about humans 22 pairs are autosomes Both chromosomes are homologs 1 pair are sex chromosomes XX -females XY -males (~homologs) © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e Meiosis Used to make gametes Eggs and sperm Chromosome number is halved (haploid) Zygote is diploid after fertilization Meiosis 2n n Fertilization 2n Mitosis 2n © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

Meiosis I vs. Mitosis In Anaphase, Sister Chromatids Separate, diploid Prophase Metaphase Anaphase Telophase and cytokinesis Mitosis Meiosis In Anaphase, Sister Chromatids Separate, diploid Sister Chromatids remain attached, Homologs separate haploid © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

Meiosis II (prophase II not shown) Metaphase II Anaphase II Telophase II & Cytokinesis © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e Meiosis vs. Mitosis Mitosis 1 cell into 2 Meiosis 1 cell into 4 Mitosis daughter cells remain diploid Meiosis cells become haploid Mitosis in anaphase, sister chromotids separate. Meiosis in anaphase I, homologs separate, cells destined to become haploid. Mitosis for cell replication including asexual reproduction, meiosis for sexual reproduction © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

Telophase I/Prophase II Complication I have kept things simple. In some species, cytokinesis begins before telophase I or II end. Different species perform different things in telophase I, cytokinesis, & prophase II of meiosis. Example, some species never reform the nuclear membrane in telophase I, so they do not degrade it in prophase II. We will keep things simple. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e Concept Quiz Meiosis insures that Each gamete receives the same genes Chromosome number is doubled in the gametes Zygotes produced by fertilization have the normal number of chromosomes All paternal chromosomes end up in the same gamete The correct answer is C. This is the key to meiosis – reduction division so that the fertilized egg restores the normal number of chromosomes. Answer A: Each gamete doesn’t receive the same genes because the homologous chromosomes are randomly assorted during meiosis I. Answer B: Chromosome number is halved in the gametes. Answer D: As in the answer for A, the homologous chromosomes are randomly assorted during meiosis I. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e Review Animation on The Cell Cycle mitosis on web © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e Review Mitosis and Cell Division Mitosis movie (shown in lab) If does not work, email me. Meiosis movie © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

© 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e In Class Activity If time, start in class activity on mitosis & meiosis. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e