BIOLOGY CONCEPTS & CONNECTIONS Fourth Edition Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Neil A. Campbell Jane B. Reece Lawrence G. Mitchell Martha R. Taylor From PowerPoint ® Lectures for Biology: Concepts & Connections CHAPTER 8 The Cellular Basis of Reproduction and Inheritance Modules 8.12 – 8.18

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Somatic cells of each species contain a specific number of chromosomes –Human cells have 46, making up 23 pairs of homologous chromosomes MEIOSIS AND CROSSING OVER 8.12 Chromosomes are matched in homologous pairs Chromosomes Centromere Sister chromatids Figure 8.12

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Cells with two sets of chromosomes are said to be diploid Gametes are haploid, with only one set of chromosomes 8.13 Gametes have a single set of chromosomes

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings At fertilization, a sperm fuses with an egg, forming a diploid zygote –Repeated mitotic divisions lead to the development of a mature adult –The adult makes haploid gametes by meiosis –All of these processes make up the sexual life cycle of organisms

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The human life cycle Figure 8.13 MEIOSISFERTILIZATION Haploid gametes (n = 23) Egg cell Sperm cell Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis, like mitosis, is preceded by chromosome duplication –However, in meiosis the cell divides twice to form four daughter cells 8.14 Meiosis reduces the chromosome number from diploid to haploid

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings In the first division, meiosis I, homologous chromosomes are paired –While they are paired, they cross over and exchange genetic information –The homologous pairs are then separated, and two daughter cells are produced

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.14, part 1 MEIOSIS I : Homologous chromosomes separate INTERPHASEPROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Nuclear envelope Chromatin Sites of crossing over Spindle Sister chromatids Tetrad Microtubules attached to kinetochore Metaphase plate Centromere (with kinetochore) Sister chromatids remain attached Homologous chromosomes separate Draw pix on pgs. 162 and 163 in your text.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis II is essentially the same as mitosis –The sister chromatids of each chromosome separate –The result is four haploid daughter cells

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.14, part 2 MEIOSIS II : Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow Sister chromatids separate TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings For both processes, chromosomes replicate only once, during interphase 8.15 Review: A comparison of mitosis and meiosis

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.15 MITOSISMEIOSIS PARENT CELL (before chromosome replication) Site of crossing over MEIOSIS I PROPHASE I Tetrad formed by synapsis of homologous chromosomes PROPHASE Duplicated chromosome (two sister chromatids) METAPHASE Chromosome replication 2n = 4 ANAPHASE TELOPHASE Chromosomes align at the metaphase plate Tetrads align at the metaphase plate METAPHASE I ANAPHASE I TELOPHASE I Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I ; sister chromatids remain together No further chromosomal replication; sister chromatids separate during anaphase II 2n2n2n2n Daughter cells of mitosis Daughter cells of meiosis II MEIOSIS II Daughter cells of meiosis I Haploid n = 2 nnnn

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes Random fertilization also increases variation in offspring

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.16 POSSIBILITY 1POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1Combination 2Combination 3Combination 4

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci 8.17 Homologous chromosomes carry different versions of genes

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.17A, B Coat-color genesEye-color genes BrownBlack CE ce WhitePink CE ce CE ce Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Crossing over is the exchange of corresponding segments between two homologous chromosomes Genetic recombination results from crossing over during prophase I of meiosis –This increases variation further 8.18 Crossing over further increases genetic variability

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.18A Tetrad Chaisma Centromere

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings How crossing over leads to genetic recombination Figure 8.18B Tetrad (homologous pair of chromosomes in synapsis) Breakage of homologous chromatids Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types Coat-color genes Eye-color genes

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Spermatogenesis Spermatogenesis, or sperm production, begins around puberty and continues for the remainder of a man's life A young healthy man produces several hundred million sperm per day.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Spermatogenesis

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24 days

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Oogenesis During embryonic development, diploid cells in the ovaries called oogonia divide by mitosis to produce primary oocytes. The primary oocytes are diploid. Primary oocytes start meiosis. They complete interphase and prophase I. They are “frozen” at the end of prophase I and remain this way until the female reaches puberty. A female is born with about 2 million primary oocytes. By the time she reaches puberty, about 400,000 are left.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Oogenesis cont…. After puberty, each month one of the primary oocytes is selected for ovulation (release from the ovary). Just prior to ovulation, the primary oocyte is “unfrozen” and completes meiosis I, forms the first polar body, and is then frozen in meiosis II at metaphase II. After ovulation, if a sperm penetrates the secondary oocyte (egg) after it is released from the ovary, the secondary oocyte will be stimulated to complete meiosis II forming one more polar body and a mature egg. The first polar body formed from meiosis one may also complete meiosis II to form another polar body.

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Prophase I

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Fertilization takes place in the fallopian tubes

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