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Understanding Meiosis: The Cellular Basis of Inheritance

Explore the process of meiosis, chromosome pairing, and genetic recombination. Discover how cells divide to form haploid gametes, contributing to genetic variation.

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Understanding Meiosis: The Cellular Basis of Inheritance

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  1. CHAPTER 8The Cellular Basis of Reproduction and Inheritance Modules 8.12 – 8.18

  2. MEIOSIS AND CROSSING OVER 8.12 Chromosomes are matched in homologous pairs • Somatic cells of each species contain a specific number of chromosomes • Human cells have 46, making up 23 pairs of homologous chromosomes Chromosomes Centromere Sister chromatids Figure 8.12

  3. 8.13 Gametes have a single set of chromosomes • Cells with two sets of chromosomes are said to be diploid • Gametes are haploid, with only one set of chromosomes

  4. 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 • At fertilization, a sperm fuses with an egg, forming a diploid zygote

  5. Haploid gametes (n = 23) Egg cell • The human life cycle Sperm cell MEIOSIS FERTILIZATION Diploidzygote (2n = 46) Multicellulardiploid adults (2n = 46) Mitosis anddevelopment Figure 8.13

  6. 8.14 Meiosis reduces the chromosome number from diploid to haploid • Meiosis, like mitosis, is preceded by chromosome duplication • However, in meiosis the cell divides twice to form four daughter cells

  7. While they are paired, they cross over and exchange genetic information • The homologous pairs are then separated, and two daughter cells are produced • In the first division, meiosis I, homologous chromosomes are paired

  8. MEIOSIS I: Homologous chromosomes separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes(withcentriolepairs) Microtubules attached tokinetochore Metaphaseplate Sister chromatidsremain attached Sites of crossing over Spindle Nuclearenvelope Sisterchromatids Tetrad Centromere(with kinetochore) Homologouschromosomes separate Chromatin Figure 8.14, part 1

  9. The sister chromatids of each chromosome separate • The result is four haploid daughter cells • Meiosis II is essentially the same as mitosis

  10. MEIOSIS II: Sister chromatids separate TELOPHASE IAND CYTOKINESIS TELOPHASE IIAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavagefurrow Sister chromatidsseparate Haploiddaughter cellsforming Figure 8.14, part 2

  11. 8.15 Review: A comparison of mitosis and meiosis • For both processes, chromosomes replicate only once, during interphase

  12. MITOSIS MEIOSIS PARENT CELL(before chromosome replication) Site ofcrossing over MEIOSIS I PROPHASE I Tetrad formedby synapsis of homologous chromosomes PROPHASE Chromosomereplication Chromosomereplication Duplicatedchromosome(two sister chromatids) 2n = 4 Chromosomes align at the metaphase plate Tetradsalign at themetaphase plate METAPHASE I METAPHASE ANAPHASE I TELOPHASE I ANAPHASETELOPHASE Sister chromatidsseparate duringanaphase Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together Haploidn = 2 Daughtercells of meiosis I 2n 2n No further chromosomal replication; sister chromatids separate during anaphase II MEIOSIS II Daughter cellsof mitosis n n n n Daughter cells of meiosis II Figure 8.15

  13. 8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring • 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

  14. 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

  15. POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Figure 8.16

  16. 8.17 Homologous chromosomes carry different versions of genes • The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci

  17. Coat-color genes Eye-color genes C E Brown Black C E C E c e c e c e White Pink Tetrad in parent cell(homologous pair ofduplicated chromosomes) Chromosomes ofthe four gametes Figure 8.17A, B

  18. 8.18 Crossing over further increases genetic variability • 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

  19. Tetrad Chaisma Centromere Figure 8.18A

  20. Coat-colorgenes Eye-colorgenes Tetrad(homologous pair ofchromosomes in synapsis) 1 Breakage of homologous chromatids • How crossing over leads to genetic recombination 2 Joining of homologous chromatids Chiasma Separation of homologouschromosomes at anaphase I 3 Separation of chromatids atanaphase II and completion of meiosis 4 Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Figure 8.18B Gametes of four genetic types

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