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Chapter 9.5 - Meiosis

Chapter 9.5 - Meiosis. College Prep Biology Mr. Martino. I. Introduction. Germ Cells: develop into sex cells (sperm and egg) Gametes: mature sex cells – sperm and egg. III. Sexual and Asexual Reproduction.

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Chapter 9.5 - Meiosis

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  1. Chapter 9.5 - Meiosis College Prep Biology Mr. Martino

  2. I. Introduction • Germ Cells: develop into sex cells (sperm and egg) • Gametes: mature sex cells – sperm and egg

  3. III. Sexual and Asexual Reproduction • Sexual Reproduction: reproductive process that includes the union of sperm and egg • Two parents – each donate a complete set of genes • Asexual Reproduction: production of offspring without the participation of sperm and egg • Single parent • Allele: unique form of the same gene (a gene)

  4. IV. Meiosis Halves Chromosome Number • Meiosis: nuclear division that divides chromosomes twice • Diploid number: (2n) pair of each type of chromosome • Homologous chromosomes: both chromosomes of a pair • Same length and shape • Genes are for same traits • Haploid number: (n) results from the second division of meiosis (sex cells)

  5. Sister chromatids: two attached chromosomes • Once they pull apart, they are considered chromosomes • Meiosis consists of 2 divisions: • Meiosis I: DNA is replicated and divides • Meiosis II: another cell division without more chromosome doubling

  6. V. Stages of Meiosis • Meiosis: the process that produces haploid gametes (sex cells) • 2 consecutive divisions: meiosis I and meiosis II • Meiosis II is essentially the same as mitosis • 4 daughter cells produced • Only 1 duplication of chromosomes, second division halves chromosome number

  7. Interphase I • Like interphase in mitosis • Each pair replicates forming a 4 chromatid structure

  8. Prophase I • Most complex phase of meiosis • Lasts 90% of meiosis • homologous chromosomes form tetrads (4 chromatids)

  9. Structure of a Tetrad • Tetrad = structure consisting of 2 pairs of chromosomes • 2 chromosomal pairs = 4 sister chromatids • Visible during Prophase I only

  10. Metaphase I • Tetrads align on metaphase plate

  11. Anaphase I • Begins as homologous chromosomes (still attached at centromere) separate and move towards opposite poles • Only tetrads are separated

  12. Telophase I and Cytokinesis • Chromosomes arrive at opposite poles • Cytokinesis results in 2 diploid cells • No DNA replication occurs between meiosis I and meiosis II

  13. Prophase II • Chromosomes must condense again since there was a pause prior to meiosis II

  14. Metaphase II • Chromosomes are aligned at metaphase plate

  15. Anaphase II • Begins when centromeres of sisters chromatids separate

  16. Telophase II and Cytokinesis • Nuclei form at opposite poles • Cytokinesis occurs • Forms 4 haploid daughter cells

  17. VI.From Gametes to Offspring • Spermatogenesis: sperm formation • results in 4 viable sperm • Oogenesis: egg formation • First meiotic division results in a polar body and a secondary oocyte • Second division results in 3 polar bodies and 1 egg • Polar bodies are reabsorbed – egg is viable

  18. VII.Meiosis and Mitosis Compared • Chromosomes of both replicate only once • Mitosis consists of one cell division and two diploid cells • Meiosis entails two cell divisions and four haploid cells • Mitosis and meiosis II are virtually identical

  19. 9.6 Meiosis Increases Genetic variation among offspring • Genetic variation – differences among the gene combinations in the offspring due to sexual reproduction • Assortment of Chromosomes – the aligning of chromosomes during metaphase I is random; which results in many possible chromosome combinations

  20. Crossing Over – the exchange of genetic material in between homologous chromosomes; during prophase I when tetrads are present

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