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CELL CYCLE

CELL CYCLE. Meiosis. Chromosome Pairing. Humans have 46 chromosomes arranged in 23 pairs 22 of the 23 pairs are homologous , meaning that the 2 chromosomes in each pair are similar with regard to shape, size, banding pattern, etc. The 23 rd pair is homologous in females but not in males.

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CELL CYCLE

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  1. CELL CYCLE Meiosis

  2. Chromosome Pairing • Humans have 46 chromosomes arranged in 23 pairs • 22 of the 23 pairs are homologous, meaning that the 2 chromosomes in each pair are similar with regard to shape, size, banding pattern, etc. • The 23rd pair is homologous in females but not in males

  3. Haploid vs. Diploid • The body cells of every species have a characteristic number of chromosomes • Diploid: any cell that contains two complete sets of chromosomes (2n) • Haploid: a cell with only one complete set of chromosomes; these are also known as sex cells or gametes (n)

  4. Haploid vs. Diploid (cont.) • Gametes must be haploid • If they weren’t, the chromosome number would double 46 +46 = 92; 92 + 92 =184 • Even one extra chromosome can cause serious defects

  5. Meiosis • To keep the number of chromosomes stable from generation to generation, haploid cells are produced by meiosis • Meiosis is a type of cellular reproduction in which the number of chromosomes is reduced by half, so that the daughter cells are haploid • Haploid gametes combine to create a diploid offspring which grows by mitosis into an adult • As an adult, the offspring will produce haploid gametes

  6. Phases of Meiosis • Meiosis is divided into 2 parts: • Meiosis I • Meiosis II • These 2 parts are divided by interkinesis

  7. Meiosis I • Prophase I • Chromosomes condense • Each homologous pair becomes attached to one another forming a TETRAD [recall that tetrads do not form in mitosis]

  8. Meiosis I (cont.) • Metaphase I • same as mitosis • Anaphase I • same as mitosis • Telophase I • same as mitosis

  9. Interkinesis • Cytokinesis occurs forming 2 new cells • Each chromosome in the haploid cells consists of 2 sister chromatids

  10. Meiosis II • Prophase II • Spindle fibers form again • Chromosomes condense • Metaphase II • Spindle fibers line up the chromosomes in the center of the cell

  11. Meiosis II (cont.) • Anaphase II • Sister chromatids are separated and pulled to opposite poles of the cell • Telophase II • Nuclei form • Spindle fibers break down

  12. Cytokinesis • Just as in mitosis, meiosis is followed by cytokinesis • Cytokinesis produces 4 haploid cells • Each cell contains one copy of one complete set of chromosomes (n)

  13. Human Cytokinesis • In the human male, cytokinesis divides the cytoplasm evenly creating 4 sperm cells • In the human female, cytokinesis divides the cytoplasm unevenly so that one cell receives almost all of the cytoplasm

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