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10.2 MEIOSIS

10.2 MEIOSIS. Martin 2022. GENES, CHROMOSOMES, AND NUMBERS. A. Dipolid and Haploid Cells: Diploid Cells: cell that have two of each kind of chromosome 2n Found in all “body” cells Haploid Cells: only have 1 of each kind of chromosome n Found ONLY IN SEX CELLS (sperm and egg)

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10.2 MEIOSIS

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  1. 10.2 MEIOSIS Martin 2022

  2. GENES, CHROMOSOMES, AND NUMBERS • A. Dipolid and Haploid Cells: • Diploid Cells: cell that have two of each kind of chromosome • 2n • Found in all “body” cells • Haploid Cells: only have 1 of each kind of chromosome • n • Found ONLY IN SEX CELLS (sperm and egg) • Sex cells also known as gametes

  3. GENES, CHROMOSOMES, AND NUMBERS • B. Homologous Chromosomes: • Two chromosomes in each pair of a diploid cell • Each pair has genes for the same trait • Genes arranged in the same order, but may be different in their alleles (dominant or recessive)

  4. GENES, CHROMOSOMES, AND NUMBERS • Why Meiosis?: • Meiosis: a type of cell division that allows gametes (sex cells) to form, so that after fertilization, offspring have the same number of chromosomes as their parents • Sperm: male gametes • Egg: female gametes

  5. GENES, CHROMOSOMES, AND NUMBERS • Meiosis is used to create cells for sexual reproduction: • Male gamete (23) fertilizes female gamete (23) and produces a zygote (offspring with 46 chromosomes) • Remember, this is different from mitosis • Mitosis = 2n  2n • Meiosis = 2n  n

  6. THE PHASES OF MEIOSIS • PMAT I, then PMAT II • Has 2 cell divisions that lead to 4 haploid cells • During PROPHASE I • 2 homologous chromosomes pair so tightly that crossing overcan occur • Where non-sister chromatids can break off and exchange genetic information • Can occur at ANY LOCATION on a chromosome, at several locations, and at the same time

  7. Crossing Over

  8. THE PHASES OF MEIOSIS • METAPHASE I • Homologous chromosomes line up along the midline/equator as a pair (tetrads) • ANAPHASE I • Homologous chromosomes separate • METAPHASE II • Chromosomes line up independently/randomly along equator • ANAPHASE II • Sister chromatids are separated

  9. MEIOSIS PROVIDES FOR GENETIC VARIATION • This variation can be because of crossing over during metaphase I • Can also be due to genetic recombination • the reassortment of chromosomes and the genetic information they carry by crossing over or independent segregation of homologous chromsomes

  10. NONDISJUNCTION • Sometimes there’s an error in meiosis • Nondisjunction: • The failure of homologous chromsomes to separate during meiosis • Leaves 1 gamete with an extra chromosome, and 1 gamete with 1 missing chromosome (22, 24) • Trisomy 21 = Down Syndrome

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