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Transcript
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) Sex cells also known as gametes 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) 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 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 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 over can 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 Crossing Over 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 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 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