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Meiosis 15 October, 2004 Text Chapter 13

Meiosis 15 October, 2004 Text Chapter 13

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Meiosis 15 October, 2004 Text Chapter 13

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  1. Meiosis15 October, 2004Text Chapter 13

  2. Asexual Reproduction In asexual reproduction, individuals give rise to genetically identical offspring (clones). All cell division in this process is mitotic.

  3. Important Terms (review) genome: all of a cell’s DNA chromosome: individual DNA molecule chromatin: chromosomal DNA and its protein scaffold sister chromatids: the two identical copies of a duplicated chromosome centromere: the point of attachment for two sister chromatids somatic cells: (most cells) have two copies of each chromosome (diploid) gametes: (reproductive cells; sperm and eggs) have one copy of each chromosome (haploid) locus: the location of a gene on a chromosome (sometimes = gene)

  4. More Important Terms homologous chromosomes: the two similar but nonidentical chromosomes that make up a pair. sex chromosomes: in humans, X and Y chromosomes. Not truly homologous autosomes: all other chromosomes fertilization: the uniting of two gametes to form a zygote (n + n goes to 2n) meiosis: the process of gametogenesis (2n goes to n + n + n + n)

  5. Describing Chromosomes

  6. Timing of Meiosis and Fertilization Some organisms have free-living multicellular haploid phases.

  7. Chromosomes and Meiosis Meiosis reduces the chromosome number from diploid to haploid. First, the DNA in a diploid cell is duplicated in S phase. Cells resulting from the first meiotic division are haploid duplicated. The products of meiosis are haploid gametes.

  8. Meiosis I with Crossing Over

  9. Meiosis II - The products of meiosis are genetically different from each other and from the parent cell.

  10. Mitosis and Meiosis Meiosis differs from mitosis in the number of divisions, the process of synapsis, and the number and genetic composition of daughter cells

  11. Independent Assortment Independent assortment of chromosomes during the first meiotic division contributes to genetic variation. Here, there are 22 = 4 possible gametes In humans, there are 223 = about 8 million possible gametes Random fertilization allows for (8 million)2 = 64 trillion possible offspring from any two parents. Crossing over occurs two to three times per chromosome, and increases the number of possible offspring even further.