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MEIOSIS NOTES 1. Diploidy • Normal cells are diploid (2n): there are two of each chromosome ◦ This means that there are 2 copies of every gene. • Diploidy is useful because 2 copies of every gene means that there a backup copy if one gets mutated. ◦ Mutations are very frequent in the cells of large organisms. We wouldn’t survive with just one copy of each gene. 2. The Problem of Doubling • However, new organisms arise from the merger of two parental cells. If these parental cells were diploid, the offspring would have twice as much DNA as the parents. ◦ This would would mean that over just three subsequent generations, the offspring would have 368 chromosomes. • We know that offspring have the same amount of DNA as their parents, so how is it that the parents only pass on half the genetic material? 3. Haploidy • The answer is that the parental cells that merge to give rise to the offspring are not diploid. They are haploid. ◦ Haploid (1n): Having only one member of each chromosome pair. • Cells become haploid via meiosis. • Meiosis: Process where a diploid cell with two pairs of each chromosome gives rise to haploid sex cells (gametes) with only one of each chromosome. 4. Homologous Pairs • Homologous Chromosomes (homologues): Pair of chromosomes with the same sequence of genes (one chromosome in a homologous pair come from the mother while the other comes from the father). ◦ Just because homologous chromosomes have the same order of genes, that is not to say that the genes are identical. ▪ e.g., if a specific spot on the homologous pairs has a gene for blood type, one homologue's gene may code for the A blood type, the other homologue's gene may code for B blood type 5. Meiosis • Before meiosis starts, the DNA must be copied. This happens during Interphase. • Interphase: Consists of G1, S and G2 phases ◦ The chromosomes are replicated during the S phase • Meiosis consists of two stages: ◦ Meiosis I: ▪ Crossing over ▪ Reduction of chromosome # (from 2n to 1n) ▪ Homologous chromosomes are separated from one another ◦ Meiosis II: ▪ Division of chromosomes ▪ Sister chromatids are separated from one another 6. Meiosis I • Prophase I: There is more going on in meiotic Prophase than in mitotic Prophase ◦ Nuclear membrane & nucleoli disappear ◦ Chromatin condenses to form chromatids ◦ Centrioles migrate to opposite ends and spindle fibers form ◦ Homologous chromosomes pair up and cross over ▪ Crossing Over • Crossing over is the exchange of genetic material between homologous chromosomes • Homologous chromosomes pair up to form tetrads and exchange pieces of their DNA. ◦ Tetrad: Two homologous chromosomes paired up • This process allows for a reshuffling of genetic material. • Metaphase I: Tetrads line up along the middle of the cell. • Anaphase I: The tetrads separated, but the centromeres remain in tact. ◦ This means that the sister chromatids remain attached to one another and it is the homologous pairs that are separated. ◦ In this step, the chromosome number is reduced by half. The cell went from being diploid to being haploid. • Telophase I: ◦ Nuclear membrane reforms. ◦ Chromatids de-condense to form chromatin. ◦ Nucleolus reappears ◦ Spindle apparatus breaks down. • Cytokinesis I: ◦ The cell divides to form two cells. ◦ Each of these cells contains one chromosome of each homologous pair. 7. Interkinesis (AKA Interphase II) • Rest phase between Meiosis I and Meiosis II. • May be long, brief or non-existent (the DNA is NOT replicated during this phase if it occurs). 8. Meiosis II • Prophase II: ◦ Nuclear membrane & nucleoli disappear ◦ Chromatin condenses to form chromatids. ◦ Sister chromatids are attached to one another by a centromere ◦ Centrioles migrate to opposite ends & spindle fibers form • Metaphase II: Chromosomes line up along middle of cell. • Anaphase II: ◦ The centromere of each chromosome splits ◦ The sister chromatids are separated and pulled to opposite ends by the spindle fibers. • Telophase II: ◦ Nuclear membrane reforms. ◦ Chromatids de-condense to form chromatin. ◦ Nucleolus reappears ◦ Spindle apparatus breaks down. • Cytokinesis II: The cell divides 9. Meiosis in Human Males • In males, each of the four cells produced by meiosis will become sperm cells. ◦ The process of making sperm is always occurring from puberty onward. 10. Meiosis in Human Females • Meiosis starts when the female is a fetus, but is halted during prophase I. • During puberty, when ovulation occurs, the egg that is to be released finishes meiosis I. • This produces two different types of cells: an oocyte and a polar body. The polar body may or may not undergo meiosis II; either way, it will eventually die. • The oocyte will begin meiosis II, but stops after prophase II. It will not finish meiosis II until it is fertilized by a sperm. If the oocyte is fertilized, the cell contents are unevenly divided to give rise to two different cells: a large ovum (which will develop into a new human being) and a small polar body which will die. Sister Chromatids { Homologous Chromosomes Protein causes freckles Centromere Protein does not cause freckles Protein causes freckles Causes Friedrich Ataxia Centromere Protein for A type Causes Friedrich Ataxia Protein for A type Centromere Protein for B type CROSSING OVER Step 1: The homologous pairs of each chromosome find each other through random motion Step 2: The sequences on each homologous pair are lined up Step 3: The DNA breaks on both chromosomes and the DNA is physically exchanged Step 4: The chromosomes now have different combinations of genes. Does not cause Friedrich Ataxia OVERVIEW OF MEIOSIS XX XX Crossing Over XX XX Meiosis I X X X X Meiosis II > > Meiosis in Males > < < > < < Meiosis in Females