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Meiosis

Meiosis. AP Biology Unit 3. Meiosis. Process that occurs to form haploid cells from diploid cells Forms gametes– sperm and egg. Homologous Chromosomes. A pair of chromosomes containing the same sets of genes One chromosome from each parent

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Meiosis

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  1. Meiosis AP Biology Unit 3

  2. Meiosis • Process that occurs to form haploid cells from diploid cells • Forms gametes– sperm and egg

  3. Homologous Chromosomes • A pair of chromosomes containing the same sets of genes • One chromosome from each parent • Don’t necessarily contain identical genetic material • Ex. You get one chromosome #4 from mom and one chromosome #4 from dad

  4. Homologous Chromosomes vs. Sister Chromatids • Homologous chromosomes contain the same information, but aren’t identical to one another • Sister chromatids are identical copies of the same chromosome.

  5. Meiosis Overview • Chromosomes are replicated to form sister chromatids before meiosis • 2 stages • Meiosis I  homologous pairs are separated • Meiosis II  sister chromatids are separated • Results in 4 haploid daughter cells • In humans, this means there are 23 chromosomes in each haploid cell

  6. No DNA is copied between Meiosis I and II

  7. Metaphase I: homologous pairs line up in the middle of the cell Anaphase I: homologous pairs pulled apart

  8. Metaphase II: Chromosomes line up single file

  9. Unique to Meiosis • Synapsis and Crossing Over in Prophase I • Synapsis = when homologous chromosomes line up next to one another  form a tetrad • Crossing over = the non-sister chromatids in the pair exchange genetic material

  10. Crossing Over and Genetic Diversity • Crossing Over leads to more genetic diversity in a species • Why? • By exchanging some genetic material, the haploid cells formed through meiosis are no longer identical– more variety

  11. Unique to Meiosis • Homologous Chromosomes line up next to each other (form tetrads) • Homologous chromosomes, NOT sister chromatids, separate from each other during Anaphase I

  12. Sexual vs. Asexual Reproduction

  13. Example of Asexual Reproduction-- Budding

  14. Recombination • Another name for Crossing Over • Occurs in Prophase I when homologous chromosomes pair up • Results in genetic variation • In humans, 1-3 crossover events per chromosome

  15. Recombination Frequency • Likelihood of crossing over between 2 genes is directly related to the distance between them • If 2 genes are close to one another on a chromosome there is less of a chance they’ll be separated by crossing over • If 2 genes are far from one another on a chromosome there is a greater chance they’ll be separated by crossing over

  16. Recombination Frequency • When 2 genes are separated by crossing over we say they have recombined. • High recombination frequency = likely to be separated by crossing over • Low recombination frequency = not likely to be separated by crossing over

  17. Recombination Frequency • If 2 genes have a high recombination frequency, are they close together or far apart? • Far apart– greater % chance of being separated

  18. Linkage Maps • A map (diagram of relative positions of genes) can be made by considering recombination frequencies

  19. Sample Problem • What is the order of genes given the following recombination frequencies?

  20. Answer • Step 1: Start by determining which genes are closest together • C & D are closest  5% recombination freq. C--- (5) --- D • Now, determine the position of one more gene relative to these, let’s say gene A • A is closer to C (10%) compared to D (15%) A ----- (10)------ C --- (5) --- D

  21. Answer (continued) • Now, put in the final gene • B is farthest from A, then C, then D according to the numbers A ----- (10) ------- C --- (5) --- D ---- (25) ---- B

  22. Nondisjunction • Meiotic spindle works incorrectly. • Chromosomes fail to separate properly. • May occur during meiosis I or II • Meiosis I: homologous chromosomes don’t separate • Meiosis II: sister chromatids don’t separate

  23. Polyploidy vs. Aneuploidy • Polyploidy = having too many chromosomes ( more than 2 copies/sets of a chromosome) • Aneuploidy = having an abnormal number of chromosomes (either too few or too many)

  24. Trisomic vs. Monosomic • Trisomic = having 3 copies of a chromosome • Monosomic = only having 1 copy of a chromosome

  25. Meiosis Problems • Extra or missing parts of chromosomes can also occur due to errors in crossing over

  26. Why is this a problem? • Why might an increase of DNA result in physical problems? • Proteins overproduced, not enough regulators to regulate all genes, etc. • Why might a decrease of DNA result in physical problems? • Missing genes or portions of genes, regulatory proteins not made, etc

  27. Karyotypes • Karyotypes are used to determine if there are chromosomal abnormalities • Only tells you if there are the right number of chromosomes • Does NOT tell you if specific genes are mutated

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