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Meiosis

Explore the intricate process of meiosis, where haploid cells form from diploid cells creating genetic diversity through crossing over and recombination. Learn about homologous chromosomes, unique phases of meiosis, and genetic abnormalities. Discover key concepts such as recombination frequency and karyotypes, crucial in understanding genetic variation and chromosomal disorders.

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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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