Meiosis

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Meiosis

  1. 1. Meiosis
  2. 2. You have body cells and gametes DNA in these cells is passed on to offspring DNA in these cells is NOT passed on to offspring
  3. 3. You have body cells and gametes DNA in these cells is passed on to offspring DNA in these cells is NOT passed on to offspring
  4. 4. Ploidy: Number of sets ofchromosomes in a cell Haploid (n)-- one set chromosomes Diploid (2n)-- two sets chromosomes Most plant and animal adults are diploid (2n) Eggs and sperm are haploid (n)
  5. 5. Your cells have autosomes andsex chromosomes
  6. 6. Summary of chromosomecharacteristics Diploid set for humans; 2n = 46 Autosomes - homologous chromosomes, one from each parent (humans = 22 sets of 2) Sex chromosomes (humans have 1 set of 2)  Female-sex chromosomes are homologous (XX)  Male-sex chromosomes are non- homologous (XY)
  7. 7. Mitosis The nucleus divides to form 2 nuclei  Prophase  Metaphase  Anaphase  Telophase Cytokinesis – the cytoplasm of the cell is divided in half and the cell membrane grows to enclose each cell
  8. 8. Asexual (vegetative)reproduction A form of duplication using only mitosis. Example,  a new plant grows out of the root or  a shoot from an existing plant. Produces only genetically identical offspring since all divisions are by mitosis.  Offspring called clones meaning that each is an exact copy of the original organism  Method of reproduction is rapid and effective  Since the offspring are identical, there is no mechanism for introducing diversity
  9. 9. Meiosis2 divisions of the nucleus  Meiosis I  Meiosis IIA form of cell division that halves thenumber of chromosomes whenforming specialized reproductivecells, such as gametes or spores
  10. 10. Formation of Diploid & Haploid Cells Mitosis Meiosis 2n 2n 2n 2n n n n n n n2 diploid cells produced 4 haploid cells produced
  11. 11. Sexual Reproduction Formation of new individual by a combination of two haploid sex cells (gametes). Gametes for fertilization usually come from separate parents  Female- produces an egg  Male - produces sperm The new individual is called a zygote, with two sets of chromosomes (diploid). Meiosis is a process to convert a diploid cell to a haploid gamete, and cause a change in the genetic information to increase diversity in the offspring.
  12. 12. What is Meiosis I? Chromosomes in a diploid cell re- segregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity!
  13. 13. Prophase I  homologous chromosomes pair, a step unique to meiosis  Chromosomal condensation allows these to be viewed in the microscope.  Note that there are 2 chromosomes and 4 chromatids, with one chromosome coming from each parent.
  14. 14. Metaphase I  Paired chromosomes (four chromatids) align at the equator  The orientation is random, with either parental homologue on a side.  This means that there is a 50-50 chance for the daughter cells to get either the mothers or fathers homologue for each chromosome.
  15. 15. Anaphase I  Paired chromosomes separate.  Chromosomes, each with two chromatids, move to separate poles.  Each of the daughter cells is now haploid (23 chromosomes)
  16. 16. Telophase I  Nuclear envelopes may reform, or the cell may quickly start meiosis II.
  17. 17. Cytokinesis  Analogous to mitosis where two complete daughter cells form.
  18. 18. Meiosis I
  19. 19. Meiosis II chromatids of each chromosome are no longer identical because of recombination. chromatids separate from one another
  20. 20. Meiosis II
  21. 21. Comparing Meiosis & Mitosis Chromosome behavior  Mitosis: Homologous chromosomes independent  Meiosis: Homologous chromosomes pair forming bivalents until anaphase I Chromosome number- reduction in meiosis  Mitosis- identical daughter cells  Meiosis- daughter cells haploid Genetic identity of progeny:  Mitosis: identical daughter cells  Meiosis: daughter cells have new assortment of parental chromosomes  Meiosis: chromatids not identical, crossing over
  22. 22. Meiotic Errors Nondisjunction- homologues dont separate in meiosis 1  Usually embryo lethal  Trisomy 21, exception leading to Downs syndrome  Sex chromosomes  Turner syndrome: monosomy X  Klinefelter syndrome: XXY Translocation and deletion: transfer of a piece of one chromosome to another or loss of fragment of a chromosome.

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