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Meiosis

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  • 1. Meiosis
  • 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. 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. 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. Your cells have autosomes andsex chromosomes
  • 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Telophase I  Nuclear envelopes may reform, or the cell may quickly start meiosis II.
  • 17. Cytokinesis  Analogous to mitosis where two complete daughter cells form.
  • 18. Meiosis I
  • 19. Meiosis II chromatids of each chromosome are no longer identical because of recombination. chromatids separate from one another
  • 20. Meiosis II
  • 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. 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.