1 / 20

Problem:

Problem:. Our cells have 46 chromosomes—so why don’t fertilized eggs have 92 chromosomes?. Answer:. Meiosis. Meiosis. The form of cell division by which gametes, with half the number of chromosomes, are produced. Diploid (2n)  haploid (n) Meiosis is sexual reproduction.

chapa
Télécharger la présentation

Problem:

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Problem: Our cells have 46 chromosomes—so why don’t fertilized eggs have 92 chromosomes?

  2. Answer: Meiosis

  3. Meiosis • The form of cell divisionby which gametes, with half the number of chromosomes,are produced. • Diploid (2n)  haploid (n) • Meiosis is sexual reproduction. • Two divisions (meiosis I and meiosis II).

  4. sperm n=23 n=23 egg 2n=46 zygote Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg

  5. Meiosis • Sex cells divide to produce gametes(sperm or egg). • Gametes have half the # of chromosomes. • Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis • Meiosis is similar to mitosis with some chromosomal differences.

  6. n=23 human sex cell sperm n=23 n=23 2n=46 haploid (n) n=23 n=23 diploid (2n) n=23 meiosis I meiosis II Spermatogenesis

  7. Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes

  8. Meiosis I (four phases) • Cell division that reduces the chromosome number by one-half. • four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

  9. Homologous chromosomes sister chromatids sister chromatids Tetrad Prophase I - Synapsis

  10. Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata. • Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. • Chiasmata (chiasma) are the sites of crossing over.

  11. Tetrad nonsister chromatids chiasmata: site of crossing over Crossing Over - variation variation

  12. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

  13. OR metaphase plate metaphase plate Metaphase I

  14. Anaphase I

  15. Telophase I

  16. Meiosis II • No interphase II (or very short - no more DNA replication) • Remember: Meiosis II is similar to mitosis

  17. Prophase II • same as prophase in mitosis

  18. metaphase plate metaphase plate Metaphase II • same as metaphase in mitosis

  19. Anaphase II • same as anaphase in mitosis • sister chromatids separate

  20. Telophase II

More Related