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Meiosis Formation of Gametes (Eggs & Sperm)

Meiosis Formation of Gametes (Eggs & Sperm). Facts About Meiosis. Preceded by interphase which includes chromosome replication Two meiotic divisions --- Meiosis I and Meiosis II Called Reduction- division Original cell is diploid (2n) Four daughter cells produced that are haploid (1n).

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Meiosis Formation of Gametes (Eggs & Sperm)

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  1. MeiosisFormation of Gametes(Eggs & Sperm)

  2. Facts About Meiosis • Preceded by interphase which includes chromosome replication • Twomeiotic divisions --- Meiosis I and Meiosis II • Called Reduction- division • Original cell is diploid (2n) • Four daughter cellsproduced that are haploid (1n)

  3. Facts About Meiosis • Daughter cells contain half the number of chromosomes as the original cell • Produces gametes (eggs & sperm) • Occurs in the testes in males (Spermatogenesis) • Occurs in the ovaries in females (Oogenesis)

  4. More Meiosis Facts • Start with 46 double stranded chromosomes (2n) • After 1 division - 23 double stranded chromosomes (n) • After 2nd division - 23 single stranded chromosomes (n) • Occurs in our germ cells that produce gametes

  5. Why Do we Need Meiosis? • It is the fundamental basis of sexual reproduction • Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote

  6. Fertilization – “Putting it all together” 2n = 6 1n =3

  7. Replication of Chromosomes • Replication is the process of duplicating a chromosome • Occurs prior to division • Replicated copies are called sister chromatids • Held together at centromere Occurs in Interphase

  8. SisterChromatids (same genes, same alleles) Homologs (same genes, different alleles) A Replicated Chromosome Gene X Homologs separate in meiosis I and therefore different alleles separate.

  9. Meiosis I MeiosisII Meiosis: Two Part Cell Division Sister chromatids separate Homologs separate Diploid Diploid Haploid

  10. Spindle fibers Nucleus Nuclear envelope Meiosis I: Reduction Division Early Prophase I (Chromosome number doubled) Late Prophase I Metaphase I Anaphase I Telophase I (diploid)

  11. Prophase I • Late prophase • Chromosomes condense. • Spindle forms. • Nuclear envelope fragments. • Early prophase • Homologs pair. • Crossing over occurs.

  12. Tetrads Form in Prophase I Homologous chromosomes(each with sister chromatids)   Join to form a TETRAD Called Synapsis

  13. Crossing-Over • Homologous chromosomesin a tetrad cross over each other • Pieces of chromosomes or genes are exchanged • Produces Genetic recombination in the offspring

  14. Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment

  15. Metaphase I Homologous pairs of chromosomes align along the equator of the cell

  16. Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres.

  17. Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two.

  18. Sister chromatids carry identical genetic information. Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Meiosis II Only one homolog of each chromosome is present in the cell. Gene X

  19. Meiosis II: Reducing Chromosome Number Prophase II Metaphase II Telophase II Anaphase II 4 Identical haploid cells

  20. Prophase II Nuclear envelope fragments. Spindle forms.

  21. Metaphase II Chromosomes align along equator of cell.

  22. Anaphase II Equator Pole Sister chromatids separate and move to opposite poles.

  23. Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two.

  24. Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome

  25. Comparing Mitosis and Meiosis

  26. Comparison of Divisions

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