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Meiosis Formation of Gametes

Meiosis Formation of Gametes. Facts About Meiosis. Daughter cells contain half the number of chromosomes as the original cell (haploid/ monoploid /1n ) Produces gametes Egg and sperm production Occurs in the testes in males spermatogenesis Occurs in the ovaries in females Oogenesis.

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Meiosis Formation of Gametes

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  1. MeiosisFormation of Gametes

  2. Facts About Meiosis • Daughter cells contain half the number of chromosomes as the original cell (haploid/monoploid/1n) • Produces gametes • Egg and sperm production • Occurs in the testes in males • spermatogenesis • Occurs in the ovaries in females • Oogenesis

  3. 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 • Promotes genetic diversity (aka a better gene pool)

  4. Fertilization: Putting it all together 2n = 6 1n =3

  5. Types of Cells • Somatic Cells • Somatic cells are body cells. • They comprise your hair, skin, intestines, etc. • Germ Cells • Will become sperm and egg cells.

  6. from mom from dad child too much! meiosis reduces genetic content Meiosis Forms Haploid Gametes • Meiosis must reduce the chromosome number by half • Fertilization then restores the 2n (diploid) number The right number!

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

  8. 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)

  9. 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

  10. SisterChromatids Same genes, Same alleles Homologs Same genes, different alleles A Replicated Chromosome Gene: specific region of DNA that codes for a particular protein. Allele: any alternative form of a gene Gene X Homologs separate in meiosis I and therefore different alleles separate.

  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 Synapsis = putting the tetrads together

  13. Crossing-Over • Sister chromatidsin a tetrad cross over each other • Pieces of chromosomes or genes are exchanged • Produces genetic recombination in the offspring • These are called homologous chromosomes

  14. Homologous Chromosomes During Crossing-Over

  15. Crossing-Over

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

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

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

  19. 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

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

  21. Prophase II Nuclear envelope fragments. Spindle forms.

  22. Metaphase II Chromosomes align along equator of cell.

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

  24. Telophase II Nuclear envelope assembles. Chromosomes uncoil. Spindle disappears. Cytokinesis divides cell into two.

  25. 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

  26. Gametogenesis

  27. Spermatogenesis • Occurs in the testes • Two divisions produce 4 spermatids • Spermatids mature into sperm • Men produce about 250,000,000 sperm per day

  28. Spermatogenesis

  29. Oogenesis • Occurs in the ovaries • Two divisions produce 3 polar bodies that die and 1 egg • Polar bodies die because of unequal division of cytoplasm • Immature egg called oocyte • Starting at puberty, one oocyte matures into an ovum (egg) every 28 days

  30. First polar body may divide (haploid) a Polar bodies die X a a X X a X X Mitosis Meiosis I Meiosis II A (if fertilization occurs) Oogonium (diploid) A Primary oocyte (diploid) X Ovum (egg) X A Mature egg Secondary oocyte (haploid) A X Second polar body (haploid) Oogenesis

  31. Comparing Mitosis and Meiosis

  32. Comparison of Divisions

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