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Chromosomes

Chromosomes. Prokaryotic Chromosome. The DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell membrane. Eukaryotic Chromosomes. All eukaryotic cells store genetic information in chromosomes

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Chromosomes

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  1. Chromosomes

  2. Prokaryotic Chromosome • The DNA of prokaryotes (bacteria) is one, circular chromosomeattached to the inside of the cell membrane

  3. Eukaryotic Chromosomes • All eukaryotic cells store genetic information in chromosomes • Most eukaryotes have between 10 and 50 chromosomes in their body cells • Human body cells have 46 chromosomes or 23 identical pairs

  4. Eukaryotic Chromosomes • Each chromosome is composed of a single,tightly coiled DNA molecule • Chromosomes can’t be seen when cells aren’t dividingand are called chromatin

  5. Chromosomes in Dividing Cells • Duplicated chromosomes are called chromatids & are held together by the centromere Called Sister Chromatids

  6. Karyotype • A picture of the chromosomes from a human cell arranged in pairs by size • First 22 pairs are called autosomes • Last pair are the sex chromosomes • XX female or XY male

  7. Boy or Girl? The Y Chromosome Decides Y - Chromosome X - Chromosome

  8. Cell Reproduction

  9. Types of Cell Reproduction • Asexual reproduction involves a single cell dividing to make 2 new, identical daughter cells • Mitosis & binary fission are examples of asexual reproduction • Sexual reproduction involves two cells (egg & sperm) joining to make a new cell (zygote) that is NOT identical to the original cells • Meiosis is an example

  10. MeiosisFormation of Gametes(Eggs & Sperm)

  11. Facts About Meiosis • Preceded by interphase which includeschromosome replication • Twomeiotic divisions --- • Meiosis I and Meiosis II Original cell is diploid (2n) • Four daughter cellsproduced that are monoploid or haploid (1n)

  12. Facts About Meiosis • Daughter cellscontain half the number of chromosomesas the original cell • Producesgametes(eggs & sperm) • Occurs in the testes in males (Spermatogenesis) • Occurs in the ovaries in females (Oogenesis)

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

  14. 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

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

  16. 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

  17. 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.

  18. 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 number The right number!

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

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

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

  22. Tetrads Form in Prophase I Homologous chromosomes(each with sister chromatids)   Join to form aTETRAD Called Synapsis

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

  24. Homologous Chromosomes During Crossing-Over

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

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

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

  28. 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

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

  30. Prophase II Nuclear envelope fragments. Spindle forms.

  31. Metaphase II Chromosomes align along equator of cell.

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

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

  34. 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

  35. Meiosis Animation http://www.johnkyrk.com/meiosis.html

  36. Comparing Mitosis and Meiosis

  37. Comparison of Divisions

  38. Gametogenesis Oogenesis or Spermatogenesis

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

  40. Spermatogenesis in the Testes Spermatid

  41. Spermatogenesis

  42. 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

  43. Oogenesis in the Ovaries

  44. 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

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