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THE BIRDS and BEES LESSON

THE BIRDS and BEES LESSON. Everything you didn’t already know about sexual reproduction. Overview of Meiosis Meiosis -form of cell division where there are two successive (back to back) rounds of cell division. In meiosis a……

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THE BIRDS and BEES LESSON

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  1. THE BIRDS and BEES LESSON Everything you didn’t already know about sexual reproduction

  2. Overview of Meiosis Meiosis-form of cell division where there are twosuccessive (back to back) rounds of cell division. In meiosis a…… Diploid (parent) cell with (2n) or “double set” of chromosomes produces 4 haploid cells with one (n) or “single set” of chromosomes. Diploid # in humans is 46 double (2n), the haploid # is 23 (n) The chromosome # is reduced by ½ in two divisions: After 1 division - 23 double stranded chromosomes (n) (interphase skipped) After 2nd division - 23 single stranded chromatids -Meiosis occurs in our germative cells (cells that produce our gametes ) Females- Oocyte  eggs (ovum) Males- Spermatocyte  sperm

  3. Body cells have 2 of each chromosome = diploid (2n) How many total chromosomes? 46

  4. Sperm/egg cells have 1 of each chromosome = haploid (1n) How many chromosomes? 23

  5. from mom from dad child too much! Why do we need meiosis? • Process keeps the chromosomal number in • organisms from doubling from one generation to the next • during sexual reproduction. Egg Sperm Zygote 46 chromosomes + 46 chromosomes = 92 Chromosomes

  6. Meiosis reduces genetic content by half Why Meiosis is Needed Zygote Egg Sperm 23 chromosomes + 23 chromosomes = 46 Chromosomes • - to bring two haploid gametes together to form a diploid zygote. • n (mom) + n (dad) = 2n (offspring)

  7. Overview of Meiosis continued.. HOMOLOGOUS CHROMOSOMES The members of each pair are called homologous because they are similar in genetic content Each member of each pair came from a parent during sexual reproduction One chromosome is maternal (from the mother) One is paternal (from the father) Homologous chromosomes are equivalent and contain the same kinds of genes in the same order . They may have differentversions of genes or “alleles” because they come from different parents

  8. A A a a B B b b C C c c D D d d E E e e F F f f What is a Homologous Chromosome? • SAME SIZE • SAME SHAPE • CARRY GENES for the SAME TRAITS • BUT ______________! • Don’t have to have the same versions(alleles) NOT IDENTICAL Chromosomes are made of sister chromatids. Same genes with the identical alleles (versions) http://sps.k12.ar.us/massengale/genetics%20tutorial.htm

  9. Meiosis During Meiosis, three important things occur that did not occur in mitosis: • Chromosomes pair up prior to division forming Homologous pairs of chromosomes • Homologous chromosomes exchange genetic information in an event called “Crossing Over” • The newly scrambled chromosomes separate and go into different daughter cells so that each contains only one chromatid of each pair of homologous chromosomes • The result is 4 cells each having unique DNA with gene combinations

  10. 1st Major Differences between Mitosis in Meiosis Metaphase of Mitosis Chromosomes line up individually along the equatorial plane Metaphase of Meiosis I Homologous chromosomes “pair-up” prior to division forming a (grouping 4 chromatids) known as a TETRAD (homologous pairs of chromosomes)

  11. What happens to the pairs? In meiosis the homologous pairs of chromosomes will separate in anaphase of the 1st division (Meiosis I), and in the second division (Meiosis II) individual chromatids will separate Maternal Chromosome Paternal Chromosome Single Paternal Chromatid During Meiosis II individual chromatids separate Homologous chromosomes separate in meiosis I and therefore different alleles also separate.

  12. 2nd Major Differences between Mitosis in Meiosis Prophase I of Meiosis Dyad pair or “tetrad” During Meiosis I homologous chromosomes “pair up” forming a Tetrad (grouping of 4 chromatids) Homologous chromosomes exchange genetic information in an event called “Crossing Over”

  13. A A a a B B b b C C c c D D d d E E e e F F f f CROSSING OVER (recombination) • Occurs in prophase of meiosis I • Generates diversity by reordering the alleles of genes within homologous chromosomes • The resulting Chromatids have DNA that is unique or “recombinant”

  14. CROSSING OVER (recombination) A a a A B • Occurs in prophase of meiosis I • Generates diversity by reordering the alleles of genes within homologous chromosomes b b B c C C c D D d d E E e e Letters denote genes Case denotes alleles (versions) F F f f • Creates chromosomes with new combinations of alleles for genes A to F.

  15. CROSSING OVER (recombination) a A a A B b B • Occurs in prophase of meiosis I • Generates diversity by reordering the alleles of genes within homologous chromosomes b c c C C D D d d E E e e Letters denote genes Case denotes alleles F F f f • The resulting Chromatids have DNA that is unique or “recombinant”

  16. Meiosis I Meiosis II Diploid germ cell Haploid gametes What is different between Meiosis and Mitosis? chromatid Homologous pairs PMATC PMATC Single chromosome TWO divisions resulting in Four Cells Result: Each new daughter cell contains only One chromatid (DNA) of each of the original homologous pairs.

  17. Spindle fibers Nucleus Nuclear envelope Prophase I (early) (diploid) Prophase I (late) (diploid) Metaphase I (diploid) Anaphase I (diploid) Telophase I (diploid) Meiosis I : the reduction division Tetrad

  18. Prophase I • Late prophase • Homologous chromosomes pair • Tetrads form with four chromatids . • Crossing over occurs- an event unique only to meiosis. • Early prophase • Chromosomes condense. • Spindle forms. • Nuclear envelope/membrane • fragments and disappears.

  19. Metaphase I Homologous pairs/Tetrads align along the equator of the cell.

  20. Anaphase I Homologues/tetrads separate and move to opposite poles. Sister chromatids remain Attached at their centromeres.

  21. Telophase I Nuclear envelopes reassemble. Spindle disappears. Chromosomes decondense Cytokinesis divides cell into two daughter cells.

  22. Four nonidentical haploid daughter cells Prophase II *(haploid) Metaphase II (haploid) Anaphase II (haploid) Telophase II (haploid) Meiosis II : the equational division

  23. Prophase II Haploid Daughter cells Chromosomes Condense Nucleus disappears Spindle apparatus forms

  24. Metaphase II Chromosomes line-up along equator of cell. Two sister chromatids attached at the centromeres

  25. Anaphase II Sister chromatids separate and move to opposite poles.

  26. Telophase II Nucleus reorganizes and nuclear envelope/membrane reforms. Chromatids which are now called “chromosomes” decondense. Spindle disappears. Cytokinesis divides each cell into two forming four haploid cells.

  27. Results of meiosis Gametes Four haploid cells One copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome

  28. CHROMOSOMAL MUTATIONS ____________________________: Change in the ______________ or ____________ of chromosomes structure number

  29. Section 12-4 Figure 12–20 Chromosomal Mutations Deletion Duplication Inversion Translocation

  30. Homologous chromosomes ________________ during MEIOSIS = _________________________ One cell gets 2 copies of the chromosome the other cell gets none. fail to separate NONDISJUNCTION

  31. Mistakes in Meiosis

  32. Normal Meiosis Nondisjunction http://web.udl.es/usuaris/e4650869/docencia/gen_etica/meioferti2.html http://www.tokyo-med.ac.jp/genet/anm/domov.gif

  33. Nondisjunction

  34. Nondisjunction Since it happens to a sperm or egg, the new baby can end up with _____________ of a chromosome = __________________ OR only ___________ of a chromosome = ___________________ 3 copies TRISOMY one copy MONOSOMY

  35. Body cells have 2 of each chromosomeexcept one pair = Monosomy (2n-1) How many chromosomes? 45

  36. Body cells have 2 of each chromosomeexcept one pair has 3 copies = Trisomy (2n+1) How many chromosomes? 47

  37. Human Abnormalities caused by Non-Disjunction______________________________________________________ Down syndrome Klinefelter syndrome Turner syndrome

  38. KARYOTYPE A __________ is a picture of an organism’s chromosomes

  39. Karyotype(need cells from baby) Image from: http://members.tripod.com/~yenial/chromosome.html Can tell __________________ chromosomes __________ Some _____________________ Can’t see _______________ mutations MISSING/EXTRA GENDER DELETIONS/INSERTIONS single gene

  40. SEX DETERMINATION XX = female male Xy =

  41. Chromosomes that determine the sex of an organism = _________________ All other chromosomes = _________________ http://www.angelbabygifts.com/ Sex chromosomes autosomes Humans have two sex chromosomes and _____ autosomes y X 44

  42. Who decides? Mom can give X Dad can give X or y X X X X X X X X y X y y Dad SO ____ determines sex of the baby. If dad gives X with mom’s X = girl If dad give y with mom’s X = boy

  43. Nondisjunction of Autosomal chromosomes Normal male Normal female

  44. Down syndrome (= ____________) TRISOMY 21

  45. Nondisjunction of Autosomal Chromosomes Male with trisomy 21 (Downs syndrome)

  46. Down syndrome (Trisomy 21) 1 in 800 births Similar facial features Slanted eyes Protruding tongue

  47. Down syndrome (Trisomy 21) Simian line on palm

  48. Down syndrome (Trisomy 21) Most common chromosomal abnormality 50% have heart defects that need surgery to repair Mild to severe mental retardation Increases susceptibility to many diseases Risk of having a child with Down syndrome increases with age of mom

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