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Life cycle of the fungus Neurospora

N. N. N. N. N. N. N. N. N. N. N. N. Life cycle of the fungus Neurospora. Spore. Spore is haploid (N). Hyphae is haploid. Mitosis. Mitosis. N. N. N. N. N. N. N. N. N. N. 2N. 2N. 2N. N. N. N. N. N. N. 2N. 2N. Mitosis. 2N. 2N. Additional mitosis after meiosis.

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Life cycle of the fungus Neurospora

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  1. N N N N N N N N N N N N Life cycle of the fungus Neurospora Spore Spore is haploid (N) Hyphae is haploid Mitosis Mitosis

  2. N N N N N N N N N N 2N 2N 2N N N N N N N 2N 2N Mitosis 2N 2N Additional mitosis after meiosis Meiosis Fruiting body is diploid

  3. N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N 2N 2N 2N N N N N N N 2N 2N Mitosis 2N 2N Life cycle of the fungus Neurospora Ascus Each ascus contains an octad of spores Additional mitosis after meiosis The octads are ORDERED Meiosis Fruiting body is diploid

  4. a A A M I T O S I S a A Neurospora a A A A 1 A 2 a a A A A a a 3 a 4 a A a A a a A MI a MII

  5. a A a A a A a a A A a A a a A a A a A A a A a a A A a a A a A a a A a A A a a A A a A A a a A A a 1st division segregation A A M I T O S I S a A a A A A 1 A 2 a a A A A a a 3 a 4 a A a A a a A MI a MII CO 2,3 CO 1,3 CO 1,4 CO 2,4 1st division segregation (= MI segregation) Remember that each class can appear inverted relative to the attachment point to the fruiting body 2nd division segregation

  6. a A A M I T O S I S a A Neurospora a A A A 1 A 2 a a A A A a a 3 a 4 a A a A a a A MI a MII 1st division segregation (= MI segregation) Octads

  7. A CO 2,3 CO 1,3 CO 1,4 CO 2,4 a A a A a a A A a a a a A A a a A a A A A a a A a a A A A A a A a a A A a a a A A a Ordered Tetrads a A A a a A a A a a A a A A a A A a a A A a A A a a A A a 1st division segregation 2nd division segregation A A a M I T O S I S A A 1 a A A 2 a A a 3 a A a 4 a a a A MI MII Octads Tetrads

  8. A a a a A A A a A a a A A A A a a a A a A a a A A 126 132 Total = 300 9 11 10 12 M I T O S I S A A 1 A 2 a A a 3 a 4 a a MI MII Can calculate map distance What distance ? 2) How to calculate ? Tetrads 1st division segregation 2nd division segregation

  9. A a a a A A A a A a a A A A A a a a A a A a a A A 126 132 Total = 300 9 11 10 12 M I T O S I S A A 1 A 2 a A a 3 a 4 a a MI MII Can calculate map distance What distance ? 2) How to calculate ? Between gene and centromer 1st division segregation 2nd division segregation

  10. Can calculate map distance How to calculate ? 9+11+10+12 100 = ?? a a a A A A 300 a A a a A A A A a a a A a A a a A A Therefore the correct distance is: 126 132 Total = 300 9 11 10 12 0.5 (9+11+10+12) 100 = 7 map units 300 What distance ? 2) How to calculate ? Between gene and centromere MI segregation MII segregation WRONG ! Each MII tetrad (octad) has only HALF of its spores as recombinants !!

  11. Map distance between a gene and the centromere in ordered tetrads a a a A A A a A a a A A A A a a a A a A a a A A 126 132 Total = 300 9 11 10 12 MI segregation MII segregation Each MII tetrad (octad) has only HALF of its spores as recombinants !! 0.5 (9+11+10+12) 100 = 7 map units 300 0.5 (number of MII tetrads) In general: 100 = Total number of tetrads

  12. Two genes on different chromosome AB AB Meiosis of two genes in ordered tetrads B A A B b a A B B A ab ab a b A B a b a b a b Parental Di-type (PD) Ab Ab b A A b A b A b a B aB aB a B a B a B Non-Parental Di-type (NPD)

  13. AB AB A B b a B A A B Ab ab B A a b A B a b a b a b Parental Di-type (PD) Ab Ab b A A b A b A b a aB aB B a B a B a B Non-Parental Di-type (NPD) Frequency of PD = Frequency of NPD

  14. Assume CO between A and its centromer CO 1,3 CO 1,4 CO 2,4 CO 2,3 AB aB aB AB aB AB AB aB Ab ab Ab ab ab Ab ab Ab CO 1,3 CO 1,4 CO 2,4 CO 2,3 Ab ab ab Ab ab Ab Ab ab AB aB AB aB aB AB aB AB Tetra-type (T) A B b a B A AB AB A B a b ab ab a b Tetra-type (T) PD b A Ab Ab A b a B aB aB a B NPD Frequency: PD = NPD > T

  15. CO 1,3 CO 1,4 CO 2,4 CO 2,3 B A AB aB aB AB aB AB AB aB AB AB A B a b Ab ab Ab ab ab Ab ab Ab ab ab a b Tetra-type (T) PD CO 1,3 CO 1,4 CO 2,4 CO 2,3 b A Ab Ab Ab ab ab Ab ab Ab Ab ab A b a B aB aB AB aB AB aB aB AB aB AB a B Tetra-type (T) NPD Frequency: PD = NPD > T Calculate map distance:1) Which distance ? 2) How do we know which gene has recombined ?

  16. CO 1,3 CO 1,4 CO 2,4 CO 2,3 B A AB aB aB AB aB AB AB aB AB AB A B a b Ab ab Ab ab ab Ab ab Ab ab ab a b Tetra-type (T) PD CO 1,3 CO 1,4 CO 2,4 CO 2,3 b A Ab Ab Ab ab ab Ab ab Ab Ab ab A b a B aB aB AB aB AB aB aB AB aB AB B Tetra-type (T) NPD Frequency: PD = NPD > T 0.5 (Number of T tetrads) Calculate map distance: 100 Total number of tetrads

  17. What if both genes can recombine away from their respective centromers ? B A AB AB A B ? a b ab ab a b PD b A Ab Ab Will NOT be given in EXAM A b a B aB aB a B NPD Frequency: PD = NPD > T

  18. Di-type Parental (PD) Lets allow CO between the two genes CO 1,3 CO 1,4 CO 2,4 CO 2,3 A B AB Ab Ab AB AB Ab Ab AB A B b aB ab aB ab ab aB a ab aB b a Two genes on the same chromsome A B b a A B AB AB A B A B ab ab b a A B b a b a b a Tetra-type Where is NPD ? What should it look like ?

  19. Non-Parental Ab Ab Di-type aB aB (NPD) Two genes on the same chromsome A B b a A B AB AB A B A B ab ab b a A B b a b a b Parental Di-type a (PD) CO 1,3 CO 1,4 CO 2,4 CO 2,3 A B AB Ab Ab AB AB Ab Ab AB A B Tetra-type (T) b aB ab aB ab ab aB a ab aB b a Where is NPD ? What should it look like ? How did it occur ? A specific double cross-over: 2,3 AND 1,4

  20. Two genes on the same chromsome A B b a A B AB AB A B A B ab ab b a A B b a b a b a Parental Di-type (PD) CO 1,3 CO 1,4 CO 2,4 CO 2,3 A B AB Ab Ab AB AB Ab Ab AB A B Tetra-type (T) b aB ab aB ab ab aB a ab aB b a Non-Parental Ab Ab PD > T >> NPD Di-type Two genes on the same chromsome aB aB Map distance: 0.5 T + NPD (NPD) 100 Total number of tetrads

  21. Yeast have UNORDERED tetradsTherefore calculation of map distances is more complicated

  22. When does crossing over occur ? After DNA replication (4-strand stage) ? A B A B A B A B A b a b a a b a b B a b Or before DNA replication (2-strand stage) ? A b A b A B A b A b a a b a B a B B a B Need to examine TOGETHER all 4 products of single/same meiosis

  23. Prediction in terms of type of tetrads resulting from cross-over Tetra-type ONLY Non-Parental Di-type Conclusion: The occurrence of TETRA-TYPE tetrads proves that crossing over took place AFTER DNA replication

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