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

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

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

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

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

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

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

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

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

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

Map distance between a gene and the centromere in ordered tetrads a A a A a A A a A a A a Total = 300 126 132 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

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

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

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

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

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

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

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

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

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

Yeast have UNORDERED tetrads Therefore calculation of map distances is more complicated

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

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