1. 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

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

3. 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

4. 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

5. 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 a1 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

6. 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

7. 2nd division segregationM 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

8. 2nd division segregationM 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

9. 2nd division segregationM 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

10. 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 ?
300
100 = ??
WRONG !
Each MII tetrad (octad) has only HALF of its spores as recombinants !!
Therefore the correct distance is:
0.5 ( )
300
100 = 7 map units

11. 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 ( )
100 = 7 map units
300
0.5 (number of MII tetrads)
In general:
100 =
Total number of tetrads

12. 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)

13. 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

14. 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

15. 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 ?

16. 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

17. 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

18. Two genes on the same chromsomeB 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 ?

19. Two genes on the same chromsomeB 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

20. Two genes on the same chromsome Two genes on the same chromsomeB 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: T + NPD
(NPD)
100
Total number of tetrads

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

22. 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) ?

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