Post on 03-Jan-2016
description
Life cycle of the fungus Neurospora
N NN NN N N N N NN N
Mitosis Mitosis
Spore
Spore is haploid (N)
Hyphae is haploid
N N N NN NN N N N N N
2N
2N 2N
2N 2NMeiosis
N
N
N
N
Fruiting body is diploid
2N2NMitosis
Additional mitosis after meiosis
N N N NN NN N N N N N
2N
2N 2N
2N 2NMeiosis
Fruiting body is diploid
Life cycle of the fungus Neurospora
2N2NMitosis
Additional mitosis after meiosis
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
Ascus
Each ascus contains an octad of sporesThe octads are ORDERED
Neurospora
A a
a
a
A
A
A
A
a
a
MITOSIS
A
AA
A
a
aa
a
A
AA
A
a
aa
a
MI MII
12
34
A a
a
a
A
A
A
A
a
a
MITOSIS
MI MII
12
34
CO 2,3 CO 1,3 CO 1,4AAaaAAaa
aaAAAAaa
aaAAaaAA
AAaaaaAA
CO 2,4
2nd division segregation
aaaa
AAAA A
AAA
aaaa
1st divisionsegregation
A
AA
A
a
aa
a
A
AA
A
a
aa
a
1st division segregation(= MI segregation)
Remember that each class can appear inverted relative to the attachment point to the fruiting body
Neurospora
A a
a
a
A
A
A
A
a
a
MITOSIS
A
AA
A
a
aa
a
A
AA
A
a
aa
a
MI MII
12
34
1st division segregation(= MI segregation)
Octads
A a
a
a
A
A
A
A
a
a
MITOSIS
MI MII
12
34
A
A
a
a
A
A
a
a
AAaa
aaaa
AAAA A
AAA
aaaa
1st divisionsegregation
aaAA
Octads
TetradsAAaaAAaa
CO 2,3aaAAAAaa
aaAAaaAA
CO 1,3 CO 1,4AAaaaaAA
CO 2,4
2nd division segregation
AaAa
aAAa
AaaA
aAaA Ordered
Tetrads
AAaa
A a
a
a
A
A
A
A
a
a
MITOSIS
MI MII
12
34
2nd division segregation1st divisionsegregation
aaAA
AaAa
aAAa
AaaA
TetradsaAaA
Total = 300126 132 9 11 10 12
Can calculate map distance
1) What distance ?2) How to calculate ?
AAaa
A a
a
a
A
A
A
A
a
a
MITOSIS
MI MII
12
34
2nd division segregation1st divisionsegregation
aaAA
AaAa
aAAa
AaaA
aAaA
Total = 300126 132 9 11 10 12
Can calculate map distance
1) What distance ?2) How to calculate ?
Between gene and centromer
AAaa
MII segregationMI segregation
aaAA
AaAa
aAAa
AaaA
aAaA
Total = 300126 132 9 11 10 12
Can calculate map distance
1) What distance ?2) How to calculate ?
Between gene and centromere
How to calculate ?
9+11+10+12
300100 = ?? WRONG !
Each MII tetrad (octad) has only HALF of its spores as recombinants !!
Therefore the correct distance is:0.5 (9+11+10+12)
300100 = 7 map units
AAaa
MII segregationMI segregation
aaAA
AaAa
aAAa
AaaA
aAaA
Total = 300126 132 9 11 10 12
Map distance between a gene and the centromere in ordered tetrads
Each MII tetrad (octad) has only HALF of its spores as recombinants !!
0.5 (9+11+10+12)300
100 = 7 map units
In general:0.5 (number of MII tetrads)
Total number of tetrads100 =
Meiosis of two genes in ordered tetrads
A a
a
a
A
A
B b
Two genes on different chromosome
B
Bb
ba
a
A
A
B
B
b
b
AB
AB
ab
ab
a
a
A
A
B
B
b
b
Ab
Ab
aB
aB
a
A
AB
B
b
b
Di-type
Di-type
Parental
Non-Parental
(PD)
(NPD)
a
A a
a
a
A
A
B b
B
Bb
b
a
a
A
A
B
B
b
b
AB
AB
Ab
ab
a
a
A
A
B
B
b
b
Ab
Ab
aB
aB
a
A
AB
B
b
b
Di-type
Di-type
Parental
Non-Parental
(PD)
(NPD)
Frequency of PD = Frequency of NPD
a
A a
a
a
A
A
B b
B
Bb
b
ABAB
abab
AbAb
aBaB
a
A
AB
B
b
b
PD
NPD
Frequency: PD = NPD > T
ABaB
Abab
CO 2,3
aBAB
Abab
CO 1,3 CO 1,4
aBAB
abAb
CO 2,4
ABaB
abAb
Tetra-type (T)
Abab
ABaB
abAb
ABaB
CO 2,3 CO 1,3 CO 1,4 CO 2,4
abAb
aBAB
Abab
aBAB
Tetra-type (T)
Assume CO between A and its centromer
a
a
a
A
A
B
Bb
b
ABAB
abab
AbAb
aBaB
a
A
AB
B
b
b
PD
NPD
Frequency: PD = NPD > T
ABaB
Abab
aBAB
Abab
CO 2,3 CO 1,3 CO 1,4 CO 2,4
aBAB
abAb
ABaB
abAb
Abab
ABaB
abAb
ABaB
CO 2,3 CO 1,3 CO 1,4 CO 2,4
abAb
aBAB
Abab
aBAB
Tetra-type (T)
Tetra-type (T)
Calculate map distance:1) Which distance ? 2) How do we know which gene has recombined ?
a
a
a
A
A
B
Bb
b
ABAB
abab
AbAb
aBaB
a
A
AB
B
b
b
PD
NPD
Frequency: PD = NPD > T
ABaB
Abab
aBAB
Abab
CO 2,3 CO 1,3 CO 1,4 CO 2,4
aBAB
abAb
ABaB
abAb
Abab
ABaB
abAb
ABaB
CO 2,3 CO 1,3 CO 1,4 CO 2,4
abAb
aBAB
Abab
aBAB
Tetra-type (T)
Tetra-type (T)
Calculate map distance: 0.5 (Number of T tetrads)
Total number of tetrads100
a
a
A
A
B
Bb
b
ABAB
abab
AbAb
aBaB
a
A
AB
B
b
b
PD
NPD
Frequency: PD = NPD > T
What if both genes can recombine away from their respective centromers ?
?Will NOT be given in EXAM
a
A aB bABABabab
Di-typeParental (PD)
A B
A Ba ba b
A B
A B
a ba b
ABAb
aBab
CO 2,3 CO 1,3 CO 1,4 CO 2,4AbAB
aBab
AbAB
abaB
ABAb
abaB
Tetra-type
Where is NPD ? What should it look like ?
Two genes on the same chromsome
A B
A Ba ba b
Lets allow CO between the two genes
A aB bABABabab
Di-typeParental (PD)
A B
A Ba ba b
A B
A B
a ba b
A B
A Ba ba b
ABAb
aBab
CO 2,3 CO 1,3 CO 1,4 CO 2,4AbAB
aBab
AbAB
abaB
ABAb
abaB
Tetra-type
Where is NPD ? What should it look like ?
(T)
AbAb
aBaB (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 chromsomeA aB b
ABABabab
Di-typeParental (PD)
A B
A Ba ba b
A B
A B
a ba b
A B
A Ba ba b
ABAb
aBab
CO 2,3 CO 1,3 CO 1,4 CO 2,4AbAB
aBab
AbAB
abaB
ABAb
abaB
Tetra-type
AbAb
aBaB (NPD)
(T)
Di-type
Non-ParentalPD > T >> NPDTwo genes on the same
chromsomeMap distance: 0.5 T + NPD
Total number of tetrads100
Yeast have UNORDERED tetrads
Therefore calculation of map distances is more complicated
When does crossing over occur ?
A
a
B
b
A
a
B
b
A
a
B
b
A
a
b
B
A
a
b
B
A
a
b
B
A b
a B
A
a
B
b
A
a
b
B
A B
a 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) ?
Conclusion:The occurrence of TETRA-TYPEtetrads proves that crossing overtook place AFTER DNA replication
Prediction in terms of type of tetrads resulting from cross-over
ONLY Non-Parental Di-type
Tetra-type