Transcript No Slide Title - People.vcu.edu

Genetics News
Mutation - Overview
Mechanism of mutation
Spontaneous
Induced
Duplication/Insertions
Mechanism (example: lacI)
Fragile X syndrome
Repair of mutation
Bypass replication
Excision repair
Mismatch repair
Mutation and its Avoidance
Thrust
Parry
DNA damage
Reversal of lesion
Base substitution
Bypass of lesion
Base mimicry
General excision pathway
Intercalation
Specific excision pathway
Mismatch repair pathway
Routes to Mutation
DNA Damage
UV
Pyrimidine dimers
T-T
C-T
T-C
Repair Strategy #1
Reversal of mutation
Light
DNA photolyase
Repair Strategy #2
Bypass of mutation
DNA Polymerase III
Faithful
Error-correcting
SOS response
Induced by DNA damage
Elicits repair enzymes
Recruits DNA Pol I
Repair Strategy #2
Bypass of mutation
SOS-induced genes
Modified DNA
Polymerase III
Resourceful
Innovative
Repair Strategy #2
Bypass of mutation
Modified DNA
Polymerase III
Resourceful
Innovative
When in doubt, A
5'-AGATTGACCTAGATAACATT
5'-AGATTGACCTAGAT
3'-CCTAACTGGATCTATTGTAA
3'-CCTAACTGGATCTATCGTAA
Study Question 9
Bypass replication across pyrimidine
dimers generally produces which kind of
base substitution:
Transitions?
Transversions?
Transitions vs Transversions
Transition: Purine
Purine; Pyrimidine
G
A
A
C
C
T
Transversion: Purine
Pyrimidine
Pyrimidine; Pyrimidine
Purine
G
T
T
G
C
C
C
C
A
C
C
G
Study Question 9
Bypass replication across pyrimidine
dimers generally produces which kind of
base substitution:
Transitions?
Transversions?
Study Question 10
DNA Pol I mutant is viable.
Mutation rate is lower in mutant.
If viable and lower mutation rate, why have Pol I???
Routes to Mutation
Base modification
• Exposure to chemical mutagen
HNO2
G A G T C
U G A G T
• Reaction with base to change
base pairing
C T C A G C T C A
• Replication of modified base
Routes to Mutation
Base modification
U G A G T
G A G T C
C T C A A C T C A
G A G T C G A G T
C T C A G C T C A
• Exposure to chemical mutagen
• Reaction with base to change
base pairing
• Replication of modified base
• Mutation is permanent
Routes to Mutation
Base mimicry
G A G T C G A G T
C T C A G CBrU C A
• Incorporation of base mimic
• Replication of modified base
Routes to Mutation
Base mimicry
G A G T C G A G T
• Incorporation of base mimic
C T C A G C T C A
• Replication of modified base
G A G T C G G G T
• Change of pairing properties
(tautomerization)
C T C A G CBrU C A
• Mutation is permanent
Problem: Deamination of cytosine
What’s the threat?
C/G to T/A transition
Could you diagnose the problem?
Strategy #3: Repair Lesion
Specific Excision Repair
Excision of base by
specific glycosylase
Excision of strand by
AP endonuclease
AP
Apurinic
Apyrimidinic
Study Question 11
Two mutations:
1. Ura-glycosylase
2. AP endonuclease
Combine both into one strain.
Phenotype?
Phenotype of Ura-glycossylase- mutant?
Phenotype of AP endonuclease- mutant?
Phenotype of double mutant?
Problem: Error in Replication
5'-AGATTGACCTAGATTATTGTTACTGACCGCATATC-3’
3'-TCTAACTGGATCTAG TAACAATGACTGGCGTATAG-5’
Is it too late?
Could you diagnose the problem?
Problem: Error in Replication
5'-AGATTGACCTAGATTATTGTTACTGACCGCATATC-3’
3'-TCTAACTGGATCTAGTAACAATGACTGGCGTATAG-5’
Is it too late?
Could you diagnose the problem?
Strategy #2: Repair of Lesion
Methyl-directed Mismatch Repair
Old DNA strand
Marked by methylation
Methylation at adenine
of GATC
New DNA strand
Unmarked
Replication with standard
nucleotides
Mark is temporary
Now can you diagnose the lesion?
Strategy #2: Repair of Lesion
Methyl-directed Mismatch Repair
MutS, H, L: protein
Ssb: Single-stranded binding
protein
Study Question 12
Phenotype of E. coli that can’t methylate GATC?
Study Question 13
Phenotype of E. coli that overmethylates GATC?
Mutagenesis
Slipped-strand mispairing
Wild-type lacI
Replication
Mispairing
Completion of replication
2nd round of replication
Study Question 4
Draw a diagram showing how mispairing could
lead to deletion of a repeat.
Fragile X Syndrome
SQ5: Is the trait
dominant?
Recessive?
SQ6: Why do
females have two
numbers?