Genetics News

Mutation - OverviewMechanism of mutation

SpontaneousInduced

Duplication/InsertionsMechanism (example: lacI)

Fragile X syndrome

Repair of mutationBypass replicationExcision repairMismatch repair

Mutation and its Avoidance

Thrust

DNA damage

Base substitution

Base mimicry

Intercalation

Parry

Reversal of lesion

Bypass of lesion

General excision pathway

Specific excision pathway

Mismatch repair pathway

UV

Pyrimidine dimers

T-T C-T T-C

Routes to MutationDNA Damage

Light

Repair Strategy #1Reversal of mutation

DNA photolyase

DNA Polymerase III

Faithful Error-correcting

SOS response

Induced by DNA damage Elicits repair enzymes Recruits DNA Pol I

Repair Strategy #2Bypass of mutation

Modified DNA Polymerase III

Resourceful Innovative

Repair Strategy #2Bypass of mutation

SOS-induced genes

Modified DNA Polymerase III

Resourceful Innovative

5'-AGATTGACCTAGAT3'-CCTAACTGGATCTATTGTAA5'-AGATTGACCTAGATAACATT3'-CCTAACTGGATCTATTGTAA5'-AGATTGACCTAGAT3'-CCTAACTGGATCTATCGTAA5'-AGATTGACCTAGATAACATT3'-CCTAACTGGATCTATCGTAA

Repair Strategy #2Bypass of mutation

When in doubt, A

Study Question 9

Bypass replication across pyrimidine dimers generally produces which kind of base substitution:

Transitions? Transversions?

Transitions vs Transversions

Transition:

G A A C C T

Transversion:

G T T G C C

C C A C C G

Purine Purine; Pyrimidine Pyrimidine

Purine Pyrimidine; Pyrimidine Purine

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

G A G T C G A G T C T C A G C T C A

HNO2

U

Base modification

• Exposure to chemical mutagen

• Reaction with base to change base pairing

• Replication of modified base

Routes to Mutation

G A G T C G A G TU

Base modification

• Exposure to chemical mutagen

• Reaction with base to change base pairing

C T C A G C T C A

• Replication of modified baseG A G T C G A G T

C T C A A C T C A

• Mutation is permanent

Routes to Mutation

G A G T C G A G T

Base mimicry

• Incorporation of base mimic

• Replication of modified baseC T C A G CBrU C A

Routes to Mutation

G A G T C G A G T

Base mimicry

• Incorporation of base mimic

• Replication of modified base

• Mutation is permanent

G A G T C G G G T

C T C A G CBrU C A

C T C A G C T C A

• Change of pairing properties (tautomerization)

Problem: Deamination of cytosine

What’s the threat? C/G to T/A transition

Could you diagnose the problem?

Strategy #3: Repair LesionSpecific Excision Repair

Excision of base by specific glycosylase

Excision of strand by AP endonuclease

APApurinic

Apyrimidinic

Study Question 11Two mutations:

1. Ura-glycosylase2. 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

Is it too late?

Could you diagnose the problem?

5'-AGATTGACCTAGATTATTGTTACTGACCGCATATC-3’3'-TCTAACTGGATCTAGTAACAATGACTGGCGTATAG-5’

Strategy #2: Repair of LesionMethyl-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 LesionMethyl-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 13Phenotype of E. coli that overmethylates GATC?

MutagenesisSlipped-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?