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©M J Larkin Biology & Biochemistry. The Queen’s University of Belfast.
D. Types of mutationD. Types of mutation
• 1. Point mutation.– Affecting non coding regions: e.g. Promoter/operator
– e.g. -10 TATAAT sequence in promoter
– Affecting CODING SEQUENCE or Open Reading Frame (ORF) sequence.
• 1. Point mutation.– Affecting non coding regions: e.g. Promoter/operator
– e.g. -10 TATAAT sequence in promoter
– Affecting CODING SEQUENCE or Open Reading Frame (ORF) sequence.
AAT DNAUUA mRNALeu amino acid
CUA GUA AUA UCA UUC UUG UUU UCALeu Val Ile Ser Phe Leu Phe Ser
UGA UAAStop Stop
Can mutate to…..
©M J Larkin Biology & Biochemistry. The Queen’s University of Belfast.
D. Types of mutation. Cont…….D. Types of mutation. Cont…….
– STOP leads to truncated protein and termination of translation
– (Note: Transcription termination involves inverted repeats and role of rho protein factor)
– Some codons are NONSENSE (i.e. normally no tRNA)
– UAG Amber (after discoverer Bernstein :German for Amber)
– UAA Ochre
– UGA Opal
• 2. Frameshifts. Addition or deletion of bases leading to altered sequence beyond the sequence change.
– STOP leads to truncated protein and termination of translation
– (Note: Transcription termination involves inverted repeats and role of rho protein factor)
– Some codons are NONSENSE (i.e. normally no tRNA)
– UAG Amber (after discoverer Bernstein :German for Amber)
– UAA Ochre
– UGA Opal
• 2. Frameshifts. Addition or deletion of bases leading to altered sequence beyond the sequence change.
©M J Larkin Biology & Biochemistry. The Queen’s University of Belfast.
D. Types of mutation. Cont…….D. Types of mutation. Cont…….
• 3. Deletions – Arise spontaneously where there are direct repeats of over 4-
5 base pairs. More frequent when there is more sequence repeated (see later excision of Transposons and IS elements)
• 3. Deletions – Arise spontaneously where there are direct repeats of over 4-
5 base pairs. More frequent when there is more sequence repeated (see later excision of Transposons and IS elements)
i.e.
3’5’
5’3’
Gap maybe up to a few 1000 bps
Replication slippage
Recombination anddeletion
©M J Larkin Biology & Biochemistry. The Queen’s University of Belfast.
E. Reversion and SuppressionE. Reversion and Suppression
• Back mutation to original sequence is rare• Suppressor mutations relieving mutations within
coding regions more common• Leads to insertion of amino acid at stop codon
• Back mutation to original sequence is rare• Suppressor mutations relieving mutations within
coding regions more common• Leads to insertion of amino acid at stop codon
e.g.
CAGGln
UAGSTOP Amber
GUC
GlntRNA
AUC
Mutation in another tRNAleads to incorporation of adifferent amino acid
Protein may ormay or be functional
©M J Larkin Biology & Biochemistry. The Queen’s University of Belfast.
F. Mechanisms of mutationF. Mechanisms of mutation
• Spontaneous mutation. ERRORS IN REPLICATION• Usually only AT GC pairing allowed• Repair systems are present• How do mis-matches or mis-pairings happen ?• Alternative Tautomeric forms of A and T occur
– (“In evolution we are all prisoners of simple chemistry”)
• Spontaneous mutation. ERRORS IN REPLICATION• Usually only AT GC pairing allowed• Repair systems are present• How do mis-matches or mis-pairings happen ?• Alternative Tautomeric forms of A and T occur
– (“In evolution we are all prisoners of simple chemistry”)
Dale Ch 3
©M J Larkin Biology & Biochemistry. The Queen’s University of Belfast.
F. Mechanisms of mutation. Cont….F. Mechanisms of mutation. Cont….
N
N
O
CH3H
H
H
O
2 x H Bonds
Thymine in its KETO formWill bind to AdenineTA pairing as expected duringreplication
N
N
O
CH3
H
H
O
3 x H Bonds
Thymine in its ENOLform. Binds Guianine.TG pairing during replication
H
©M J Larkin Biology & Biochemistry. The Queen’s University of Belfast.
F. Mechanisms of mutation. Cont….F. Mechanisms of mutation. Cont….
Adenine in its AMINO formWill bind to Thymine. TA pairing as expected.
Adenine in its IMINOform. Binds Cytosine.AC pairing.
Still 2 x H Bonds
N
NH
N H
2 x H Bonds
N
NH2
N
SIMILARLY
Forms in equilibrium with about 1 in 104 or 105 molecules in the ENOL or IMINO forms.Therefore natural transition per base per generation is also about 1 in 104 to 105.
©M J Larkin Biology & Biochemistry. The Queen’s University of Belfast.
F. Mechanisms of mutation. Cont….F. Mechanisms of mutation. Cont….
Must also have segregation after replication
AT
GT
GC
AT
1st replication2nd replication
This would happen about 1 in 104 to 105 !The potential for many deleterious mutations is highi.e. GENETIC LOAD is too high
Dale Ch 3
©M J Larkin Biology & Biochemistry. The Queen’s University of Belfast.
• WORKING AT ANOTHER LEVEL AFTER REPLICATION IS Mismatch repair.
– A form of excision repair (see later). DNA adenine methylation is involved in recognising sections of DNA to be repaired after replication.
• DNA can also become damaged due to a variety of influences.
– Chemical mutagens
– Ionising radiation (such as X rays, rays and UV254nm)
• Repair of this damage rapidly can lead to mutagenic effects.
• WORKING AT ANOTHER LEVEL AFTER REPLICATION IS Mismatch repair.
– A form of excision repair (see later). DNA adenine methylation is involved in recognising sections of DNA to be repaired after replication.
• DNA can also become damaged due to a variety of influences.
– Chemical mutagens
– Ionising radiation (such as X rays, rays and UV254nm)
• Repair of this damage rapidly can lead to mutagenic effects.
F. Mechanisms of mutation. Cont….F. Mechanisms of mutation. Cont….
Dale Ch 3
©M J Larkin Biology & Biochemistry. The Queen’s University of Belfast.
• Chemical mutagens– Base analogues: substitute for normal base but are not
proofread - mispairing • 2-aminopurine: Adenine analogue which pairs with
cytosine• 5-bromouracil: thymine analogue pairs with guanine
– Modification of bases: Do not require replication and induce DNA repair.• Nitrous acid (deaminates); hydroxylamine (reacts with
cytosine)• Alkalyating agents transfer alkyl groups to bases. Very
powerful; ethyl methane sulphonate (EMS); N-methyl, N-nitrosoguanidine (NTG)
– Intercalating agents: additions or deletions caused• acridine orange; ethidium bromide
• Chemical mutagens– Base analogues: substitute for normal base but are not
proofread - mispairing • 2-aminopurine: Adenine analogue which pairs with
cytosine• 5-bromouracil: thymine analogue pairs with guanine
– Modification of bases: Do not require replication and induce DNA repair.• Nitrous acid (deaminates); hydroxylamine (reacts with
cytosine)• Alkalyating agents transfer alkyl groups to bases. Very
powerful; ethyl methane sulphonate (EMS); N-methyl, N-nitrosoguanidine (NTG)
– Intercalating agents: additions or deletions caused• acridine orange; ethidium bromide
F. Mechanisms of mutation. Cont….F. Mechanisms of mutation. Cont….
©M J Larkin Biology & Biochemistry. The Queen’s University of Belfast.
AG or CT MismatchAG or CT Mismatch
PolymerasePolymerase
Removal of mismatch
Polymerisation in 5’ - 3’ direction
3’ - 5’ digestion by polymerase
F. Mechanisms of mutation. Cont….F. Mechanisms of mutation. Cont….Proofreading carried out by the 3’ to 5’ exonuclease activityof the DNA Polymerases III and I.
THIS WORKS AT ONE LEVEL
