D. Types of mutation

1
D. 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.

AAT DNA UUA mRNA Leu amino acid
Can mutate to..
CUA GUA AUA UCA UUC UUG
UUU UCA Leu Val Ile
Ser Phe Leu Phe
Ser
UGA UAA Stop Stop
2
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.

3
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)

i.e.
Replication slippage
Recombination and deletion
4
E. 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

e.g.
UAG STOP Amber
CAG Gln
AUC
Mutation in another tRNA leads to incorporation
of a different amino acid
Protein may or may or be functional
5
F. 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)

? Dale Ch 3
6
F. Mechanisms of mutation. Cont.
2 x H Bonds
H
3 x H Bonds
O
CH3
N
N
H
O
H
Thymine in its ENOL form. Binds Guianine. TG
pairing during replication
Thymine in its KETO form Will bind to Adenine TA
pairing as expected during replication
7
F. Mechanisms of mutation. Cont.
SIMILARLY
Still 2 x H Bonds
2 x H Bonds
Adenine in its IMINO form. Binds Cytosine. AC
pairing.
Adenine in its AMINO form Will bind to Thymine.
TA pairing as expected.
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.
8
F. Mechanisms of mutation. Cont.
This would happen about 1 in 104 to 105 ! The
potential for many deleterious mutations is
high i.e. GENETIC LOAD is too high
? Dale Ch 3
9
F. Mechanisms of mutation. Cont.

• 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.

? Dale Ch 3
10
F. Mechanisms of mutation. Cont.

• 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

11
F. Mechanisms of mutation. Cont.
Proofreading carried out by the 3 to 5
exonuclease activity of the DNA Polymerases III
and I.
Removal of mismatch
Polymerisation in 5 - 3 direction
THIS WORKS AT ONE LEVEL