Mutant: An organism with an altered DNA sequence in one or several genes.

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Mutants and Mutations

• Mutant An organism with an altered DNA sequence
  in one or several genes.
• Mutation Any heritable alteration in the genetic
  material.

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Genotype
Genetic composition of a mutant
lacZ1 hisC527
Phenotype
Observable properties of a mutant
Lac-
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Random Mutation vs Directed Change

• Random mutation
• Mutation does not have a specific purpose.
• Directed-change
• Mutation is created to fit a specific environment
  condition

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Luria Delbrück Experiment
Search for mutants that are resistant to T1 phage
Culture no. No. of mutant 1 1 2 0
3 3 4 0 5 0 6 5 7
0 8 5 9 0 10
6 11 107
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Lederbergs Experiment
With antibiotic
Without antibiotic
Duplication
(Streptomycin)
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Mutations

• Spontaneous mutations
• Mutagens

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Spontaneous Mutation
DNA replication errors G sometime pairs with
T Insertion and deletion DNA repair
errors Repair of damaged DNA is prone to
errors. Modification of nucleotides Spontaneo
us deamination of cytosine to uracil
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Cystosine
Uracil
GC ---gt AU
Uracil glycosylase to remove uracil in DNA
structure.
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Mutagens
Mutagen A chemical or physical agent that
increase the frequency of mutation.
UV irradiation Form pyrimidine dimers. Base
analogs 2-aminopurine Bromo-uracil Alkylating
agents adding CH3- or CH3CH2- group to
bases. Hydroxylamine N-methyl-N-nitrosohuanidi
ne Ethlymethane sulfonate Ethylethane sulfonate
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(Mutagens continue)
Intercalating agents Insert between DNA base
pairs. Oxidative damages Nitrous
acid Reactive free radicals (H2O2, superoxide,
hydroxyl radicals).
Some mutagens have been used for cancer treatment.
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The Ames Test
Bruce Ames. Bacterium Salmonella typhimurium
His-. Revertants
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Ames Test Experiment
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Mutations at DNA level
Single base mutation Transition
mutation Purine replaced by purine, or
pyrimidine replaced by pyrimidine AT --gt GC
GC--gt AT. Transversion mutation Purine
replaced by pyrimidine Pyrimidine replaced by
purine AT --gt CG GC--gt CG.
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Insertion and Deletion
A. Strand slippage during replication B.
Intra-molecular recombination
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Strand slippage induced deletion
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Strand slippage induced insertion
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Intra-molecular recombination-mediated deletion
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Intra-molecular recombination-mediated insertion
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Mutations at protein level
Single base mutation Silent mutation Multiple
codons for one amino acid e.g. GAG (Glu)--gt GAA
(Glu) Mis-sense mutation e.g. GAG (Glu)--gt
GAC (Asp) Nonsense mutation e.g. GAG (Glu)--gt
TAG (stop)
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Mutations at Protein Level-2
Frame shift mutation
5' AUG UGG GGA CCC AAG GGU AGC CCC ... 3'
(wild-type) met trp gly pro
lys gly ser pro ... 5' AUG UGG
GGG ACC CAA GGG UAG CCC C.. 3' (mutant)
met trp gly thr gln gly stop
Insertions and deletions.
Mutations could occur in DNA regions not coding
for proteins
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Bacterial Phenotypes
Auxotrophic mutants (v.s. prototrophic wild
type) Examples a) requires certain
nutrition to grow e.g. His- b) can not use a
specific sugar as energy source e.g.
Lac- Resistant mutants (v.s. sensitive wild
type) Examples a) resistant to antibiotics b)
resistant to toxic substances c) resistant to pH
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Bacterial Phenotypes
Hypersensitive mutants Examples a)
Hypersensitive to radiation b) Hypersensitive to
oxygen Conditional mutants Temperature-sensitive
mutants Permissive temperature Non-permissiv
e temperature