MUTATIONS
WILD-TYPE ORGANISM: The organism that carries the normal, unaltered gene.
MUTANT ORGANISM: The organism with the altered gene.
Eg.: lac
+
E.coli is wild, while lac
-
is a mutant
MUTATION: Any change in the base sequence of DNA
TYPES OF MUTATIONS
Point Mutations
Multiple Mutations
Say, the wild type gene is: CGACCTAGT
Base Substitution: CGACCGAGT (Pt Mut)
Base Addition: CGACCATAGT (Pt Mut)
Base Deletion: CGACC?AGT (Pt Mut)
Base Rearrangement: CGATACCGT (Multiple Mut)
Mutations, in terms of a.a
Missense Mutation:
If there is an a.a substitution
Temperature-sensitive (Ts) Mutation:
Substitution of an a.a, producing a protein that is active at one temp and inactive
at another.
Nonsense / Chain-termination Mutation:
Substitution of a non-sense codon leading to no further a.a being added and
causing end of translation
SILENT MUTATION:
When an a.a substitution has no detectable effect on the phenotype of a cell.
Eg. In a hydrophobic cluster / pocket of a protein, replacement of a non-polar a.a as Leu
by another non-polar a.a as Ile
LEAKY MUTATION
When an a.a substitution is only partially disruptive w.r.t. functionality of the protein.
Leu Ile might be fine
But, Leu a bulkier Phe will not be favorable
Leads to slight stereochemical changes in the hydrophobic cluster
Hence, reduction, rather than loss of activity, of that enzyme is resulted.
TRANSITIONS:
The original and the altered base pairs maintain the original Pu-Py or Py-Pu
orientation,
viz., If TA, then, CG or If AT then GC
TRANSVERSIONS:
A change of Pu-Py orientation occurs.
viz., Pu-Py Py-Pu and Py-Pu Pu-Py
MUTAGENESIS
MUTAGENESIS: The process of formation / production of a mutation.
MUTAGEN: The physical or chemical agent that causes mutations. Eg. Nitrous Acid
Classification of Mutagenesis
Induced Mutagenesis
Base-analogue Mutagenesis
Chemical Mutagenesis
Mutagenesis by Intercalating Agents
Spontaneous Mutagenesis
Back Mutation / Reverse Mutation / Reversion
Induced Mutagenesis
Occurs by the addition of a physical agent or a chemical reagent (mutagen)
Spontaneous Mutagenesis
Occurs in nature, say, due to the errors of replication; No known mutagen is responsible for this.
INDUCED MUTAGENESIS
Base-analogue Mutagenesis
Base-analogue: A compound sufficiently similar to one of the 4 bases, such that it can
be added during DNA Replication.
Mode of action: Substitutes for a standard base during replication and causes a new bp to
appear in daughter cells, later on.
Eg. 5-Bromouracil: AT becomes GC & GC becomes AT
Eg. 2-Aminopurine: AT becomes GC
Chemical Mutagenesis
Mode of Action: Chemical alteration of a standard base so that a new base appears in daughter
cells, later on.
Eg. Nitrous Acid: Converts amino groups to keto groups by oxidative deamination.
C becomes U, A becomes Hypoxanthine & G becomes Xanthine
Mutagenesis by Intercalating Agents
Mode of Action: Unknown
INTERCALATION: The process by which substances form stacked arrays and are also able to
stack with a base pair; the insertion occuring between bases in adjacent pairs.
Eg. Substituted Acridines like Acridine Orange, Proflavine, & Acriflavine: 3-ringed molecules
with dimns same as a Pu-Py pair.
SPONTANEOUS MUTAGENESIS
Back Mutation / Reverse Mutation / Reversion
The process in which the wild type phenotype is regained from the mutant state.
Eg.
Wild type E.coli is leu
+
( ?? )
Mutate leu gene leu
--
E.coli.
Plate, say, 100 leu
--
E.coli onto Agar plate lacking Leucine No colonies
Plate, say, 10
10
leu
--
E.coli onto Agar plate lacking Leucine Few colonies
These colonies have E.coli that can produce their own Leucine and grow in Leucine-less media.
These cells are called leu
+
Revertants.
