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TOPIC – MUTATIONS AND MOLECULAR BASIS
OF MUTATION
Change in one or a few base pairs in a
gene.
 These are very small changes when
compared with the large-scale mutations.
 Point mutations can be divided into two
general categories:

› Base-pair substitutions
› Base-pair insertions or deletions (frame shift
mutations)

A base-pair substitution is the replacement of
one nucleotide and its partner in the
complementary DNA strand with another
pair of nucleotides.
THEY ARE DIVIDED INTO TWO TYPES:
 TRANSITION
 TRANSVERSION


Whenever a purine is
replaced by a purine (A
-G) or a pyrimidine is
replaced by pyrimidine
(C-T).
They therefore involve
bases of similar shape.


Transversions are
interchanges of purine
for pyrimidine
bases, and vice versa.
They therefore involve
exchange of onering and tworing structures.
Whenever there is ADDITION OR DELETION
of one or more than one base pair, it leads
to change in the reading frame of the triplet
codon.
 Proteins are built incorrectly.

Back Mutation is always II Mutation :
Whenever a wild type phenotype changes to a
mutant type this is known as FORWARD
MUTATION. If a second mutation occurs at
exactly at the same point where the first
mutation has occurs and reverse back the effect
of first mutation ( changes mutant to wild type
). This is called BACK MUTATION.

SUPPRESSES THE EFFECT OF FIRST MUTATION.
 Whenever a mutation suppresses the effect of
first mutation in the DNA, The second mutation
occurring at some other point as compared to
the first mutation. This is known as
SUPPRESSOR MUTATION.

SOMATIC MUTATIONS:
Whenever a mutation occurs in somatic cell , it
will be carried by all the other cell derived
from this somatic cell .
o They will not pass to progeny. They will
remain consistent to specific region or tissue.




OCCURS IN GAMETES (MAY NOT BE EXPRESSED UNTIL
OFFSPRINGS).
Mutation occurring in the reproductive cells or the
gametes or the germ cell is known GERM TYPE
MUTATION.
This type of mutation is carried to the next progeny.
They are mainly caused during DNA
replication or by incorporation of
incorrect nucleotide in the growing DNA
chain.
 They occur naturally by changes in DNA
sequence during replication.
 Caused by certain Mutagenic agents
present in the environment

Caused by changes in DNA brought about
some environmental factor called MUTAGEN.
E.g. – UV light, x-rays, gamma rays etc.
 EXAMPLE OF MUTAGENS :
1) Ultraviolet light.
2) Alpha, beta , gamma radiations.
3) Radioactive dust.





Each of the bases in DNA can appear in one of
several forms, called tautomers, which are isomers
that differ in the positions of their atoms and in the
bonds between the atoms.
The forms are in equilibrium. The keto form of each
base is normally present in DNA whereas
the imino and enol forms of the bases are rare.
The ability of the wrong tautomer of one of the
standard bases to mispair and cause a mutation in
the course of DNA replication. was first noted by
Watson and Crick when they formulated their
model for the structure of DNA.
Structures of bases are not static. Hydrogen atoms
move from one position to another. Such changes
are known as TAUTAMERIC SHIFT.
ADENINE AND CYTOSINE are present in stable
AMINO FORM. But sometimes changes into
less stable IMINO FORM.
 THYMINE AND GUANINE are present in
stable KETO FORM. But may change into less
stable ENOL FORM.
 These shifts are reversible.
 Tautameric shift can lead to TRANSITION AND
TRANSVERSION.
