Mutations introduce heritable changes in an organism's genetic material. They occur spontaneously due to errors in DNA replication or due to exposure to mutagens. There are two main types of mutations: spontaneous mutations, which occur without external causes, and induced mutations, which are caused by mutagenic agents. Spontaneous mutations result from replication errors or transposable elements, and include transitions, transversions, and frameshift mutations. Induced mutations are caused by mutagens that damage DNA, such as base analogs, agents causing specific mispairing, intercalating agents, and substances like UV radiation. Mutations can have various effects that may or may not result in an observable change in phenotype, depending on whether they are

1. MUTATION
Ankita Behera
M.Sc Microbiology
St. George College of Management & Science
Bengaluru North University

2. • Inheritance is based on genes that are faithfully transmitted from
generation to generation in micro-organisms(in all organisms).
• Despite the biochemical mechanism that facilitate such transmission
faithfully, sudden change in the sequence of nucleotide bases in genes
can and do occur.
• These changes called “MUTATION”,introduce variability into the gene
pool and are heritable
• Therefore, a mutation can be define as “sudden and heritable change in
the nucleotide sequence of a gene”
• Mutation always change the GENOTYPE(the genetic constitution of the
cell)and the PHENOTYPE(the expression of the genotype in observable
of the micro-organisms.

3. • Mutation is an important phenomenon because it is the ultimate source
of genetic variation and provides the raw material for evolution . Thus ,
mutations are essential to provide new genetic variability to allow micro-
organisms to evolve and adapt to environmental changes.
• At the same time if mutations occurred too frequently they would totally
disrupt the transmission of Genetic information from generation to
generation.
• It occurs only rarely in nature, for example-in a microbial population
typically contains about one in cells that carries a detectable mutation in
any given gene but exposure of micro-organisms to certain powerful
mutagens increases dramatically the frequency of their mutant cells by
as much as 10*5 fold.

4. OCCURRENCE OF MUTATIONS:-
Mutations occur in 2 ways
1}spontaneous mutation
2}induced mutation

5. SPONTANEOUS MUTATION :-
• Spontaneous mutation are those that arise occasionally in the absence
of a known cause i.e , without exposure to external agents.
• These mutations may result from errors in DNA replication or from the
action of transposons or even from the effect of sone mutagens agents
present in the environment.
• Some most commonly operation spontaneous mutation mechanism
are:-
• 1}transition and transversion mutations
• 2}Frameshift mutations

6. 1}TRANSITION AND TRANSVERSION
MUTATION :-• The bases of nucleotides in DNA strands exist in tautomeric form i.e , they exist as
structural isomers that are in chemical equilibrium and readily change into one another.
• Bases exist in keto form and they can take on either an IMINO or ENOL form by error
resulting in a rare tautomeric form.
• These tautomeric shifts alter the hydrogen bonding characteristics of the bases
allowing purine for purine or pyrimidine for pyrimidine substitution which can eventually
result in a stable alteration of the nucleotide sequence.such substitutions are
“TRANSITION MUTATI”.
• In transversion mutation, a pyrimidine substitutes a purine or a purine substitutes a
pyrimidine.
• These mutations are rarer due to steric problems of pairing pyrimidine with pyrimidine
and purine with purine.

8. 2}FRAMESHIFT MUTATIONS:-
• It is usually caused by the deletions of DNA segments resulting in an altered codon
reading frame.
• It occurs where there is a short repeated nucleotide sequence.
• In such location, the pairing of template and its complementary strand which can be
displaced by the distance of the repeated sequence leading to additions or deletion of
bases in the complementary strand.
• These spontaneous frameshift mutations originate from lesion in DNA as well as from
REPLICATION errors.
• For example , it is possible for purine nucleotides to be depurinated.
• this results in the formation of an “APRINIC SITE” which will not base pair normally
and may cause a transition type mutation after the next round of replication.
• Cytosine can be deaminated to uracil which is then removed to form an
“APYRIMIDINIC SITE”.

10. INDUCED MUTATION
•Any agent that directly damages DNA
changes its chemistry or interefers with
repairs mechanisms will induce mutation
and the agents which cause such actions
are known as “MUTAGENS”.
•They can be classified according to their
mechanisms of action:-

11. I}BASE ANALOGS
• These are similar to normal nitrogenous bases and can be
incorporated into the growing polynucleotide chain during
replication.
• Once in place , these compounds typically exhibit base
pairing properties different form the bases they replace and
cause mutation.
• 5-Bromouracil is a widely used base analog of thymine.
• It undergoes tautomeric shift from the normal keto form to an
enol more frequently than does a normal base .
• Then the enol forms hydrogen bonds and directs the
incorporation .

13. B}SPECIFIC MISPAIRING
• This action is caused when a mutagen changes a base structure and therefore , alters
its base pairing characteristics.
• Mutagens are selective and they preferentially react with some bases and produce a
specific kind of DNA damage.
• Example:- Methyl- nitroguanidine is an
alkylating agent that adds methyl
group to guanine causing it to
mispair with thymine
• Ethylmethane sulfonate and hydroxylamine are example which cause mispairing.

15. C}INTERCALATING AGENTS
• This agent distort DNA to induce single nucleotide pair
(insertion and deletion ).
• These mutagens are planar and insert themselves
between the stacked bases of the helix.
• This results in a mutation and possibly through the
formation of a loop in DNA.
• Ex:-Proflavin and Acridine orange

17. D}OTHER MUTAGENS
• UV radiation generates Cyclobutane type dimero between
adjacent pyrimidines.
• Aflatoxin B1 and other Benzopyrene derivatives cause
damage to DNA which is lethal but may trigger a repair
mechanism that restores much of the damaged genetic
material .

18. THE EXPRESSION OF MUTATIONS
• The expression of a mutation can be observed if it produces a
detectable , altered phenotype.
• A mutation from the most prevelant gene form the wild type to a mutant
form is called a “FORWARD MUTATION”.
• A second mutation may make the mutant appear to be a wild type
organism again such mutation is called “reversion mutation”or “BACK
MUTATION”.In these organism reverts back to its original phenotype .A
true mutation converts the mutant nucleotide sequence back to the wild
type arrangement.
• The wild type phenotype can be regained by second mutation in a
different gene called “SUPPRESSOR MUTATION” which overcomes the
effect of first mutation but sometimes , mutation may takes place and do
not alter the phenotype.
• When a large deletion and insertion mutations exist and affect only one
base pair in a given location called”POINT MUTATION”.

19. THE DIFFERENT TYPES OF POINT
MUTATIONS ARE:-

20. I}SILENT MUTATION:-
• It is a kind of mutation that could not be detected without use
of nucleic acid sequencing techniques.
• Mutations can occurs and have no visible effect because of
code degeneracy.
• For example-if codon CGU is changed to CGC it will still code
for arginine even though a mutation had occurred.
• The expression of this mutation would not be detected
except at the level of the DNA or mRNA.
• Since , there would be no change in the protein there would
be no change in the phenotype of the organism.

22. II}MISSENSE MUTATION
• This type of mutation involves a single base
substitution in the DNA that changes a codon for one
amino acid into a codon for another .

23. III}NONSENSE MUTATIONS:-
• It involves the early termination of translation and
therefor results in a shortened polypeptide.
• Such mutation involve for conversion of a same codon
to a nonsense codon or stop codon.
• Depending on the relative location of mutation the
phenotypic expression may be more or less severely
affected.

25. IV}FRAMESHIFT MUTATIONS
• This mutation arise from the insertion or deletion of
one or two base pairs within the coding region of the
gene.
• These mutations normally are very deleterious and
yield mutant phenotype resulting from the synthesis of
non-functional proteins.

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