sri harshini degree&pg college

1. B.Sc BIOTECHNOLOGY – Semester VI
SRI HARSHINI DEGREE & PG COLLEGE.
ONGOLE-523001.
Dr Vemu Anil kumar M.Sc, Ph.D.nanobiotechanil@gmail.com

2. Mutations :
• Change in the Nucleotide sequence of DNA.
• Occurs in both Somatic cells and Gametes.
Mutagenesis :
• It is a process by which the genetic information of an organism is
changed, resulting in a mutation.
• Spontaneous mutations.
• Induced mutations.
Mutagens :
• Agents which cause Mutations.

3. MUTATIONS
Spontaneous
(Natural)
Tautomerism
Depurination
Deamination
Slipped strand
mispairing
Induced (External
agents)
Physical mutagens
Radiations
Non-Ionising: UV
rays
Ionising: X rays,γ
rays
Chemical
mutagens
Nitric acid
Hydroxyl amine
Alkalyting agents

4. I. Spontaneous Mutations :
• Damages in DNA or Replication errors.
• Cause is not exactly known.
• Errors in base pairing occurs even after Proof reading.
1. Tautomerism:
• Base is changed by Repositioning of a H-atom by altering H-Bonding
resulting in Incorrect base pairing during replication.
• Errors in base pairing occurs even after Proof reading.
2. Depurination:
• Loss of Purine bases to form Apuric site (AP site).
• The β-N-glycosidic bond is hydrolytically cleaved to release a Purine base.

5. 1. Tautomerism: 2. Depurination:

6. 3. Deamination:
• Removal of Amino group.
• Normal Base convert in to Base containing Keto group.
4. Slipped strand mispairing (SSM / replication slippage):
• Occurs during replication.
• It involves denaturation and displacement of the DNA strands, resulting in
mispairing of the complementary bases.
• Leads to Insertion or Deletion.

7. 3.Deamination

8. 4. Slipped strand mispairing

9. II. Induced Mutations :
• Artificially caused mutations.
1. Physical Mutagens :
a. Non ionising radiations: U.V radiations.
• Two nucleotide bases in DNA, Cytosine and thymine are most damaged
by radiation and change their properties.
• Adjacent pyrimidine bases are covalently joined as a Pyrimidine dimer.
• Cause oxidative damage to DNA.
b. Ionising radiations: x- rays, γ- rays, β- rays.
• High penetration capacity.
• Produce reactive atoms or molecules – Damage to N Base and Sugar.
• Crosslinking of DNA to itself.
• Cause cancer or death.

11. 2. Chemical Mutagens :
i. Nitrous acid ii. Hydroxyl amine iii. Alkylating agent - Ethyl methyl sulphate.
i. Nitrous acid :
• Replaces Amino group with Hydroxyl group.
• Reacts with Adenine to form Hypoxanthine (similar to Guanine).
• N-Base, pairs with Cytosine instead of Thymine after 2 replications.

13. ii. Hydroxyl amine :
• Reacts with Amine group of Cytosine to change its Tautomerism and pairs with
Adenine instead of Guanine.
• Mismatch pairing.

14. iii. Alkylating agent:
• Ethyl methyl sulphate (EMS) or Ethyl ethane sulphate (EES).
• EMS with Guanine makes Alkylated guanine - analogue to Adenine.
• Remove alkylated Guanine – Depurination.
• Cause Deletion mutations.

15. Types of Muations
I. Point Mutations or Single base mutation :
• Single nucleotide is Substituted, Inserted or Deleted. Either by Transition or by
Transversion.
Consequences of Point Mutations :
1. Silent mutations.
2. Missense mutations.
3. Non – sense or Termination mutations.

16. 1. Silent mutations :
• Condon containing CHANGED BASE codes for the same Amino acid.
• Normal protein and mutant protein contain same Amino acid sequence.
Eg : AGT - UCA codes for Serine. Third Base is changed by Transition.
AGC - UCG also codes for Serine. Degeneracy of Genetic code.
2. Missense mutations :
• Condon containing CHANGED BASE codes for different Amino acid.
Eg : GCA - CGU codes for Arginine. Second Base is changed by Transversion.
GAA - CUU codes for Leucine.
2. Non – sense or Termination mutations :
• Condon containing CHANGED BASE becomes a Non-sense codon.
Eg : AGT - UCA codes for Serine. Second Base is changed by Transverstion.
ATT - UAA - Non sense codon.

18. II. Frame shift base mutation :
• It is a genetic mutation caused by Indels (insertions or deletions) of a number
of nucleotides in a DNA sequence.
• An Inserted or deleted nucleotide further alters the triplet codons and shifts
the reading frame - Framing error or a reading frame shift.
• It can make protein synthesis stop prematurely by forming a stop codon.
• It resulting in a Non-functional protein that often disrupts the biochemical
processes of a cell.
• Crohn’s disease, cystic fibrosis and some forms of cancer.
• Usually changes to the genetic material in every cell, it is rare to find a cure.

20. III. Auxotrophic mutation :
• Producing a strain with loss of ability to synthesise an essential metabolite. It
may be one of the protein amino acids, nucleic acid base, vitamin etc.
• Mutant strain that will proliferate only when the medium is supplemented with
some specific substance (not required by wild-type organisms).
• Auxotrophic gene markers are used in Molecular genetics.
IV. Suppressor mutation :
• It is a secondary mutation that Partially or Completely restores a lost function
by primary mutation.
• It is used for identifying new genetic sites which effect a biological process of
interest.

21. 1. Back mutation : It causes reversion. A change in a nucleotide pair in a mutant
gene that restores the original sequence to produce original phenotype (Wild).
2. Intragenic mutations : It is a mutation that occurs in the same gene where the
first mutation had occurred.
• Used to study the fundamental nature of Genetic code.
3. Intergenic mutations : It happens in a location somewhere else in the DNA,
which is not in the same gene where the original mutation had happened.
• Used in identifying and studying interactions between the molecules such as
proteins.