Spontaneous mutations occur naturally without any apparent cause. It arises from a variety of sources- Errors in DNA replication, Spontaneous lesions or by Transposable genetic element. These mutations results in several human diseases.

1. Spontaneous mutation
Presented by:
Megha Varshini Gowda B R

2. Content
• Introduction
• Sources of spontaneous mutation
• Errors in DNA replication
a. Tautomeric shift
b. Insertion and deletion of base
c. Trinucleotide repeat expansion (TNRE)
• Spontaneous lesions
a. Depurination
b. Deamination
c. Oxidatively damaged bases
• Transposable genetic element
• Conclusion
• Reference

3. Introduction
• Spontaneous mutations occur naturally without any apparent
cause.
• Salvador Luria and Max Delbrück in 1943 designed fluctuation
test to illustrate random mutation.
• Replica plating developed by Joshua and Esther Lederberg in
1952 to demonstrate the presence of mutants before selection.
• Strand-slippage hypothesis was given by George Streisinger in
1966.
• The rate of spontaneous mutation for a particular gene is 10-5 to
10-9 per cell generation.
• It arises from a variety of sources-
1. Errors in DNA replication
2. Spontaneous lesions
3. Transposable genetic element

4. Errors in DNA replication
It occurs when an illegitimate nucleotide pair forms in DNA
synthesis, leading to base substitution.
Tautomeric shift:
• Each bases in DNA can appear in one of several forms-
tautomers.
• Tautomers are the isomers that differ in the positions of their
atoms and in the bonds between the atoms.
Base In normal state pairs
with
In tautomeric state pairs
with
A T C
T A G
G C T
C G A
Table 1: Formation of base pairs in normal and tautomeric state.

5. Amino form of Adenine and Cytosine are more stable
Keto form of Guanine and Thymine are more stable
Figure 1: Tautomeric forms of (a) Adenine and Cytosine (b) Guanine and Thymine.
a.
b.

6. • Tautomeric shift in incoming base (substrate transition) or the
base already in the strand(template transition) results in
mispairing.
Fig.2: Hydrogen bonded A:C and G:T base pairs.

7. • Mismatch could be repaired via proofreading activity of DNA
Polymerase or via mismatch repair system.
• If it fails, one of the progeny will be a mutant type.
Fig.3: Mechanism by which tautomeric shifts in the bases in DNA cause mutations.

8. Insertion and deletion
• If a newly synthesized strand loops out it results in the addition of
an extra nucleotide and looping of template strand results in deletion
of nucleotide base in newly synthesized strand. Region with
repeated sequences are prone to such errors.
Fig.4: Strand slippage during replication results in addition or deletion of base pairs.

9. • Insertion or deletion of one or more nucleotides during replication
leads to frameshift mutation.
• As the reading frame shifts the outcome of frameshift mutation is
complete alteration of the amino acid sequence of a protein.
Fig.5: Frameshift mutation by nucleotide insertion.

10. Trinucleotide repeat expansion (TNRE)
• TNRE are the hotspots of mutation.
• It is a phenomenon in which a repeated sequence of three
nucleotides can readily increase in number from one generation to
another.
• DNA polymeraese slips off the DNA after the repeat sequence is
synthesized.
• A hairpin is quickly formed and DNA polymerase hops back onto
the DNA and continues with DNA replication.
• Depending on how the DNA is repaired, this may result in
trinucleotide repeat expansion.
• TNRE results in human diseases such as Spinal and bulbar
muscular atrophy-(CAG), Huntington disease-(CAG), Fragile X
syndrome(CGG, GCC)

11. Fig.6: Possible mechanism for trinucleotide expansion.

12. Spontaneous lesions
• Naturally occurring damage to the DNA (spontaneous lesions)
causes mutation.
Depurination :
• Refers to removal of purine base from its position.
• The space formed after removal is called apurinic (AP) site.
• A mammalian cell spontaneously loses about 10,000 purines
from its DNA in a 20-hour cellgeneration period at 37°C.
• In replication, the apurinic sites cannot specify a base
complementary to the original purine.
• If repair system fails, during replication any of the four bases
are added to the new strand, leading to mutation.
• The chance of causing mutation is 75%.

13. Fig.7: Depurination of DNA

14. Deamination:
Removal of amino group from the base.
• Deamination of Cytosine produces Uracil.
• Uracil-DNA glycosylase recognizes the Uracil residue in DNA
and excises them.
Fig.8: Deamination of Cytosine.
H2O
+ NH3

15. • Deamination of 5-methylcytosine generates thymine (5-
methyluracil), which is not recognized by the enzyme uracil-
DNA glycosylase and thus is not repaired.
• Therefore, C → T transitions generated by deamination are seen
more frequently at 5-methylcytosine sites, because they escape
this repair system.
Fig.9: Deamination of 5-methylcytosine.
H2O
+ NH3

16. Oxidatively damaged bases:
• Oxidative DNA damage refers to change in DNA structure that are
caused by ROS.
• Reactive Oxygen Species(ROS) such as hydrogen peroxide (H2O2),
superoxide (O2·), hydroxyl radical (OH·) are products of
metabolism in all aerobic organisms.
• DNA bases, particularly Guanine are vulnerable to oxidation which
leads to oxidized guanine products.
• Guanine on oxidation forms 8-Oxo-7-hydrodeoxyguanosine (8-oxo
dG) which base pairs with adenine during DNA replication.
Fig.10: Oxidatively damaged bases.

17. Transposable genetic element
• Transposable genetic element are DNA elements that can move from
one site in the genome to another site.
• The insertion of
a transposon into
a gene will often
render the gene
nonfunctional. If
the gene encodes
an important
product, a mutant
Phenotype is likely
to result.
Fig.11: Mutation caused by insertion of transposable genetic element.

18. • Crossing over between homologous transposons located at different
positions in a chromosome.
• If two transposons in the same orientation pair and cross over, the
segment between them will be deleted.
Fig.12: Formation of a deletion by intrachromosomal recombination between two
transposons in the same orientation.

19. Common causes of
Spontaneous mutation
Description
Errors in DNA
replication
A mistake by DNA polymerase may cause point mutation
Tautomeric shift Spontaneous changes in base structure can cause mutations if
they occur immediately prior to DNA replication
Depurination The linkage between purines and deoxyribose can spontaneously
break
Deamination Cytosine and 5-methylcytosine can spontaneously deaminate to
create Uracil and Thymine
Toxic metabolic
products
Reactive oxygen species are chemically reactive and can alter the
structure of DNA
Transposable elements Transposable elements can insert themselves into the sequence of
a gene.
Summary and Conclusion
• Spontaneous mutations can be generated by different sources.
Replication errors and spontaneous lesions generate most of the
base-substitution and frameshift mutations. These mutations results
in several human diseases.

20. References
• Snustad, D.P., Simmons, M.J. and Jenkins, J.R. (1997).
Principles of Genetics. 6th edn. John Wiley & Sons, Inc. N.Y.
Pp.320-324, 497-498.
• Tamarin, R.H. (1985). Principles of Genetics. 7th edn. TATA
McGRAW HILL edn. Pp.328-332.
• Griffiths A.J.F, Miller J.H, Suzuki D.T, et al.(2000). An
Introduction to Genetic Analysis. 7th edition. W. H. Freeman,
New York. Pp.422-438
• Brooker R.J. (2004). Genetics –analysis and principles. 2nd edn.
Addison Wesley Longman Inc. California. Pp.318-325
• https://www.nature.com/scitable/topicpage/dna-replication-and-
causes-of-mutation-409