Mutations can occur spontaneously during DNA replication or be induced by environmental factors like chemicals or radiation. Spontaneous mutations arise from errors in DNA replication or chemical changes to bases like deamination, while induced mutations are caused by mutagens that damage DNA like radiation, base analogs, or intercalating agents. Both spontaneous and induced mutations can lead to changes in the genetic code through base substitutions, insertions, or deletions.

1. By Shariqa Aisha
University of Kashmir
Department of Bioresources
Molecular mechanism of mutations
Spontaneous and induced mutations
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2. Learning Objectives
To know about the basics of mutations.
 To learn about the types of mutation.
 To understand the molecular mechanism spontaneous mutation.
 To learn the basics of induced mutations.
 To get a glimpse of chemical and physical mutagens involved in induced
mutations.

3. Introduction
 The term ‘mutation’ was first used by Hugo de Vries in 1901, for the
variations he observed In the evening primose, Oenothera lamarckiana.
 Mutations in its broad sense also include the chromosomal aberrations
but in most of the modern literatures mutation refers to only gene
mutation.
 Gene mutation can be defined as the change in the genetic material(
generally DNA) .
 Mutation is an alteration in the genetic material (the genome) of a cell
of a living organism or of a virus that is more or less permanent and
that can be transmitted to the cell’s or the virus’s descendants.
Hugo de Vries

4. Introduction
Mutations can occur spontaneously when DNA is being replicated during cell
division, but also can be induced by environmental factors, such as chemicals
or ionizing radiation (such as UV rays).
Mutation in the DNA of a body cell of a multicellular organism (somatic
mutation) may be transmitted to descendant cells by DNA replication and
hence result in a sector or patch of cells having abnormal function, an example
being cancer.
 Mutations in egg or sperm cells (germinal mutations) may result in an
individual offspring all of whose cells carry the mutation, which often confers
some serious malfunction, as in the case of a human genetic disease such as
cystic fibrosis

5. Types of Gene Mutations
 Gene mutations can be of different types:
Base pair substitution mutation: It is a change
from one base pair to another in DNA. It is are of
two types_
i) Transition mutation: It is a mutation from one
purine-pyrimidine base pair to the other
purine-pyrimidine base pair, such as A_T to
G_C.
ii) Transversion mutation: It is a mutation from a
purine-pyrimidine base pair to a pyrimidine-
purine base pair, such as G_C to C_G or A_T to
C_G

6. INSERTIONS AND DELETIONS
Insertion
 Insertions are mutations in which extra
base pairs are inserted into a new place
in the DNA.
 Insertions of foreign DNA sequences
into a gene disrupt its function, as in
hemophilia A caused by insertion of
repetitive sequence into the F8C gene.
Deletion
 It can be large gene deletions as large as
whole gene deletions e.g., Alpha
thalassemia.
 It can be partial gene deletions e.g.,
Duchene muscular dystrophy.

7. Spontaneous and Induced Mutations
 Mutagenesis, the creation of mutations, can occur spontaneously or can be induced.
Spontaneous mutations
 Mutations that happen naturally are called spontaneous, such as those occurring
during enzymatic process of DNA replication.
 They are random in nature and produce genetic variations in a population, which
provide the raw material for evolution.
 Spontaneous mutations also can result from the movement of transposable genetic
elements.
 In humans, the spontaneous mutation rate for individual genes varies between
10−4 and 4 × 10-6 per gene per generation.
 Most spontaneous errors are corrected by cellular repair systems.
 Only some errors remain uncorrected as permanent changes.

8. Causes of Spontaneous Mutation
DNA REPLICATION ERRORS
 Mispairing in the course of replication is a source of spontaneous base substitution.
 All of the bases in DNA can exist 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 found in DNA, whereas the enol forms of the
bases are rare.
 The complementary base pairing of the enols is different from that of the keto forms.
 The mispairing of enol form to that of keto form results in tautomeric shifts.

9. DNA POLYMERASES INACCURACY
 DNA polymerase makes errors in replication e.g., inserting a T opposite to a G.
 Most of the errors are repaired by the proofreading function of the replication
complex but some errors escape and become permanent.
SPONTANEOUS CHEMICAL CHANGES
 Depurination and deamination of particular bases are two common chemical events
that produce spontaneous mutations.
 These events create lesions_ damaged sites in the DNA.
DEPURINATION:
 It refers to the removal of purine base from DNA when the bond hydrolysis between
the base and the deoxyribose sugar, resulting in an apurinic site.
 Depurination occurs because the covalent bond between the sugar and purine is
much less stable than the bond between sugar and pyrimidine and is very prone to
breakage.

11. DEAMINATION
 It is the removal of an amino group from a base e.g., the deamination of cytosine
produces uracil, which is not a normal base in DNA, although it is normal base in RNA.
 A repair system replaces most of the uracils in DNA , thereby minimizing the mutation
consequences of cytosine deamination.
 One of the repair enzymes in the cell, uracil-DNA glycosylase, recognizes the uracil
residues in the DNA that arise from deaminations and excises them, leaving a gap that is
simultaneously filled in.
 However, if uracil is not replaced, an adenine will be incorporated into the new DNA
strand opposite it during replication, eventually resulting in a GC to TA transition
mutation.

12.  Similarly, 5-methyl cytosine( certain bases in prokaryotes and eukaryotes are normally
methylated) results into thymine after deamination.
 5-methyl deamination mutations are less likely to be corrected, locations of 5-methyl in the
genome often appear as mutational hot spots_ that is, nucleotides where a higher than
average frequency of mutation occurs.

13. OXIDATIVELY DAMAGED BASES
 Damage of bases through oxidative damage is another source of spontaneous lesions.
 Active oxygen species, such as superoxide radicals( O2 D), hydrogen peroxide ( H2O2),
and hydroxyl radicals(OHD), are produced as by-products of normal aerobic metabolism.
 These oxidative species can cause oxidative damage to DNA, as well as precursors of
DNA( such as GTP), resulting in mutation.

14. ALKYLATION
 It is the another type of spontaneous DNA lesion.
 It is the addition of alkyl ( methyl, ethyl, occasionally propyl) groups to the bases or
backbones of DNA.
Alkylation can occur through reaction compounds such as S-adenosyl methionine with
DNA.
INDUCED MUTATIONS
 Mutation can be induced by exposing organisms to physical mutagens, such as radiation,
or to chemical mutagens.
 Induced mutations play an important role in the study of mutations.
 Since the rate of spontaneous mutation is so low, geneticists use mutagens to increase the
frequency of mutation so that a significant number of organisms have mutations in the
gene being studied.
 The mutagens can be divided into two types:

15. 1. CHEMICAL MUTAGENS: A large number of chemicals are reported to be mutagenic.
Some important ones are:
BASE ANALOGS
 These chemically structurally resemble purines and pyrimidines and may be incorporated
into DNA in place of the normal bases during DNA replication.
(a) 5- Bromouracil(5-BU)
 It resembles thymine( has Br atom instead of methyl group) and will be incorporated into
DNA and pair with adenine like thymine.
 The presence of Bromine atom significantly alters the distribution of electrons in the base
ring, so 5-BU can frequently change to either the enol form or an ionized form.
 The ionized form pairs with guanine and causes A-T to G-C transitions in the course of
replication.

17. (b) 2- Aminopurine
 It is an adenine analog which can pair with T or in protonated form mispair with C.
 It causes A: T to G: C or G: C to A: T transitions.

18. BASE MODIFYING AGENTS
These are chemicals that act as mutagens by
modifying the chemical structure and properties of
bases
(a) Nitrous acid ( HNO3)
 It causes conversion of C to U, G to xanthine, and
A to hypoxanthine through oxidative deaminations.
 U in DNA pairs with A, xanthine in DNA pairs with
C and hypoxanthine in DNA pairs with C and cause
AT and GC transitions.
(b) Nitrosoguanidine, methyl methanesulphonate,
ethyl methanesulphonate
 Chemical mutagens that react with bases and add
methyl or ethyl groups.
Mutation by Ethyl methanesulphonate

19. INTERCALATING AGENTS
Ethudium bromide, Acridine Orange, Proflavin, IRC-191, etc.
 These are flat, multiple ring molecules which interact with bases of DNA and insert
between them.
 The insertion causes a stretching of the DNA duplex so that DNA polymerase inserts an
extra base opposite an intercalated molecule.
 The result is that intercalating agents cause frameshift mutations.
PHYSICAL MUTAGENS
IONIZING RADATION
 X- and γ- rays are energetic enough that they produce reactive ions ( charged atoms and
molecules) which react with biological molecules.
 Ionizing radiation produces a range effects on DNA both through free radical effects and
direct action
i) Breaks in one or both strands.
ii) Damage to/loss of bases.
iii) Crosslinking of DNA to itself or proteins.

20. ULTRAVIOLET RADIATIONS
 UV radiation is less energetic and therefore,
non-ionizing, but its wavelengths are
preferentially absorbed by bases of DNA and by
aromatic amino acids of proteins.
 UV is normally classified in terms of its
wavelength_
i) UV-C ( 180-290) : germicidal_ most energetic
and lethal, it is not found in sunlight because it
is absorbed by the ozone layer.
ii) UV-B (290-320) : major lethal/mutagenic
fraction of sunlight. It produces Pyrimidine
dimers.
iii) UV-A(320nm_ visible): near UV, also has
deleterious effects (primarily because it causes
oxygen radicals)

21. SUMMARY
 Mutations can result in changes in heritable traits.
 Mutation is the process that alters the sequence of base pairs in a molecule. The
alteration can be as simple as a single base-pair substitution, insertion, or deletion
etc.
 Mutagenesis, the creation of mutations, can occur spontaneously or can be induced.
Mutations that happen naturally are called spontaneous mutations.
 Gene mutations may be caused by exposure to a variety of chemicals called
chemical mutagens, a number of which exist in the environment and can cause
genetic diseases in humans and other organisms.

22. Links and Books of the Title Lecture
 iGenetics A Molecular Approach_ Peter J. Russell
 Arihant_ Life Sciences
 Principles of Genetics_ Eldon John Gardner and Michael J. Simmons
 http://www.sciencedirect.com
 https://evolution.berkeley.edu
 https://www.britannica.com
 http://www.ncbi.nllm.nih.gov
 http://www.reseacrhgate.net