Mutations are changes in the nucleotide sequence of genes. They can be point mutations like substitutions, insertions, or deletions of single nucleotides, or large-scale mutations involving larger chromosomal changes. Mutations can arise spontaneously from errors in DNA replication or DNA damage, or be induced by mutagens like chemicals, radiation, or viruses. Point mutations can cause silent, missense, or nonsense changes to proteins, while frameshift mutations alter the reading frame. Mutations can have harmful, beneficial, or no effects depending on their location and type of genetic change.

1. MUTATIONS
Namrata Chhabra
M.D Biochemistry

2. INTRODUCTION
● A mutation is a permanent change
in the nucleotide sequence of a
gene.
● Mutations may be either gross, so
that large areas of chromosome are
changed,
● or may be subtle with a change in
one or a few nucleotides.
Mutations
Large
scale
Point
mutations

3. Causes of Mutations
SPONTANEOUS
INDUCED BY
MUTAGENS

4. 1) Spontaneous mutations
● Tautomerism - A base is changed by the repositioning of a hydrogen atom.
● Depurination - Loss of a purine base (A or G).
● Deamination - Changes a normal base to an atypical base; C → U, (which can be
corrected by DNA repair mechanisms), or spontaneous deamination of 5-
methylcytosine (irreparable), or A → HX (hypoxanthine).
● Transition - A purine changes to another purine, or a pyrimidine to a pyrimidine.
● Transversion - A purine becomes a pyrimidine, or vice versa.

5. Spontaneous Mutations
SPONTANEOUS
MUTATIONS
Tautomerism
Deamination
Depurination
Transition
Transversion

6. Tautomerism

7. Tautomerism

8. Deamination of bases

9. Depurination
and
deamination

10. Transition and transversion

11. 2) Induced by Mutagens
● Chemicals
o Nitroso compounds
o Hydroxylamine NH2OH
o Base analogs
o Simple chemicals (e.g. acids)
○ Alkylating & methylating agents (e.g.
N-ethyl-N-nitrosourea (ENU)

12. 2) Induced by Mutagens
● Chemicals
o Polycyclic aromatic hydrocarbons e.g. benzopyrenes
o DNA intercalating agents (e.g. ethidium bromide)
o DNA cross linker (e.g. platinum)
o Oxidative damage caused by oxygen(O)radicals

13. Chemical Mutagens

14. 2) Induced by Mutagens
❖ Radiations
● Ultraviolet radiation (non-ionizing radiation) - excites electrons to
a higher energy level. DNA absorbs ultraviolet light. Two nucleotide
bases in DNA - cytosine and thymine-are most vulnerable to excitation
that can change base-pairing properties. UV light can induce adjacent
thymine bases in a DNA strand to pair with each other, as a bulky
dimer.
● Ionizing radiation

15. Radiations

16. Radiations
● The Chernobyl disaster was a nuclear
accident that occurred on April 26,
1986.
● It is considered the worst nuclear
power plant accident in history.
● A Russian publication concludes that
985,000 excess cancers occurred
between 1986 and 2004 as a result of
radioactive contamination.

17. 2) Induced by Mutagens
❖ Biological-
○ Viruses (DNA or RNA viruses)
○ Transposons

18. Mutagens

19. Classification of mutations
Structurally, mutations can be classified as:
A) Point mutations
● include single nucleotide changes
● often caused by chemicals or
● malfunction of DNA replication

20. Classification of point mutations
Point
mutations
Substitution
Insertion
Deletion

21. A) Point mutations
i) Substitution-exchange a single nucleotide for another.
● Most common is the transition that exchanges a purine
for a purine (A ↔ G) or a pyrimidine for a pyrimidine, (C
↔ T).
● A transition can be caused by nitrous acid, base
mispairing, or mutagenic base analogs.

22. A) Point mutations
● Less common is a transversion, which exchanges a purine for
a pyrimidine or a pyrimidine for a purine (C/T ↔ A/G).
● An example of a transversion is adenine (A) being converted
into a cytosine (C).

23. Point
mutations

24. A) Point mutations
ii) Insertions add one nucleotide into the DNA.
● They are usually caused by transposable elements or
● errors during replication of repeating elements (e.g. AT
repeats).

25. Insertional
mutagenesis
● Insertions in the coding
region of a gene may
alter splicing of the
mRNA (splice site
mutation), or
● cause a shift in the
reading frame (frame
shift), both of which
can significantly alter
the gene product.

26. A) Point mutations
iii) Deletions remove
one nucleotide from the
DNA.
Like insertions, these
mutations can alter the
reading frame of the
gene.

27. Insertion and
deletion
mutations

28. B) Large-scale mutations
B) Large-scale mutations in chromosomal structure, including:
a) Amplifications (or gene duplications) leading to multiple
copies of all chromosomal regions, increasing the dosage of
the genes located within them.
b) Deletions of large chromosomal regions, leading to loss of
the genes within those regions.

29. B) Large-scale mutations
c) Mutations whose effect is to juxtapose previously separate pieces of DNA,
potentially bringing together separate genes to form functionally distinct fusion
genes (e.g. bcr-abl). These include:
○ Chromosomal translocations: interchange of genetic parts from
nonhomologous chromosomes.
○ Interstitial deletions: an intra-chromosomal deletion that removes a
segment of DNA from a single chromosome, thereby apposing previously
distant genes.
○ Chromosomal inversions: reversing the orientation of a chromosomal
segment.

30. Gene amplification

31. Effects of mutations
Although the initial change may not occur in the template strand of
the double-stranded DNA molecule for that gene, after replication,
daughter DNA molecules with mutations in the template strand
will segregate and appear in the population of organisms.
If the nucleotide sequence of the gene containing the mutation is
transcribed into an RNA molecule, then the RNA molecule will
possess a complementary base change at this corresponding
locus.

32. Single-base changes in the mRNA molecules
Single-base changes in the mRNA molecules may have one of
several effects when translated into protein:
1. Silent mutations-There may be no detectable effect because
of the degeneracy of the code.This would be more likely if the
changed base in the mRNA molecule were to be at the third
nucleotide of a codon.

33. Single-base changes in the mRNA molecules
Silent mutations (contd.)
● Because of wobble, the translation of a codon is least sensitive
to a change at the third position.
● E.g. valine has 4 codons GUU, GUC, GUA, or GUG, the change in
the third nucleotide will have the incorporation of same amino
acid, thus there will not be any effect on the functional capacity
of the protein.

35. Single-base changes in the mRNA molecules
(2) A missense effect will occur when a different amino acid is incorporated
at the corresponding site in the protein molecule. This mistaken amino acid—
or missense, depending upon its location in the specific protein—might be
acceptable, partially acceptable, or unacceptable to the function of that
protein molecule.
Most single-base changes would result in the replacement of one amino acid
by another with rather similar functional groups. This is an effective
mechanism to avoid drastic change in the physical properties of a protein
molecule.

36. Single-base changes in the mRNA molecules
(2) A missense effect (contd.)
● If an acceptable missense effect occurs, the resulting protein
molecule may not be distinguishable from the normal one.
● A partially acceptable missense will result in a protein molecule
with partial but abnormal function.
● If an unacceptable missense effect occurs, then the protein
molecule will not be capable of functioning in its assigned role.

37. a) Acceptable Missense mutations
● The sequencing of a large number of hemoglobin mRNAs and genes from
many individuals has shown that the codon for valine at position 67 of the
chain of hemoglobin is not identical in all persons who possess a normally
functional chain of hemoglobin.
● The codon changes by point mutation from GUU (of valine) to GAU of Aspartic
acid in Hb Bristol.
● Similarly in Hb Sydney the codon changes from GUU to GCU for Alanine.
● Both Hb Bristol and Hb Sydney are normal Hb variants with normal oxygen
carrying capacity.
● Thus these are acceptable mutations.

38. a) Acceptable Missense mutations
Hemoglobin Hikari has been found in at least two families of
Japanese people.
This hemoglobin has asparagine substituted for lysine at the 61
position in the chain. The corresponding transversion might be
either AAA or AAG changed to either AAU or AAC.
The replacement of the specific lysine with asparagine apparently
does not alter the normal function of the chain in these individuals.

39. b) Partially acceptable Missense mutations
● A partially acceptable missense mutation is best exemplified by
hemoglobin S, which is found in sickle cell anemia.
● Here glutamic acid, the normal amino acid in position 6 of the chain, has
been replaced by valine.
● The corresponding single nucleotide change within the codon would be
GAA or GAG of glutamic acid to GUA or GUG of valine.

40. Partially acceptable Missense mutations
● Clearly, this missense mutation hinders normal
function and results in sickle cell anemia when the
mutant gene is present in the homozygous state.
● The glutamate-to-valine change may be considered to
be partially acceptable because hemoglobin S does
bind and release oxygen, although abnormally.

41. c) Unacceptable Missense Mutations
● The hemoglobin M mutations generate molecules that allow the Fe2+ of
the heme moiety to be oxidized to Fe3+, producing methemoglobin.
● Here the single nucleotide change alters the properties of a protein to
such an extent that it becomes non- functional.
● Hb M results from histidine to tyrosine substitution.
● Distal Histidine of alpha chain of Globin is replaced by Tyrosine.
● The codon CAU is changed to UAU with the resultant incorporation of
Tyrosine and formation of MetHb.
● Met hemoglobin cannot transport oxygen.

42. (3) A nonsense mutation
A nonsense codon may appear that would then result in the
premature termination of a peptide chain and the production of
only a fragment of the intended protein molecule.
The probability is high that a prematurely terminated protein
molecule or peptide fragment will not function in its assigned role.

43. Mutations

44. (3) A nonsense mutation
● The codon UAC for Tyrosine may be mutated to UAA or UAG, both
are stop codons. Beta Thalassemia is an example of nonsense
mutation.
● In certain conditions as a result of mutational event the stop
codon may be changed to normal codon (UAA to CAA) .
● This results in the elongation of protein to produce “Run on
polypeptides”. The resultant protein is a functionally abnormal
protein.

45. Hb Constant spring

46. Frameshift Mutations
● A frame shift mutation is a mutation caused by inserts or deletes of a
number of nucleotides from a DNA sequence.
● Due to the triplet nature of gene expression by codons, the insertion
or deletion can disrupt the reading frame, or the grouping of the
codons, resulting in a completely different translation from the
original.
● The earlier in the sequence the deletion or insertion occurs, the more
altered the protein produced is.

47. Frameshift mutations

48. Frameshift Mutations
● If three nucleotides or a multiple of three are deleted from a
coding region, the corresponding mRNA when translated will
provide a protein from which is missing the corresponding
number of amino acids.
● Because the reading frame is a triplet, the reading frame will
not be disturbed for those codons distal to the deletion

49. Triplet deletion
A triplet deletion removes
exactly one amino acid from
the polypeptide ,the most
common mutation in cystic
fibrosis is Delta F508 (i.e.
deletion of amino acid number
508 (a phenylalanine, F).

50. Trinucleotide expansion
● The commonest inherited cause of mental retardation is a
syndrome originally known as Martin-Bell syndrome.
● Patients are most usually male, have a characteristic elongated
face and numerous other abnormalities including greatly
enlarged testes.
● In 1969 the name of the syndrome was changed to the fragile X
syndrome.

51. Triplet
expansion

52. Trinucleotide expansion
● The mutation was tracked down to a trinucleotide expansion in
the gene now named FMR1 (Fragile site with Mental
Retardation).
● A number of diseases have now been ascribed to trinucleotide
expansions.
● These include Huntington's disease and Myotonic dystrophy.

53. Gene deletions
● Alpha Thalassemia is
an example of Gene
deletion.
● The clinical
manifestations are as
per the number of
genes deleted.

54. Consequences of Mutations
Harmful mutations
● Changes in DNA caused by mutation can cause errors in protein sequence, creating
partially or completely non-functional proteins.
● To function correctly, each cell depends on thousands of proteins to function in the
right places at the right times.
● When a mutation alters a protein that plays a critical role in the body, a medical
condition can result.
● A condition caused by mutations in one or more genes is called a genetic disorder.

55. Consequences of Mutations
● If a mutation is present in a germ cell, it can give rise to offspring that
carries the mutation in all of its cells.
● This is the case in hereditary diseases.
● On the other hand, a mutation can occur in a somatic cell of an
organism.
● Such mutations will be present in all descendants of this cell, and
certain mutations can cause the cell to become malignant, and thus
cause cancer.

56. Consequences of Mutations
Often, gene mutations that could cause a genetic disorder are
repaired by the DNA repair system of the cell.
Each cell has a number of pathways through which enzymes
recognize and repair mistakes in DNA.
Because DNA can be damaged or mutated in many ways, the
process of DNA repair is an important way in which the body
protects itself from disease.

57. Consequences of Mutations
Beneficial mutations
● A very small percentage of all mutations actually have a positive effect.
● These mutations lead to new versions of proteins that help an organism
and its future generations better adapt to changes in their environment.
● For example, a specific 32 base pair deletion in human CCR5 (CCR5-Δ32)
confers HIV resistance to homozygotes and delays AIDS onset in
heterozygotes.
● The CCR5 mutation is more common in those of European descent.

58. Summary of Mutations