2. Mutation
A mutation is defined as a change in nucleotide
sequence of DNA
Mutagens are substances which can induce
mutations. These can be chemicals, radiations or
viruses
The changes that occur in DNA on mutation are
reflected in replication, transcription and
translation
Statistically, out of every 106 cell divisions, one
mutation takes place
4. Point mutation
Replacement or change in a single base
Two types
Transition : replacement of a purine by
another purine (A to G or G to A)or pyrimidine
by pyrimidine (T to C or C to T)
Transversion : replacement of a purine by
pyrimidine (A to C) or pyrimidine by Purine (T
to G)
5. Deletion
Large gene deletions e.g. alpha thalassemia
(entire gene) or homophilia (partial)
Deletion of a codon, e.g. cystic fibrosis (one
amino acid, 508th phenyl alanine is missing in
the CFTR gene
Deletion of single base, which will rise to
frame shift effect
6. Insertion
Single base additions, leading to frame-shift
effect
Trinucleotide expansions, e.g. in Huntington’s
chorea, CAG trinucleotides are repeated 30 to
300 times. This leads to a polyglutamine
repeat in the protein
Duplications. E.g.in Duchenne Muscular
Dystrophy (DMD), the gene is duplicated
7. Effect of mutation
Point mutation may lead to
– Silent Mutation
– Mis-sense Mutation
• Acceptable
• Partially acceptable
• Unacceptable
– Non-sense
Insertion or deletion of single base leads to
– Frame-shift Mutation
8. Silent Mutation
A point mutation may change the codon for
one amino acid to synonym for the same
amino acid
Mutation is silent and has no effect on the
phenotype
E.g. CUA is mutated to CUC; both code for
leucine, and so this mutation has no effect
9. Mis-sense but Acceptable Mutation
A change in amino acid may be produced in
the protein; but with no functional
consequences
Acceptable mutation
HbA-β chain 67 Val
Hb(Sidney)-β chain 67 Ala
GUU
GCU
10. Mis-sense; Partially Acceptable
Mutation
o The amino acid substitution affects the functional
properties of the protein
o HbS has abnormal electrophoretic mobility and
subnormal function, leading to sickle-cell anemia
Partially Acceptable mutation
HbA-β chain 6 Glu
HbS-β chain 6 Val
GAG
GUG
11. Mis-sense; Unacceptable Mutation
o The single amino acid substitution alters the
properties of the protein to such an extent
that it becomes nonfunctional and the
condition is incompatible with normal life
Unacceptable mutation
HbA-α chain 58 His
HbM(Boston)-α chain 58 Tyr
CAU or CAC
UAU or UAC
12. Nonsense; Terminator Codon
Mutation
The codons with the altered base may become
one of the three termination codon (UAA, UAG or
UGA) called as “nonsense codon”.
This leads to premature termination of the
protein, and so functional activity may be
destroyed. E.g. beta-thalassemia
A terminator codon is altered into a coding codon
(UAA to CAA), resulting in elongation of the
protein to produce “run on polypeptide” (Hb
Constant spring)
13. Frame-shift Mutation
This is due to addition or deletion of bases.
From that point onwards, the reading frame
shifts. A “garbled” (completely irrelevant)
protein, with altered amino acid sequence is
produced.
14.
15. Not only the sequence of amino acids distal to
the addition or deletion is garbled, there may
appear a nonsense (chain termination or run-
on-polypeptide) that are non-functional
16. Manifestations of Mutations
Lethal Mutations
The alteration is incompatible with life of the cell
or the organism
E.g. mutation producing alpha-4 Hb is lethal, and
so the embryo dies
Silent Mutations
Alteration at an insignificant region of a protein
may not have any functional effect
17. Beneficial Mutations
Beneficial spontaneous mutations are the basis of
evolution
Such beneficial mutants are artificially selected in
agriculture.
E.g. normal maize is deficient in tryptophan.
Tryptophan-rich maize varieties are now available
for cultivation
Carcinogenic Effect
The mutation may not be lethal, but may alter the
regulatory mechanism.
Such a mutation in a somatic cell may result in
uncontrolled cell division leading to cancer
18. DNA damage and DNA Repair
DNA is replicated with great fidelity (accuracy).
However, DNA can be damaged by variety of
causes resulting in several distinct types of
lesions
Various physical and chemical agents produce
base alterations; these are to be appropriately
corrected immediately
The DNA polymerase has 3’ to 5’ exonuclease
activity. Hence any mispaired nucleotide added is
immediately removed
19. • Cause of DNA damage
Misincorporation of deoxynucleotides during
replication
By spontaneous deamination of bases during normal
genetic functions
From x-radiation that cause “nicks” in the DNA
From UV irradiation that causes thymine dimer
formation
From various chemicals that interact with DNA e.g.
ozone (produced by lightning), hydrazines (present in
edible mushrooms), allylisothiocynates, aflatoxin (mold
growing on peanuts and grains), alkylating agents
(busulphan, cyclophosphamide) and free radicals
(oxidative stress)
20.
21. Types of damage to DNA
1. Single-base alteration
a. Depurination
b. Deamination of cytosine to uracil
c. Deamination of adenine to hypoxanthine
d. Alkylation of base
e. Insertion or deletion of nucleotide
f. Base-analog incorporation
22. 2. Two-base alteration
a. UV light-induced thymine-thymine (pyrimidine) dimer
b. Bifunctional alkylating agent cross-linkage
3. Chain breaks
a. Ionizing radiation
b. Radioactive disintegration of backbone element
c. Oxidative free radical formation
4. Cross-linkage
a. Between bases in same or opposite strands
b. Between DNA and protein molecules (e.g. histones)
23. Mechanism of DNA Repair
the maintenance of the integrity of DNA is
very important in order to provide correct
genetic information.
The integrity of DNA after DNA replication is
maintained by the presence of specific DNA
repair system
There are several DNA repair system
24. Mechanism of DNA Repair
Mechanism Problem Repair
Mismatch repair Copying errors (single base
or two- to five-base
unpaired loops
Methyl-directed strand
cutting, exonuclease
digestion, and replacement
Base excision-repair Spontaneous, chemical, or
radiation damage to a single
base
Base removal by N-
glycosylase, abasic sugar
removal, replacement
Nucleotide excision-repair Spontaneous, chemical, or
radiation damage to a DNA
segment
Removal of an approximately
30-nucleotide oligomer and
replacement
Double-strand break
repair
Ionizing radiation,
chemotherapy, oxidative
free radicals
Synapsis, unwinding,
alignment, ligation
25. General Mechanism
Recognition of altered base
Removal of altered base along with a few
bases around that area.
A small segment of DNA with correct base
sequence is then synthesized by DNA
polymerase beta.
Then the gap or nick is sealed by DNA ligase
26. Mismatch Repair
Mismatching of bases can occur during DNA
synthesis since proof reading is not 100%
accurate
Repair enzymes:- mismatch repair protein
complexes(MutS, MutC and MutL in Ecoli and
MSh and MLH in humans), exonucleases, DNA
polymerases and DNA ligases are involved in
mismatch repair
27. Repair process
Specific proteins scan the newly synthesized DNA, using adenine
methylation within a GATC sequence as the point of reference
The template strand is methylated, and the newly synthesized strand
is not.
This difference allows the repair enzymes to identify the strand that
contains the errant nucleotide which requires replacement.
If a mismatch or small loop is found a GATC endonuclease cuts the
strand bearing the mutation at a site corresponding to the GATC.
Exonuclease digest this strand from the GATC through the mutation,
thus removing he faulty DNA
DNA polymerase fills the gap
The last phosphodiester linkage is closed by DNA ligase
28.
29. Base Excision Repair
Involves repair of alkylated bases, repair of
deaminated bases and repair of depurination
Repair enzymes:- DNA glycosylates, AP
endonucleases, helicases, excision nuclease,
DNA polymerase and DNA ligase
30. Repair of deamination
Cytosine spontaneously deaminates to form uracil
Uracil is recognized by uracil DNA glycosidase and uracil
is excised
Creation of AP (either apurine or apyrimidine) site
consisting of only deoxyribose phosphate backbone
AP endonuclease nicks the deoxyribose phosphate
backbone
Excision nuclease removes the AP site and several
nucleotides
DNA polymerase fills the gaps
DNA ligase seals the phosphodiesterase bond
31. Repair of depurination
Depurination occurs by breaking of N-glycosyl
bond between the purine and deoxyribose
AP (apurinic site) endonucleases recognizes the
site of missing purines and nicks the deoxyribose
sugar phosphate
Phosphodiesterase excises the deoxyribose
phosphate
DNA polymerase replace the purine nucleotide
DNA ligase seals the phosphodiester bond
32. Nucleotide Excision Repair
It repairs covalent bonding between adjacent thymine bases
or adjacent thymine-cytosine bases caused by ultravoilet
light. This produces thymine dimers or thymine-cytosine
cross links. Both of these alterations produce distortions of
DNA helix
Enzymes:- excinuclease, DNA polymerase and DNA ligase. At
least 18 different proteins are involved in nucleotide
excision repair. Proteins encoded by 7 genes related to
xeroderma pigmentosum (XPA to XPG) are involved in
nucleotide excision repair. Cockayne syndrome related
genes (CSA or CSB) are involved in transcription coupled
DNA repair
33. Excinuclease detects the distortion of the
helix, nicks the damaged strand on both sides
of the lesion and removes the nucleotides
DNA polymerase fills in the gap,
using the undamaged strand as
template
DNA ligase seals the
phosphodiester bond
34. • In transcription coupled repair, RNA
polymerase is made to transverse back from
the site of lesion followed by correction of the
lesion
35. Double Strand Break Repair
It is usually caused by ionizing radiation,
oxidative stress and chemicals such as
bleomycin
It can also occur during immunoglobulin gene
arrangement
Enzymes:- Ku protein with helicase activity, DNA
dependent protein kinase, exonuclease and
ligase
36. Ku protein binds to both ends of DNA double stranded
DNA segments
Recruit DNA dependent protein kinase
DNA dependent protein kinase approximates the two
separated strands and activate Ku protein
Activated Ku protein has helicase activity and unwinds
the two ends of DNA
Approximated DNA segments form the base pairing
Extra nucleotides are removed by exonuclease
Gaps are filled by ligase
37. Clinical aspect
• Xeroderma Pigmentosum
(greek xeros – dry + derma- skin)
– Defect: nucleotide excision repair; caused by the
defect in the removal of pyrimidine dimer caused
by the defective excinuclease, mutation in XPA
gene
– Features: hypersensitivity to sunlight (UV
radiation) leading to the development of skin
lesions and skin cancer
38. • Ataxia Telangiectasia
– Defect in gene involved in DNA repair and cell
cycle
– Characterized by hypersensitivity to ionizing
radiation, cerebellar ataxia, oculocutaneous
telangiectasia and immunodeficiency. These
patients are susceptible for the development of
lymphomas
39. • Fanconi’s Anemia
– Defect in double strand break repair
– Feature: hypersensitivity to DNA cross linking
agents, bone marrow failure (aplastic anemia) and
leukemia
• Bloom’s syndrome
– Defect in double strand break repair, defective
helicase
– Feature: susceptibility to ultraviolet radiation, and
the development of leukemia
40. • Hereditary Nonpolyposis Colorectal Cancer
(HNPCC)
– Defect in mismatch repair, defective HNPCC
genes, 50-60% of HNPCC is associated with
mutation on hMSH2, hMLH1 is associated with
most of other cases
– Features: condition accounts for about 15% of
colon cancers, early development of tumors
– Identification of the genes responsible for HNPCC
permit the early detection of the condition
41. • Cockayne Syndrome
– Defect in preferential repair of he transcribed
strand, mutation in proteins CSA and CSB
– Features: neurological degeneration and growth
retardation
• Warner’s syndrome
– Inherited defect in excision repair of DNA,
defective helicase
– Characterized by accelerated aging
42. References
Harper’s Illustrated Biochemistry, 28th edition
Biochemistry by Voet and Voet, 4th edition
Medical Biochemistry, AR Aroora
Text Book of Biochemistry, DM Vasudevan
Text Book of Medical Biochemistry, MN
Chatterjea
Biochemistry, U Satyanarayana