DNA mutation ppt

1. DNA MUTATIONS

2. CONTENTS
 Introduction
 Types of mutation
 Causative agents
 DNA repair
 Damage reversal
 Screening methods
 Effects of mutation
 Genetic disorders
 Unusual genetic mutations in human
 Beneficial genetic mutation

3. Central dogma of molecular biology

4. INTRODUCTION
 Change in nucleotide sequence
 Dutch botanist, Hugo de vries introduced the word
mutation
 It can be at Chromosomal or DNA levels
 Mutation can be in
• somatic cells (cannot pass to offspring)
• germline cells (occur in cells produce gametes/eggs,
pass to offspring)

5. Point mutation: change in single base pair
A) Transition
B) Transversion
C) Missense/mis-pair mutation
D) Non-sense mutation
E) Silent mutation

6. Transition: mutation
that changes a purine
nucleotide to another
purine (A ↔ G) or a
pyrimidine nucleotide
to another pyrimidine
(C ↔ T)
Transversion:
substitution of purine
with pyrimidine or
vice versa

7. Silent mutation:
• mutations in DNA that do
not significantly alter the
phenotype of the
organism
• Effects are sometimes
completely unnoticeable
• It doesn't actually change
the amino acid sequence
of the protein that is
made

8. Missense mutation:
• Nucleotide change results in a codon that codes for
a different amino acid
• It is a type of nonsynonymous substitution

9. Nonsense mutation:
• Mutation (a change) in a
base in the DNA that
prematurely stops the
translation (reading) of
messenger RNA
• It leads to a stop codons
such as TAA, TAG, TGA etc

10. Frame shift mutation:
• Mutation caused by the addition or deletion of a base pair
or base pairs in the DNA of a gene resulting in the
translation of the genetic code in an unnatural reading
frame from the position of the mutation to the end of the
gene
• It changes reading frame resulting in a completely
different translation from the original

12. Causative agents
• Spontaneous mutation: DNA fails to copy accurately/naturally
occurring
Eg: Strand slippage
• Induced mutation: External factor affects DNA
Mutagens : agents cause mutation
– Chemical: Benzo-pyrene, sodium azide
– Physical: X-rays, UV-radiation (above 260 nm cause
pyrimidine dimers)
– Alkylating agents: ethyl methane sulfonate (EMS), methyl
methane sulfonate (MMS), diethylsulfate (DES) and
nitrosoguanidine which tend to de-acidify the acidic
properties of DNA

13. DNA repair system
 Checkpoints
 Play important role in the cell control system of cell cycle
 Sensing the defects occur through essential process- DNA replication
 Proof reading activity
 Error correcting process
 DNA polymerase : missing bases can be detected and replaced
 DNA repair systems
 Damage reversal
 Damage removal
 Damage tolerance

14. • Photoreactivation:
a) Found in bacteria, lower
eukaryotes, plants, insects
b) Photolyase enzyme catalyzes this
reaction by splitting pyramidine
dimers in presence of light
c) Visible light is utilized to break the
cyclobutane ring structure
Damage reversal:
simplest; enzymatic action restores normal
structure without breaking backbone

15. • Ligation of single strand
breaks:
a) Breaks in backbone of DNA due
to x-rays and some chemicals
can be repaired by DNA ligase
b) DNA ligase is a specific type of
enzyme, a ligase that facilitates
the joining of DNA strands
together by catalyzing the
formation of a phosphodiester
bond
c) Deficiency in DNA ligase: Bloom
syndrome

16. Damage removal : involves
cutting out and replacing a
damaged or inappropriate
base or section of
nucleotides
• Commonly employed to
remove incorrect bases (like
uracil) or damaged bases
(like 3-methyladenine): "base
excision repair“
Damage tolerance: not truly
repair but a way of coping
with damage so that life can
go on

17.  Denaturing gradient gel electrophoresis (DGGE)/
Temperature gradient gel electrophoresis (TGGE)
 Heteroduplex analysis (HET)
 Chemical cleavage method (CCM)
Screening techniques

18.  Uses temperature/chemical gradient to denature the sample as
it moves through the gel
 With increasing concentration of denaturant or temperature,
domains in the DNA dissociate according to their melting
temperature (Tm)
 100% of point mutations can be detected when heteroduplices
are generated from sense and antisense strands
 Use denaturants such as urea and formamide
 Detected by radiolabeling, by ethidium bromide or silver stain
Denaturing gradient gel electrophoresis
(DGGE)/Temperature gradient gel electrophoresis (TGGE)

19.  Generated by heat denaturation and reannealing of a mixture
of wild-type and mutant DNA molecules
 In nondenaturing polyacrylamide gels heteroduplices exhibit
distinct electrophoretic mobilities
 Proportion of point mutations detected by HET has been
estimated to ∼80%
 Detected either by ethidium bromide or silver staining after gel
electrophoresis.
Heteroduplex analysis (HET)

20. Chemical cleavage method (CCM)
Mispaired nucleotides within heteroduplices are modified by
chemical agents by using Maxam–Gilbert sequencing
Hydroxylamine reacts with mispaired cytosine residues, osmium
tetroxide with mispaired thymine residues
DNA:DNA or DNA:RNA heteroduplices are cleaved by piperidine
at the sites of chemical modification
Analyzed by gel electrophoresis, silver staining and Fluorescence
labeling

21. Effects of mutation
• Nonsense mutations- truncate the protein and make non
functional protein
• Insertions-
– may alter splicing of the mRNA (splice site mutation)
– cause a shift in the reading frame (frameshift mutation)
• Synonymous mutations- due to the degenerate nature of
the genetic code
• Loss-of-function mutations- (inactivating mutations) - result
in the gene product having less or no function (being
partially or wholly inactivated)

22. • Back mutation/reversion- point mutation that restores the
original sequence and hence the original phenotype
• Gain-of-function mutations- (activating mutations)
– change the gene product such that its effect gets
stronger (enhanced activation)
– gene product takes place by a different and abnormal
function
• Lethal mutations- leads to the death of the organism
• Harmful, or deleterious mutation- decreases the fitness of
the organism

23. Diseases caused by DNA mutation
• Hypertrophic cardiomyopathy (HCM)
• Crohn's Disease
• Cystic fibrosis
• Sickle-cell anemia

24. Hypertrophic cardiomyopathy (HCM)
• Primary disease of the myocardium in which a portion of the
myocardium is hypertrophied
• Caused by mutations in genes encoding proteins of the cardiac
sarcomere i.e. Troponin C gene (TNNC1)
• Frameshift mutation (c.363dupG or p.Gln122AlafsX30) in
Troponin C
• Functional impairment of the cardiac muscle
• Causes sudden death in young people, including trained
athletes

25. Crohn's Disease
• Any part of the gastrointestinal tract from
mouth to anus
• Mutation- insertion of a Cytosine at
position 3020, associated with NOD2 gene
• Leads to a premature stop codon,
shortening the transcribed protein
• When the protein is able to form normally,
it responds to bacterial liposaccharides,
where the 3020 in sC mutation prevents
the protein from being responsive
• Body's immune system attacks the
gastrointestinal tract causing inflammation

26. Cystic fibrosis
• Genetic disorder that affects mostly the
lungs but also the pancreas, liver, kidney
and intestine
• caused by the G542X mutation in the cystic
fibrosis transmembrane conductance
regulator (CFTR) gene
• Loss of the amino acid phenylalanine at
the 508th position on the protein

27. • caused by a point mutation in the β-globin chain of
haemoglobin
• Replaces the hydrophilic amino acid glutamic acid with the
hydrophobic amino acid valine at the sixth position
Sickle cell anaemia

28. • Association of two wild-type globin subunits with two
mutant β-globin subunits forms haemoglobin S (HbS)
Under low-oxygen conditions (being at high altitude), the
absence of a polar amino acid at position six of the β-
globin chain promotes the non-covalent polymerization
(aggregation) of haemoglobin
• Distorts red blood cells into a sickle shape and decreases
their elasticity
• Sickle-cell disease occurs when a person inherits two
abnormal copies of the haemoglobin gene, one from each
parent

29. Beneficial mutation in human
• Protection against cardiovascular disease: Apolipoprotein
A-1 Milano :(also ETC-216, now MDCO-216) is a naturally
occurring mutated variant of the apolipoprotein A1 protein
found in human HDL, the lipoprotein particle that carries
cholesterol from tissues to the liver
• Increased bone density: One of the genes that governs
bone density in human beings is called low-density
lipoprotein receptor-related protein 5, or LRP5 for short,
mutations which impair the function of LRP5 are known to
cause osteoporosis mutation leads to high density
mutation

30. • Malaria resistance: hemoglobin mutation named HbS that
makes red blood cells take on a curved, sickle-like shape.
With one copy of gene, it confers resistance to malaria
• HIV-1 resistance: ccr5-delta 32 mutation confers to HIV-1
resistance those with a double copy of the allele
(homozygous), mutation also confers resistance to plague
and smallpox while increasing susceptibility to West Nile
virus