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Shikha Yashveer1, Jayanti Tokas2, Shalini Jain3 and Hariom Yadav4
1Department of Molecular Biology and Biotechnology, 2Department of Biochemistry, CCS HAU, Hisar, Haryana, India
3Department of Biochemistry, PGIMER, Chandigarh, India4National Agri-Food Biotechnology, Mohali, Punjab, India
Email: [email protected]
Any sudden change occurring in hereditary material is called as mutation
They may be harmful, beneficial or neutral
In multicellular organism, two broad categories of mutations: Somatic mutations & germ line mutations
Somatic mutations Arise in the somatic cellsPassed on to other cells through the process of mitosisEffect of these mutations depends on the type of the cell in which they occur & the developmental stage of the organismIf occurs early in development, larger the clone of the mutated cells
Germ line mutations
They occur in the cells that produce gametes
Passed on to future generations In multicellular organisms, the term
mutation is generally used for germ line mutations
Some Facts
Term mutation was given by Devries in 1901 while studying eveningprimerose Oenothera lamarckiana
Most of these were chromosomal variations
Some were point variations Originally the term mutation was
given to both chromosomal as well as point mutations
Cont.
Recently chromosomal mutations are studied separately
The term mutation is now given only to point mutations
Definition
DNA is a highly stable molecule that replicates with amazing accuracy
Some errors of replication do occur A mutation is defined as an inherited
change in genetic information
Types of gene mutationNumber of ways to classify gene mutations: On the basis of the molecular nature of the
defect On the nature of the phenotypic effect--
amino acid sequence of the protein is altered or not
On the basis of the causative agent of the mutation
Base substitution Insertions & deletions
Base substitution:
Simplest type of gene mutation Involves the alteration of a singlenucleotide in the DNA
A base substitution usually leads to base pair substitution
GGG AGT GTA GAT CGT
CCC TCA CAT CTA GCA
CCC TCA CAT CTA GCA
GGG AGT GCA GAT CGT
A base substitution
CCC TCA CGT CTA GCA
GGG AGT GCA GAT CGT GGG AGT GTA GAT
CGT
CCC TCA CAT CTA GCA
First cycle of DNA replication
Base substitution is of two types:
Transition: Purine is replaced with a purine
Pyrimidine is replaced with a pyrimidine
Insertions & deletions:
2nd major class of gene mutation Addition or the removal, respectively, of
one or more nucleotide pair Usually changes the reading frame, altering
all amino acids encoded by codons following the mutation
Also called as frame shift mutations
cont.
Additions or deletions in the multiples of three nucleotides will lead to addition or deletion of one or more amino acids
These mutations are called in-frame insertions and deletions, respectively.
Mutations on the basis of the Phenotypic effects of mutations:
Most common phenotype in natural populations of the organism is called as wild type phenotype
The effect of mutation is considered with reference to wild type phenotype
Forward mutation: a mutation that alters the wild type phenotype
Reverse mutation (reversion): a mutation that changes a mutant phenotype back in to the wild type
Missense mutation: a base is substituted that alters a codon in the mRNA resulting in a different amino acid in the protein product
TCAAGT
UCA
TTAAAT
UUA
Ser Leu
Nonsense mutation: changes a sense codon into a nonsense codon. Nonsense mutation early in the mRNA sequence produces a greatly shortened & usually nonfunctional protein
TCAAGT
UCA
TGAACT
UGA
Ser
Stop codon
Silent mutation: alters a codon but due to degeneracy of the codon, same amino acid is specified
TCAAGT
UCA
TCGAGC
UCG
Ser Ser
Neutral mutation: mutation that alters the amino acid sequence of the protein but does not change its function as replaced amino acid is chemically similar or the affected aa has little influence on protein function.
CTTGAA
CUU
ATTTAA
AUU
Leu Ile
Loss of function mutations:
Complete or partial loss of the normal functionStructure of protein is so altered that it no longer works correctly
Mutation can occur in regulatory region that affects transcription , translation or spilicing of the protein
Frequently recessive
Gain of function mutations:
Produces an entirely new traitCauses a trait to appear in inappropriate tissues or at inappropriate times in developmentFrequently dominant
Conditional mutations: Expressed only under certain conditions
Lethal mutations: Cause the death of the organism
Suppressor mutation:Suppresses the effect of other mutation
Occurs at a site different from the site of original mutation
Organism with a suppressor mutation is a double mutant but exhibits the phenotype of un mutated wild type
Different from reverse mutation in which mutated site is reverted back into the wild type sequence
On the basis of Causative agent of mutation:
Spontaneous: Mutations that result from natural changes in DNA
Induced: Results from changes caused By
environmental chemicals & radiations Any environmental agent that increases
the rate of mutation above the spontaneous is called a mutagen such as chemicals & radiations
Chemical Mutagens: First discovery of a chemical mutagen was made by
Charlotte Auerbach
Base Analogs:Chemicals with structures similar to that of any of the four standard bases of DNADNA polymerases cannot distinguish these analogs They may be incorporated into newly synthesized DNA molecules
5-bromouracil an analog of thymine
N
N1
2
3
6
5
4
O
O
5BUBr N
N1
2
3
6
5
4
CH₃T
O
O
N
N1
2
3
6
5
4
Br
O
5BU
O
N
N1
2
3
6
5
4
Br
O
5BU
OH
Keto pairs with A
Enol mispair with G
TA
5dBU
A5dBU
5dBUG
CG
TRANISITION T C A G
GACCTG
Strand seperation
3’
5’ 3’
5’
GAC3’ 5’
CTG5’ 3’
GAC3’
5’CBG
5’
3’
CTG
5’
3’
GAC3’
5’
GAC3’ 5’
CBG5’ 3’
GAC3’ 5’
CTG5’ 3’
CBG5’ 3’
GGC3’ 5’
CBG5’ 3’
GGC3’ 5’
CBG5’ 3’
GAC3’ 5’
GGC3’
5’CCG
5’
3’replication
Incorporated error
GC
5dBU
5dBUG
5dBUA
AT
TRANISITION G AC T
2-amino purine (P)
Base analog of adenine Normally pairs with thymine May mispair with cytosine Causes a transition mutation
GTCCAG
Strand separation
3’
5’ 3’
5’GTC
3’
5’CAG
5’
3’
GTC3’ 5’
CPG5’ 3’
CAG5’ 3’
GTC3’ 5’
GTC3’ 5’
CPG5’ 3’
GTC3’ 5’
CAG5’ 3’
CPG5’ 3’
GCC3’ 5’
CPG5’ 3’
GCC3’ 5’
CAG5’ 3’
GTC3’ 5’
GCC3’ 5’
CGG5’ 3’
replication
Incorporated error
T.A C.G
TA
2AP
2APT
C2AP
CG
TRANISITION T CA G
CG
2AP
2APC
T2AP
TA
TRANISITION C TG A
Both base analogs produce transition mutationsMutations by base analogs can be reversed by treatment with the same analog or different analog
Alkylating agents:Chemicals that donate alkyl groups e.g.ehylmethanesulfonate(EMS) It adds an ethyl group to guanine and produces 6-
ethylguanine, which pairs with thymine and leads to CG:TA transitions
Also adds an ethyl group to thymine to produce 4-ethylthymine, which then pairs with guanine, leading to a TA:CG transition
Mutations produced by EMS can be reversed by additional treatment with EMS.
Mustard gas is another alkylating agent.
CG
EMS
6EGT
TA
TA
EMS
G4ET
CG
Nitrous acid: causes deamination Cytosine Uracil
N
N
CYTOSINE
1
2
3
6
5
4
NH2
O
H
N
N1
2
3
6
5
4
O
o
HNo2
URACIL
H
U G
U
G
U A
G
C
U A
U A
A
T
C.G TA
C G HNO2
5’ 3’
3’ 5’
5’ 3’
3’ 5’
5’ 3’
3’ 5’
5’ 3’
3’ 5’
3’ 5’
5’ 3’
5’ 5’
3’
3’3’
5’
5’ 3’
3’ 3’5’
5’
HT
H
T
H C
A T
H C
H C G
C
A.T G.C
A T HNO2
5’ 3’
3’ 5’
5’ 3’
3’ 5’
5’ 3’
3’ 5’
5’ 3’
3’ 5’
3’ 5’
5’ 3’
5’ 5’
3’
3’3’
5’
5’ 3’
3’ 3’5’
5’
Adenine changes into Hypoxanthin which then pairs with Cytosine
XC
X
C
X T
C G
X T
X T A
T
A.TG.C
G C HNO2
5’ 3’
3’ 5’
5’ 3’
3’ 5’
5’ 3’
3’ 5’
5’ 3’
3’ 5’
3’ 5’
5’ 3’
5’ 5’
3’
3’3’
5’
5’ 3’
3’ 3’5’
5’
Guanine changes into Xanthin which pairs with Cytosine. Xanthin can also pair with Thymine
Nitrous acid produces exclusively transition mutations
Both C.G T.A & T.A C.G transitions are produced
Thus mutations can be reversed with the nitrous acid
Hydroxl amine Specific base modifying mutagen which
adds a hydroxyl group to cytosine producing hydroxlamine cytosine which pairs with adenine instead of guanine
This Leads to C.G T.A tranisitions Acts only on cytosine thus can not revert
the mutation produced
hCG
hC
G
hC A
G C
hC
A
hC A T
A
T.AC.G
C G
5’ 3’
3’ 5’
5’ 3’
3’ 5’
5’ 3’
3’ 5’
5’ 3’
3’ 5’
3’ 5’
5’ 3’
5’ 5’
3’
3’3’
5’
5’ 3’
3’ 3’5’
5’
Cytosine changes into hydroxlamine Cytosine which pairs with Adenine instead of Guanine
NH₂OH
Oxidative reactions: Reactive forms of oxygen like superoxide
radicals, hydrogen peroxide and hdroxyl radicals produced in the course of normal aerobic metabolism or by radiation, ozone, peroxides, and certain drugs Cause damage to DNA & induce mutations by chemical changes
Oxidation converts guanine into 8-oxy-7,8-dihydrodeoxyguanine which mispairs with adenine leading to G.C T.A transversion
Intercalating agents Proflavin, acridine orange, ethidium bromide,
and dioxin They are about the same size as a nucleotide They produce mutations by sandwiching
themselves (intercalating) between adjacent bases in DNA
They distort the three-dimensional structure of the helix and cause single-nucleotide insertions and deletions in replication
These insertions and deletions frequently produce frameshift mutations
Radiations:Ionizing radiations:
In 1927, Herman Muller demonstrated that mutations could be induced by X-rays. X-rays, gamma rays, and cosmic rays are all capable of penetrating tissues and damaging DNA. They remove electrons from the atoms that they encounter, changing stable molecules into free radicals and reactive ions which then alter the structures of bases and break phosphodiester bonds in DNA.
Ionizing radiation also frequently results in double-strand breaks in DNA.
Mutation rates
The frequency with which a gene changes from the wild type to a mutant is reffered to as the mutation rate.
Expressed as the number of mutations per biological unit i.e. mutations per cell division, per gamete per round of replication
e.g. mutation rate for achondroplasia (hereditary dwarfism) is about 4 mutations per 100,000 gametes
Mutation frequency: Incidence of a specific type of mutation with in a group of individual organism
e.g. for achondroplasia, the mutation frequency in united states is about 2x10⁻⁴
