one codon even though each codon is sepecific for one amino
acid.
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? For example arginine, is coded by six codons but each of sixcodon code for only arginine.
Wobble Hypothesis.
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? Methionine is coded only by one codon.? It is due to wobble phenomenon described below.
? WOBBLE HYPOTHESIS---
? Single tRNA recognises more than
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one codon. All such codons have their third nucleotide
different from each other but first the two codon are same.
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? This is called Wobble Hypothesis.Degeneracy of codon.
Amino acids
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Number of codonArginine, leucine, serine
6 each
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Alanine, glycine, proline and
4 each
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threonineIsoleucine
3 each
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Aspartate, glutamate, cysteine,2 each
histidine, lysine, phenylalanine,
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asparagine, glutamine
Methionine and tryptophan
1 each
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? The first two bases of a wobble codon makes strong basepairings of the with the corresponding bases
anticodon on tRNA.
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? This confers most of the coding specificity. The third base of the
codons forms a loose/weak pairings with the corresponding base
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on anticodon.? This third base is called wobble ( unsteady or unstable).
? It permits rapid dissociation of the tRNA from the codon during
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protein synthesis.
FEATURES OF GENETIC CODE.
? UNIVERSALITY-
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Genetic code is considered to be universal because the
aminoacid are coded by the same codons in the proteins of all the living organism.
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? SPECIFICITY---A particular codon always codes for the same aminoacid.eg.-UGG always
codes for only tryptophan.
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? NON-OVERLAPPING--
The genetic code is read on mRNA 5' to 3' end in
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a continuous , comma-less pattern.The codon does not overlap and once thetranslation started , there is no punctuation and it is read in a continuing sequence
until a stop codon is encountered.
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The addition or deletion of one or more bases alters the message sequence in
mRNA result in different protein synthesis.( Mutation ).
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?MUTATION.
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? Mutations are defined as change in nucleotidesequence of DNA which act as templates for
transcription and transmission of genetic
information.
TYPES OF MUTATION.
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TYPES OF MUTATION.
Base substitution
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or point mutation.Frame shift.
Silent and mis-
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Deletion and
sense.
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nonsenseinsertion.
Introduction.
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? Sudden heritable change in genetic material orcharacter of an organism is known as mutation.
? Individuals showing these changes are called
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mutants.
? Factors or agent causing mutation are known as
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mutagens.? Mutation which causes change in base sequence
of a gene are known as gene mutation or point
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mutation.
POINT MUTATION
? Point mutation results from change in single
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nucleotide in the sequence. They may be due
to the following changes.
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Only one base is altered.Protein with abnormal AA sequence.
Types-
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Transition
Transversion.
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or substitution of one nucleotide in a gene.
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Sickle cell disease is the result of nucleotide substitution.
Ocurs in Haemoglobin gene.
? INSERTION-
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Trinucleotide expansion-? In Huntington's chorea , CAG trinucleotides are repeated 30
to 300 times leads to polyglutamines repeat in protein.
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? Duplication---Gene duplication results from unequal crossingover of chromosomes during meiosis and plays an important
role in evolution.
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Point mutation.
o Transition--
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o in which one purine base (A) is changed by another purine (G).o Similarly, a pyrimidine base (C) may be
changed by other pyrimidine base (T).
Point mutation.
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? Purine is never changed by pyrimidine or vice
versa.
o Transversion-- in which a purine base is changed by a
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pyrimidine and vice versa.
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Effects of Point Mutations.? Silent Mutation( no change in sequence )--
? Mutation may not lead to any change in
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sequence of resulting protein due todegeneracy or wobble effect of genetic code.
? This is most likely if the change of nucleotide
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falls in the third nucleotide of the codon.
Point mutation.
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? Missense Mutation-( change in sequence )-Mutation which results in change in sequence leading
to a different amino acid being incorporated in the
protein is called missense mutation.
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It can be acceptable, partially acceptable orunacceptable.
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? This wrong or missense amino acid may ormay not have any effect on the function
depending upon its position in the protein. Its
can be classified as:
o Acceptable missense mutation:
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o When the changed amino acid does not alterthe function, it is called acceptable missense
mutation e.g. hemogglobin Hikari.
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o Hb-Hikari? Asp replaces Lys at 61th position of
Beta chain--normal function unaltered.
o Partially acceptable missense mutation:
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o When the changed amino acid alters the function
partically which is compatible with life e.g
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Hemoglobin S, it is called partially acceptablemissense mutation.
o Sickle cell disease-Beta 6 GLU- Val.
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o Unacceptable missense mutations:? When the changed amino acid alters the function drastically
or abolishes it completely so that the protein can not perform
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its normal function and is not compatible with life.
? It is called unacceptable missense mutation.
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? Methemoglobin which cannot tranport O2 is an examples.? Alpha 58His---Tyr, non functional Hb.
Nonsense Mutation- (no protein synthesis)--
? Nonsense mutation which result in the formation of
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a stop codon also called nonsense codon ortermination codon.
This leads to the production of only a part of protein,
i.e. Incomplete protein which is not functional.
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o FRAME SHIFT MUTATION ?? The mutation due to an addition or removal of one, two
and more (non multiples of three)nucleotides in the gene
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leading to the change in the reading of nucleotidessequence is called frame shift mutation.
Frame shift Mutation.
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? Insertion or deletion of one or two nucleotides in DNA.? Whole reading frame alters.
? Deletion frame shift mutation-cystic fibrosis of pancreas.
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? Insertion frame shift mutation-Thalassaemia.
EFFECT OF FRAMESHIFT MUTATION.
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? Premature Termination of Synthesis--? At times, the insertion or deletion of a nucleotide changes
the original codon to a stop codon. In this case, as the
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reading frame reaches the stop codon, the protein synthesisstops abruptly leading to incomplete protein synthesis due
to its premature termination.
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MUTAGEN AND MUTAGENESIS.? Any agent which will increases DNA damage.
? X-ray, UV-rays, acridine orange etc are well
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known mutagens.
? Lethal Mutations?
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The alteration is incompatiblewith life of cell or the organism.
? Examples-a mutation which does not produce
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alpha chain( 4 gene deletion) will result in
intrauterine death of embryo.
? Silent mutation?
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? Alteration at an insignificantregion of protein may not have metabolic
effect.
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? Beneficial mutation---Normal maize is dificient
in tryptophan. Tryptophan ? rich maize
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varities are now available for cultivation.? Carcinogenic Effect-
Mutation may not be lethal
but alter regulatory mechanism. Such a
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mutation in a somatic cell may results in
uncontrolled cell division leading to cancer.
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