Download MBBS Genetic Code and Mutation Lecture PPT

PROPERTIES OF GENETIC CODE.

? DEGENERACY-
? Degeneracy means that an amino acid is coded by more than

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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 six

codon 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 codon

Arginine, leucine, serine

6 each

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Alanine, glycine, proline and

4 each

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threonine
Isoleucine

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 base

pairings 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 the

translation 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 nucleotide
sequence 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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nonsense

insertion.
Introduction.

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? Sudden heritable change in genetic material or

character 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 crossing

over 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 to

degeneracy 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 or

unacceptable.

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? This wrong or missense amino acid may or
may 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 alter

the 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 acceptable

missense 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 or
termination 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 nucleotides

sequence 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 synthesis

stops 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 incompatible

with 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 insignificant

region 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.