Download MBBS (Bachelor of Medicine and Bachelor of Surgery) Latest Genetic Code and Mutation Lecture PPT
PROPERTIES OF GENETIC CODE.
? DEGENERACY-
? Degeneracy means that an amino acid is coded by more than
one codon even though each codon is sepecific for one amino
acid.
? For example arginine, is coded by six codons but each of six
codon code for only arginine.
Wobble Hypothesis.
? Methionine is coded only by one codon.
? It is due to wobble phenomenon described below.
? WOBBLE HYPOTHESIS---
? Single tRNA recognises more than
one codon. All such codons have their third nucleotide
different from each other but first the two codon are same.
? This is called Wobble Hypothesis.
Degeneracy of codon.
Amino acids
Number of codon
Arginine, leucine, serine
6 each
Alanine, glycine, proline and
4 each
threonine
Isoleucine
3 each
Aspartate, glutamate, cysteine,
2 each
histidine, lysine, phenylalanine,
asparagine, glutamine
Methionine and tryptophan
1 each
? The first two bases of a wobble codon makes strong base
pairings of the with the corresponding bases
anticodon on tRNA.
? This confers most of the coding specificity. The third base of the
codons forms a loose/weak pairings with the corresponding base
on anticodon.
? This third base is called wobble ( unsteady or unstable).
? It permits rapid dissociation of the tRNA from the codon during
protein synthesis.
FEATURES OF GENETIC CODE.
? UNIVERSALITY-
Genetic code is considered to be universal because the
aminoacid are coded by the same codons in the proteins of all the living organism.
? SPECIFICITY---
A particular codon always codes for the same aminoacid.eg.-UGG always
codes for only tryptophan.
? NON-OVERLAPPING--
The genetic code is read on mRNA 5' to 3' end in
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.
The addition or deletion of one or more bases alters the message sequence in
mRNA result in different protein synthesis.( Mutation ).
?
MUTATION.
? Mutations are defined as change in nucleotide
sequence of DNA which act as templates for
transcription and transmission of genetic
information.
TYPES OF MUTATION.
TYPES OF MUTATION.
Base substitution
or point mutation.
Frame shift.
Silent and mis-
Deletion and
sense.
nonsense
insertion.
Introduction.
? Sudden heritable change in genetic material or
character of an organism is known as mutation.
? Individuals showing these changes are called
mutants.
? Factors or agent causing mutation are known as
mutagens.
? Mutation which causes change in base sequence
of a gene are known as gene mutation or point
mutation.
POINT MUTATION
? Point mutation results from change in single
nucleotide in the sequence. They may be due
to the following changes.
Only one base is altered.
Protein with abnormal AA sequence.
Types-
Transition
Transversion.
or substitution of one nucleotide in a gene.
Sickle cell disease is the result of nucleotide substitution.
Ocurs in Haemoglobin gene.
? INSERTION-
Trinucleotide expansion-
? In Huntington's chorea , CAG trinucleotides are repeated 30
to 300 times leads to polyglutamines repeat in protein.
? Duplication---Gene duplication results from unequal crossing
over of chromosomes during meiosis and plays an important
role in evolution.
Point mutation.
o Transition--
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.
? Purine is never changed by pyrimidine or vice
versa.
o Transversion-- in which a purine base is changed by a
pyrimidine and vice versa.
Effects of Point Mutations.
? Silent Mutation( no change in sequence )--
? Mutation may not lead to any change in
sequence of resulting protein due to
degeneracy or wobble effect of genetic code.
? This is most likely if the change of nucleotide
falls in the third nucleotide of the codon.
Point mutation.
? 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.
It can be acceptable, partially acceptable or
unacceptable.
? 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:
o When the changed amino acid does not alter
the function, it is called acceptable missense
mutation e.g. hemogglobin Hikari.
o Hb-Hikari? Asp replaces Lys at 61th position of
Beta chain--normal function unaltered.
o Partially acceptable missense mutation:
o When the changed amino acid alters the function
partically which is compatible with life e.g
Hemoglobin S, it is called partially acceptable
missense mutation.
o Sickle cell disease-Beta 6 GLU- Val.
o Unacceptable missense mutations:
? When the changed amino acid alters the function drastically
or abolishes it completely so that the protein can not perform
its normal function and is not compatible with life.
? It is called unacceptable missense mutation.
? 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
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.
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
leading to the change in the reading of nucleotides
sequence is called frame shift mutation.
Frame shift Mutation.
? Insertion or deletion of one or two nucleotides in DNA.
? Whole reading frame alters.
? Deletion frame shift mutation-cystic fibrosis of pancreas.
? Insertion frame shift mutation-Thalassaemia.
EFFECT OF FRAMESHIFT MUTATION.
? 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
reading frame reaches the stop codon, the protein synthesis
stops abruptly leading to incomplete protein synthesis due
to its premature termination.
MUTAGEN AND MUTAGENESIS.
? Any agent which will increases DNA damage.
? X-ray, UV-rays, acridine orange etc are well
known mutagens.
? Lethal Mutations?
The alteration is incompatible
with life of cell or the organism.
? Examples-a mutation which does not produce
alpha chain( 4 gene deletion) will result in
intrauterine death of embryo.
? Silent mutation?
? Alteration at an insignificant
region of protein may not have metabolic
effect.
? Beneficial mutation---Normal maize is dificient
in tryptophan. Tryptophan ? rich maize
varities are now available for cultivation.
? Carcinogenic Effect-
Mutation may not be lethal
but alter regulatory mechanism. Such a
mutation in a somatic cell may results in
uncontrolled cell division leading to cancer.
This post was last modified on 30 November 2021