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Genetic code and its characteristics
1
The central dogma of molecular biology describes the
two-step process, transcription and translation, by which
the information in genes flows into proteins: DNA →
RNA → protein.
Flow of genetic information
2
Biochemistry For Medics
The pathway of protein synthesis is called Translation
because the language of nucleotide sequence on
mRNA is translated in to the language of an amino
acid sequence. The process of Translation requires a
Genetic code, through which the information
contained in nucleic acid sequence is expressed to
produce a specific sequence of amino acids.
Introduction
3
Biochemistry For Medics
The letters A, G, T and C correspond to the
nucleotides found in DNA. They are organized
into codons.
The collection of codons is called Genetic
code.
For 20 amino acids there should be 20
codons.
Each codon should have 3 nucleotides to
impart specificity to each of the amino acid for
a specific codon
1 Nucleotide- 4 combinations
2 Nucleotides 16 combinations
3 Nucleotides- 64 combinations ( Most suited
for 20 amino acids)
Introduction – Genetic Code
4
Biochemistry For Medics
Genetic Code
 Genetic code is a dictionary that
corresponds with sequence of
nucleotides and sequence of Amino
Acids.
 Words in dictionary are in the form of
codons
 Each codon is a triplet of nucleotides
 64 codons in total and three out of
these are Non Sense codons.
 61 codons for 20 amino acids
5
Biochemistry For Medics
Genetic Code- Table
6
Biochemistry For Medics
Genetic Code-Characteristics
 Specificity- Genetic code is specific
(Unambiguous)
 A specific codon always codes for the
same amino acid.
e.g. UUU codes for Phenyl Alanine, it
can not code for any other amino acid.
7
Biochemistry For Medics
Genetic Code-Universal
Universal- In all living organism Genetic
code is the same.
 The exception to universality is found in
mitochondrial codons where AUA codes
for methionine and UGA for tryptophan,
instead of isoleucine and termination
codon respectively of cytoplasmic protein
synthesizing machinery.
 AGA and AGG code for Arginine in
cytoplasm but in mitochondria they are
termination codons.
8
Biochemistry For Medics
Genetic Code-Redundant
Redundant- Genetic code is Redundant, also called
Degenerate.
Although each codon corresponds to a single amino acid but
a single amino acid can have multiple codons. Except
Tryptophan and Methionine each amino acid has multiple
codons.
9
Biochemistry For Medics
Genetic Code- Non Overlapping
and Non Punctuated
 All codons are independent sets of 3
bases.
 There is no overlapping ,
 Codon is read from a fixed starting
point as a continuous sequence of
bases, taken three at a time.
 The starting point is extremely
important and this is called Reading
frame.
10
Biochemistry For Medics
Non Sense Codons
 There are 3 codons out of 64 in genetic code which do not
encode for any Amino Acid.
 These are called termination codons or stop codons or
nonsense codons. The stop codons are UAA, UAG, and
UGA. They encode no amino acid. The ribosome pauses and
falls off the mRNA.
11
Biochemistry For Medics
Initiator codon
 AUG is the initiator codon in majority
of proteins-
 In a few cases GUG may be the
initiator codon
 Methionine is the only amino acid
specified by just one codon, AUG.
12
Biochemistry For Medics
Wobbling phenomenon
 The rules of base pairing are relaxed
at the first position, so that a base can
pair with more than one
complementary base.
 Some tRNA anticodons
have Inosine at the first position.
 Inosine can pair with U, C, or A. This
means that we don't need 61 different
tRNA molecules, only half as many
are required.
13
Biochemistry For Medics
Biochemistry For Medics 14
Biochemistry For Medics 15
Wobbling phenomenon
 First two bases in Codon in m RNA(5’-3’)
base pair traditionally with the 2 nd and
3rd base of the Anticodon in t RNA(5’-3’)
 Non traditional base pairing is observed
between the third base of the codon and
1st base of anticodon.
 The reduced specificity between the third
base of the codon and the
complementary nucleotide in anticodon
is responsible for wobbling.
16
Biochemistry For Medics
Wobbling phenomenon
t RNA (first
base)
m RNA (Third
base)
Base pairing
C G Traditional
A U Traditional
U A Traditional
U G Nontraditional
G C Traditional
G U Nontraditional
I U Nontraditional
I C Nontraditional
I A Nontraditional
17
Biochemistry For Medics
Wobbling phenomenon
 Proline has 4 codons(5’-3’)
 CCU
 CCC
 CCA
 CCG
 The first three codons can be
recognized by a single t RNA having
Inosine at the first place.(IGG- 5’-3’)
18
Biochemistry For Medics
Clinical Significance
 Mutations can be well explained using
the genetic code.
A) Point Mutations
1) Silent
2) Misense
3)Nonsense
B) Frame shift mutations
19
Biochemistry For Medics
Silent Mutations
Single nucleotide change-A to G, same amino
acid is incorporated. Mutation goes unnoticed.
20
Biochemistry For Medics
Missense mutations
Single nucleotide change A to C – different amino
acid incorporated. Loss of functional capacity of
protein.
21
Biochemistry For Medics
Non sense Mutation
Single nucleotide change from C to T, stop codon is
generated (In m RNA represented by UAG), premature
termination of chain, may be incompatible with life.
22
Biochemistry For Medics
Frame sift Mutations
 Insertion or removal of a base/bases can alter the reading
frame with the resultant incorporation of different amino acids
23
Biochemistry For Medics
Genetic code- Summary
 Genetic code is unambiguous,
universal, degenerate, commaless
and non overlapping.
 UAG, UGA and UAA are termination
codons .
 AUG is the start codon
 Mutations can be well described using
genetic code
24
Biochemistry For Medics

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geneticcode.pptx

  • 1. Genetic code and its characteristics 1
  • 2. The central dogma of molecular biology describes the two-step process, transcription and translation, by which the information in genes flows into proteins: DNA → RNA → protein. Flow of genetic information 2 Biochemistry For Medics
  • 3. The pathway of protein synthesis is called Translation because the language of nucleotide sequence on mRNA is translated in to the language of an amino acid sequence. The process of Translation requires a Genetic code, through which the information contained in nucleic acid sequence is expressed to produce a specific sequence of amino acids. Introduction 3 Biochemistry For Medics
  • 4. The letters A, G, T and C correspond to the nucleotides found in DNA. They are organized into codons. The collection of codons is called Genetic code. For 20 amino acids there should be 20 codons. Each codon should have 3 nucleotides to impart specificity to each of the amino acid for a specific codon 1 Nucleotide- 4 combinations 2 Nucleotides 16 combinations 3 Nucleotides- 64 combinations ( Most suited for 20 amino acids) Introduction – Genetic Code 4 Biochemistry For Medics
  • 5. Genetic Code  Genetic code is a dictionary that corresponds with sequence of nucleotides and sequence of Amino Acids.  Words in dictionary are in the form of codons  Each codon is a triplet of nucleotides  64 codons in total and three out of these are Non Sense codons.  61 codons for 20 amino acids 5 Biochemistry For Medics
  • 7. Genetic Code-Characteristics  Specificity- Genetic code is specific (Unambiguous)  A specific codon always codes for the same amino acid. e.g. UUU codes for Phenyl Alanine, it can not code for any other amino acid. 7 Biochemistry For Medics
  • 8. Genetic Code-Universal Universal- In all living organism Genetic code is the same.  The exception to universality is found in mitochondrial codons where AUA codes for methionine and UGA for tryptophan, instead of isoleucine and termination codon respectively of cytoplasmic protein synthesizing machinery.  AGA and AGG code for Arginine in cytoplasm but in mitochondria they are termination codons. 8 Biochemistry For Medics
  • 9. Genetic Code-Redundant Redundant- Genetic code is Redundant, also called Degenerate. Although each codon corresponds to a single amino acid but a single amino acid can have multiple codons. Except Tryptophan and Methionine each amino acid has multiple codons. 9 Biochemistry For Medics
  • 10. Genetic Code- Non Overlapping and Non Punctuated  All codons are independent sets of 3 bases.  There is no overlapping ,  Codon is read from a fixed starting point as a continuous sequence of bases, taken three at a time.  The starting point is extremely important and this is called Reading frame. 10 Biochemistry For Medics
  • 11. Non Sense Codons  There are 3 codons out of 64 in genetic code which do not encode for any Amino Acid.  These are called termination codons or stop codons or nonsense codons. The stop codons are UAA, UAG, and UGA. They encode no amino acid. The ribosome pauses and falls off the mRNA. 11 Biochemistry For Medics
  • 12. Initiator codon  AUG is the initiator codon in majority of proteins-  In a few cases GUG may be the initiator codon  Methionine is the only amino acid specified by just one codon, AUG. 12 Biochemistry For Medics
  • 13. Wobbling phenomenon  The rules of base pairing are relaxed at the first position, so that a base can pair with more than one complementary base.  Some tRNA anticodons have Inosine at the first position.  Inosine can pair with U, C, or A. This means that we don't need 61 different tRNA molecules, only half as many are required. 13 Biochemistry For Medics
  • 16. Wobbling phenomenon  First two bases in Codon in m RNA(5’-3’) base pair traditionally with the 2 nd and 3rd base of the Anticodon in t RNA(5’-3’)  Non traditional base pairing is observed between the third base of the codon and 1st base of anticodon.  The reduced specificity between the third base of the codon and the complementary nucleotide in anticodon is responsible for wobbling. 16 Biochemistry For Medics
  • 17. Wobbling phenomenon t RNA (first base) m RNA (Third base) Base pairing C G Traditional A U Traditional U A Traditional U G Nontraditional G C Traditional G U Nontraditional I U Nontraditional I C Nontraditional I A Nontraditional 17 Biochemistry For Medics
  • 18. Wobbling phenomenon  Proline has 4 codons(5’-3’)  CCU  CCC  CCA  CCG  The first three codons can be recognized by a single t RNA having Inosine at the first place.(IGG- 5’-3’) 18 Biochemistry For Medics
  • 19. Clinical Significance  Mutations can be well explained using the genetic code. A) Point Mutations 1) Silent 2) Misense 3)Nonsense B) Frame shift mutations 19 Biochemistry For Medics
  • 20. Silent Mutations Single nucleotide change-A to G, same amino acid is incorporated. Mutation goes unnoticed. 20 Biochemistry For Medics
  • 21. Missense mutations Single nucleotide change A to C – different amino acid incorporated. Loss of functional capacity of protein. 21 Biochemistry For Medics
  • 22. Non sense Mutation Single nucleotide change from C to T, stop codon is generated (In m RNA represented by UAG), premature termination of chain, may be incompatible with life. 22 Biochemistry For Medics
  • 23. Frame sift Mutations  Insertion or removal of a base/bases can alter the reading frame with the resultant incorporation of different amino acids 23 Biochemistry For Medics
  • 24. Genetic code- Summary  Genetic code is unambiguous, universal, degenerate, commaless and non overlapping.  UAG, UGA and UAA are termination codons .  AUG is the start codon  Mutations can be well described using genetic code 24 Biochemistry For Medics