FEATURES OF
GENETIC CODE AND
NON SENSE CODONS
by mrs. Anita jain
Pgt biology
k.v. no.1 hindan
ghaziabad
GENETIC CODE
The relationship between the sequence of amino acids
in polypeptide chain and nucleotide sequence of mRNA
or DNA is called genetic code.
Or
The genetic representation of codon by which the
information in RNA is decoded in a polypeptide is
called genetic code.
Or
The order in which bases (nucleotides) are arranged in
RNA deciding the order in which amino acids are
arranged in proteins. (i.e. it is the relationship between
nucleotide bases and the amino acids).

It was hypothesised by George Gamow that triplet code
is operative as DNA contains only four bases while the
no. of Amino acids are 20. So it generates (4)=64 triplet
codons.

Har Gobind Khurana synthesised homopolymers
&copolymers i.e. RNA molecules with specific
combinations of bases.

Marshal Nirenberg helped in deciphering the code by
making a cell free system for protein synthesis.
For this work , Har Gobind khurana shared Nobel prize
in 1968 with Nirenberg & Holley.
CODON
Sequence of 3 nitrogenous bases (triplet) on MRNA
coding for a particular amino acid is known as codon.
Or
A triplet of bases present on MRNA coding for one
amoinoacid is called codon.
E.g. If MRNA has a triplet code AUG, for which
corresponding aminoacid in the protein would be
methionine. Similarly UUU code for phenylalanine . Thus
codons for all 20 aminoacids of proteins were found.
Many amino acid have more than one codon.
Anti-codon
A triplet of bases present on ERNA & that corresponds
with codon of MRNA is called anti-codon.
e.g. UAC (ERNA anticodon)
THE SALEINT FEATURES OF GENETIC CODE
1) The genetic code are triplet . Thus 4 bases (AUGC)
when arranged in the form of triplet code can generate
43 or 64 codons. Of these codons do not code for
aminoacid but function as Stop/ terminator codons/
Non-sense codons.
(UAA, UAG & UGA), so there are 61 codons which
code for 20 aminoacids.
2) One codon codes for only one aminoacid hence it is
‘unambiguous’ & ‘specific’.
3) Some amino acids are coded by more than one codon
hence the code is ‘degenerate’
E.G. methionine is the amino acid that is coded by only
one codon i.e. AUG.
4) AUG have dual functions as it codes for methionine & also acts
as initiation/ starf codon. It is the first amino acid in the
polypeptide chain.
5) The codons are real in a continuous fashion in 5’
direction i.e. without punctuation.
3’
6) The code is non-overlapping i.e. same letter cannot for 2
different codons.
7) The code is nearly universal i.e. the codon codes for same
amino acid in any organism from bacteria to human beings./ (i.e.
in all organism).
e.g. UUU would code for phenylalanine , in all organisms.
Comma less
Non sense codon
Non ambiguous
(UAA, UAG, UGA)
universal
Initiation Codon
(AUG)
Genetic
code
Degenerate
Linear
Non Overlapping
Triplet
First
position
U
C
A
G
third
position
Second position
U
C
A
G
UUU
UUC
UUA
UUG
phe
phe
leu
leu
UCU
UCC
UCA
UCG
ser
ser
ser
ser
UAU
UAC
UAA
UAG
tyr
tyr
stop
stop
UGU
UGC
UGA
UGG
cys
cys
stop
trp
CUU
CUC
CUA
CUG
leu
leu
leu
leu
CCU
CCC
CCA
CCG
pro
pro
pro
pro
CAU
CAC
CAA
CAG
his
his
gin
gin
CGU
CGC
CGA
CGG
arg
arg
arg
arg
AUU
AUC
AUA
AUG
ile
ile
ile
met
ACU
ACC
ACA
ACG
thr
thr
thr
thr
AAU
AAC
AAA
AAG
asn
asn
lys
lys
AGU
AGC
AGA
AGG
ser
ser
arg
arg
GUU
GUC
GUA
GUG
val
val
val
val
GCU
GCC
GCA
GCG
ala
ala
ala
ala
GAU
GAC
GAA
GAG
asp
asp
glu
glu
GGU
GGC
GGA
GGG
gly
gly
gly
gly
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
RECAPITULATION
1.Define genetic code , codon , & anti-codon.
2.What are initiating codons?
3.Mention the features of genetic code.
4.What are terminating codons?
5.Why are terminating codons also called as non sense
codons?
6.Following is the DNA sequence representing a part of
the gene TAC CCC CAC CAG TTA TAT ATA CGG
GGG CAT CAT ATG
From it derive
(i) The RNA transcript.
(ii)The spliced MRNA (assuming that all codons
containing ac represent the intron.
5. Complete the following table using alphabets (a to k)
DNA
triplet
3’
5’
MRNA
codon
5’
Anticodon
Aminoacid
3’
UAC
UGG
tryptoplan
AAG
GAC
6. Read the sequence of the nuleotides in the given
segment of MRNA and the respective amino acid
sequence in the polypeptide chain
met
phe met
pro
val
ser
X
AUG UUU AUG CCU GUU UCU UAA
MRNA
(i)
(ii)
(iii)
Provide the triplet of bases (codon) for (a) valine
(b) proline
Write the nucleotide sequence of the DNA
strand from which this MRNA was transcribed.
What does the last codon of this RNA stand for?
7. AUG GUC GGG UUU CUU AUC UAA
Given above is a sketch MRNA in 5’ – 3’ direction.
Answer the following:
a)
Make the DNA strand on which this RNA got
synthesised.
b)
How many amino acid will be coded on this MRNA?
c)
What are their anticodons for different amino acids?
d)
What happens if last ‘A’ is replaced by ‘C’? Will the
chain terminate or not? Why?
8. A MRNA strand has a series of codons of which 3 are
given (i)AUG (ii)UUU (iii)UAG
a)
What will be the DNA codes for above codons?
b)
Which amino acid will be coded?
c)
What will be their anti codon?
9. If one nucleotide (encircled) gets missed in the given
series then how many amino acid will be changed?
AUG U AU CAA AAG UAG
10. The sequence of the nucleotide on template strand of
DNA is GTG CAT TCA GCA TGA ATG TAC.
a)
Write the nucleotide sequence on coding strand.
b)
Write the MRNA sequence formed on basis of
template strand.
c)
Give the anticodons of all amino acid. Which will be
coded?
11. Given below is strand of MRNA:
AUG GAC CCC CUG AUA UUU UCA UGA
a)
Write the base sequence of one DNA strand from
which it has been transcribed.
b)
Upon translation how many amino acids will be
present in resulting peptide?
Thank you