Deciphering of genetic code and its salient features for undergraduates in biology, biochemistry and medicine

1. Genetic Code
R.C. Gupta
Professor and Head
Dept. of Biochemistry
National Institute of Medical Sciences
Jaipur, India

2. An overview of protein synthesis
A gene consists of a specific sequence of
nucleotides
Information about its amino acid
sequence is present in a gene
Every protein has got a unique amino
acid sequence

3. The base sequence in the gene encodes
a specific sequence of amino acids
When a protein is to be synthesized, the
corresponding gene is transcribed
An mRNA molecule is formed

4. The base sequence of mRNA is comple-
mentary to the sense strand of the gene
The code words on mRNA are known as
codons
The mRNA goes to cytosol and binds to a
ribosome

5. Amino acids are present in cytosol
They bind to specific tRNA molecules
This is known as charging of tRNAs

6. Each tRNA possesses an anticodon for a
particular amino acid
The anticodon is complementary to a
codon
The charged tRNAs go to the ribosome

7. Charged tRNAs find the complementary
codons on mRNA
Amino acids are joined in a sequence
directed by the codons on mRNA
This process is called translation

8. Structural gene
Transcription
3´ hnRNA
3´ mRNA
40S Subunit
of ribosome
60S Subunit
of ribosome
5´
tRNAs
DNA
Addition of cap and tail, and splicing
5´
5´
3´
a1
a2
an
+ + +
Amino acids Amino acyl tRNAs
+
a1 a2
a3
an an
a2a1
a3

9. The code words for amino acids are made
up of purine and pyrimidine bases
The bases are A, T, C and G in DNA and
A, U, C and G in RNA
How 20 amino acids could be encoded by
four bases was a mystery until 1960
Genetic code

10. It was surmised that:
• If each base acted as a code word,
only four code words would be possible
• If a pair of bases formed a code word, 42
i.e. 16 code words could be formed
• These are not sufficient to encode
20 amino acids

11. If the code word consists of 3 bases,
43 = 64 combinations are possible
Gamow was the first to suggest that each
code word is made up of three bases
This was proved in 1961 by Crick and his
associates

12. Crick et al did their experiments on phage
T4
They induced mutations in a gene by
inserting or deleting nucleotides

13. When one, two or four nucleotides were
inserted/deleted, the gene became inactive
When three nucleotides were inserted or
deleted, the gene remained active
They concluded that the smallest coding
unit in the gene is made up of three bases

14. In the same year, Nirenberg and Matthaei
deciphered the first codon
They prepared a synthetic polyribonucleo-
tide to be used in place of mRNA
The polyribonucleotide had only one base,
uracil, repeated again and again

15. The poly-U polyribonucleotide was added
to a cell-free protein-synthesizing system
The system synthesized a protein
14C-labeled amino acids were used to find
out which amino acids were incorporated in
the protein

16. The protein was found to have only
phenylalanine repeated again and again
This showed that UUU is a codon for
phenylalanine
Polyribonucleotide 5’ UUU UUU UUU UUU 3’
Peptide H2N- Phe- Phe- Phe- Phe- COOH
↓

17. Nirenberg and Matthaei also found that:
▪ Poly-C resulted in the synthesis of poly-proline
This showed that:
▪ CCC is the codon for proline
5’ CCC CCC CCC CCC 3’
↓
H2N- Pro- Pro- Pro- Pro-COOH

18. Using the same technique,
Severo Ochoa found that:
▪ Poly-A resulted in the synthesis
of poly-lysine
This proved that:
▪ AAA is the codon for lysine
5’ AAA AAA AAA AAA 3’
↓
H2N- Lys- Lys- Lys- Lys- COOH

19. Har Gobind Khorana
prepared synthetic
polyribonucleotides having
different base sequences
These were used in cell-
free protein-synthesizing
system
Amino acid sequences of
the peptides synthesized
were determined

20. When two bases, U and C, were
added alternately:
5’ UCU CUC UCU CUC 3’
H2N-Ser-Leu-Ser-Leu-COOH
This showed that UCU is the codon
for serine and CUC for leucine
The peptide synthesized had two
amino acids present alternately

21. A poly-ribonucleotide having four bases
repeated again and again was used
The peptide synthesized had four
amino acids repeated again and
again

22. These observations showed that:
UAU = Tyrosine
CUA = Leucine
UCU = Serine
AUC = Isoleucine
5’ UAUCUAUCUAUCUAUCUAUCUAUC 3’
H2N−Tyr−Leu−Ser−Ile−Tyr−Leu−Ser−Ile−COOH
↓

23. Using different base sequences,
Khorana deciphered the entire genetic
code by 1966
Of these, 61 are codons for amino
acids
It was found that all the 64 triplets are
code words

24. The remaining three codons do not code
for any amino acid
They are called nonsense codons
The nonsense codons also have a
function
They act as stop signals (chain
termination signals)

25. Codons on mRNA
UUU→Phe
UUC→Phe
UUA→Leu
UUG→Leu
UCU→Ser
UCC→Ser
UCA→Ser
UCG→Ser
UAU→Tyr
UAC→Tyr
UAA→Stop
UAG→Stop
UGU→Cys
UGC→Cys
UGA→Stop
UGG→Trp
CUU→Leu
CUC→Leu
CUA→Leu
CUG→Leu
CCU→Pro
CCC→Pro
CCA→Pro
CCG→Pro
CAU→His
CAC→His
CAA→Gln
CAG→Gln
CGU→Arg
CGC→Arg
CGA→Arg
CGG→Arg
AUU→Ile
AUC→Ile
AUA→Ile
AUG→Met
ACU→Thr
ACC→Thr
ACA→Thr
ACG→Thr
AAU→Asn
AAC→Asn
AAA→Lys
AAG→Lys
AGU→Ser
AGC→Ser
AGA→Arg
AGG→Arg
GUU→Val
GUC→Val
GUA→Val
GUG→Val
GCU→Ala
GCC→Ala
GCA→Ala
GCG→Ala
GAU→Asp
GAC→Asp
GAA→Glu
GAG→Glu
GGU→Gly
GGC→Gly
GGA→Gly
GGG→Gly

26. First base
at 5’-end
Third base
at 3’-end
U
C
A
G
Phe Ser Tyr Cys U
Phe Ser Tyr Cys C
Leu Ser Stop Stop A
Leu Ser Stop Trp G
Leu Pro His Arg U
Leu Pro His Arg C
Leu Pro Gln Arg A
Leu Pro Gln Arg G
Ile Thr Asn Ser U
Ile Thr Asn Ser C
Ile Thr Lys Arg A
Met Thr Lys Arg G
Val Ala Asp Gly U
Val Ala Asp Gly C
Val Ala Glu Gly A
Val Ala Glu Gly G
Second base
U C A G

27. UAA UAG UGA
AUG
Start Codon
Stop Codons
Codons having a special function

28. Salient features
of genetic code:
Degeneracy
Unambiguity
Universality
Absence of
punctuations
EMB-RCG

29. Degeneracy
EMB-RCG
Only tryptophan and methionine have a
single codon each
All the other amino acids are encoded by
more than one codons
Leucine, serine and arginine have six
codons each
Coding of one amino acid by multiple
codons is known as degeneracy

30. EMB-RCG
Degeneracy resides mainly in the third
base of the codon
For example, the four codons for alanine
are GCU, GCC, GCA and GCG
In these, the first two bases are common
and only the third base is different

31. Degeneracy confers an advantage
If the third base of a codon is subs-
tituted due to a mutation:
Amino acid sequence of
the protein may not change
The new codon may encode
the same amino acid

32. Unambiguity
A given codon codes only for one
particular amino acid
This is known as unambiguity of the
genetic code
Unambiguity ensures correct
translation of genetic information
EMB-RCG

33. Universality
The genetic code is identical in all
living organisms
However, some variations are found
in mitochondrial DNA
Therefore, the genetic code is nearly
universal
EMB-RCG

34. In mitochondria
AUA is the codon for methionine
instead of leucine
UGA is the codon for tryptophan
instead of being a stop signal
AGA and AGG are stop signals
instead of codons for arginine
EMB-RCG

35. Absence of punctuations
The genetic code is read continuously
from the start site
There are no commas or full stops
to indicate where a codon has ended
Therefore, insertion or deletion of a
base changes the reading frame
EMB-RCG

36. EMB-RCG
Addition of
G after the
third base
Removal of
U after the
third base
UGG UGG UGG UGG UGG
Trp Trp Trp Trp Trp
UGG GUG GUG GUG GUG
Trp Val Val Val Val
UGG GGU GGU GGU GGU
Trp Gly Gly Gly Gly
Altered code Altered code
Normal code