The genetic code is the set of rules used by living cells to translate information encoded within genetic material into proteins

1. GENETIC CODE : DECIPHERING PROPERTIESAND CODE
DICTIONARY
HEENA KAUSAR
DEPARTMENT OF MICROBIOLOGY

2. Genetic Code
 The relationship between a sequence of DNA and the
sequence of the corresponding protein is called the
genetic code.
Genetic code is a set of rules by which information
encoded in genetic material i. e. RNA and DNA is
translated into proteins.

3. Establishment of the Genetic Code
DNA is the genetic material triggered efforts to understand how
genetic instructions are stored and organized in the DNA molecule.
Genetic code showed that the DNA base sequence corresponds to the
amino acid sequence of the polypeptide specified by the gene.
That is, the nucleotide and amino acid sequences are co-linear.
 Marshall Nirenberg and Heinrich J. Matthaei were the first to reveal
the nature of genetic code in 1967.

4. What is codon…..?
Genetic code is a dictionary consists of “genetic words” called codon.
There are 64 codons (each of 4 possible nucleotides can occupy each of three
positions of the codon, so that there are 43 = 64 possible trinucleotide sequences).
Each of these codons has a specific meaning in protein synthesis.
61 codons represent 20 amino acids.
3 codons cause the termination of protein synthesis.

5. Why codon is triplet…..?
The logic behind triplet genetic code or why is the genetic code is triplet code
instead of singlet or doublet..?
As We know that there are total four nitrogenous bases- In DNA- Adenine,
Thymine, Cytosine, Guanine and In RNA- Adenine, Uracil, Cytosine, Guanine.
Total 4 base will mathematically follows the permutation and combination
technique and in triplet form they forms 64 codons.
As we already know there are total 20 Amino acids present that are coded by
these four nitrogenous bases.
Let’s us consider the following possibilities….

7. Lets see different cases for the
arrangement of N2 bases-

8. If we consider the genetic code to be singlet then it would
be impossible as there are four bases and that cannot code
for 20 amino acids. If codon is singlet is can code for 4
amino acids only.
CASE- I

9. If the genetic code is a doublet, that is , an amino acid is coded by 2
nitrogenous base on mRNA in a specific sequence, then it can form 16
codons ( 42=4*4=16), still not sufficient enough to code 20 amino acids.
So the genetic code can’t be of two letters.
CASE- II

10. If the genetic code is triplet, that is an amino acid is coded by 3
nitrogenous bases, and then it can be 64 codons.
But we have only 20 amino acids so codon are in excess. Then the
possibility is some amino acids may be coded by more than one
triplet code.
Now as the number of codon is more than number amino acid this
condition is acceptable. Since one amino acid can be there for
different codon. So the codon is taken in triplet form.
CASE- III

12. If the genetic code is a 4 letter code, that is, an amino acid is coed by 4
nitrogenous bases then it can form 256 codons (44=4*4*4*4=256).
But we have only 20 amino acid which is way too more, so it can’t be 4.
CASE- IV

13. Types of codon
1. Sense codon: The codon that code amino acid into protein.
2. Signal codons: There are two types of signal codons
•Initiation codons
•Termination codons
a. Initiation codon: AUG is the initiation codon that codes for the first amino
acid for all protein. It codes Methionine (Met) in Eukaryotes and Formyl
methionine (fMet) in Prokaryotes.
b. Termination codons : UGA, UAG, and UAA are involved in the termination
of translation is called nonsense codon or stop codon.

14. Anticodon
 tRNA is the key to deciphering the codons in mRNA.
 The base sequence of tRNA which pairs with codon of mRNA during translation
is called as anticodon.

15. Wobble hypothesis
There are more than one codon for one amino acid. This is called degeneracy of
genetic code.
To explain the possible cause of degeneracy of codons, in 1966, Francis Crick
proposed “the Wobble hypothesis”. (wobble means to sway or move
unsteadily).
The Wobble hypothesis proposes that normal base pairing can occur between
nitrogen bases in positions 1 and 2 of the codon and the corresponding bases (3
and 2) in the anticodon. Actually, the base 1 in anticodon can form non-Watson-
Crick base pairing with the third position of the codon. The hypothesis is
applicable to most (not all) tRNAs.

17. Difference between codons and anticodon…?
CODONS
1. Present in DNA or RNA.
2. Written in 5’ to 3’ direction.
3. Codons are sequentially arranged in
nucleic acid strand.
ANTICODON
1. Always present in RNA.
2. Written in 3’ to 5’ direction.
3. Anticodons are discretely arranged.

18. Properties of genetic code
1. Codons are Triplet
2. Codons are Non overlapping
3. Codons are Comma less
4. Codons are Nearly Universal
5. Codons are Serially read
6. Continuous Translation
7. Codons are Redundancy.

19. Codons are Triplet
 Codons are always in triplet.
If codons are singlet or doublet they do not code 20 amino acid.
The nucleotides of mRNA are arranged as a linear sequence of codons, each
codon consisting of three successive nitrogenous bases, i.e., the code is a triplet
codon.

20. Codons are Non-overlapping
In translating mRNA molecules the codons do not overlap but are “read”
sequentially.
Thus, a non-overlapping code means that a base in a mRNA is not used for
different codons.
B- Overlapping of codon due to one bases
A- Non overlapping codon

21. Codons are serially read
 Bases in mRNA are read sequentially in the 5’ to 3’ direction, in group of three
bases.

22. Codons are Comma less
The genetic code is comma less.
It means that after one amino acid is coded, the second amino acid will be
automatically, coded by the next three letters and that no letters are wasted as the
punctuation marks.

23. Codons are Non-ambiguous
Non-ambiguous code means that a particular codon will always code for the
same amino acid.
In case of ambiguous code, the same codon could have different meanings or in
other words, the same codon could code two or more than two different amino
acids. Generally, as a rule, the same codon shall never code for two different
amino acids.
However, there are some reported exceptions to this rule: the codons AUG and
GUG both may code for methionine as initiating or starting codon, although
GUG is meant for valine. Likewise, GGA codon codes for two amino acids
glycine and glutamic acid.

24. The code has polarity
The code is always read in a fixed direction, i.e., in the 5’→3′ direction. In other
words, the codon has a polarity. It is apparent that if the code is read in opposite
directions, it would specify two different proteins, since the codon would have
reversed base sequence.

25. Codons are nearly Universal
 Same genetic code is found valid for all organisms ranging from bacteria to
man.

26. Three kinds of mutations alter the genetic code
1. Missense mutation
2. Nonsense or stop mutation
3. Frame shift mutation

27. Missense mutation
It contains an alteration that can
change a codon specific for one
amino acid to a codon specific for
another amino acid.
A single amino acid is substituted for
another in a protein product of gene
bearing a missense mutation.

28. Nonsense or stop mutation
The nonsense mutation converts a codon (a triplet of bases) that encodes an
amino acid into a stop codon.
 That specifies the termination of translation.

29. Frame Shift mutations
Frame shift mutations includes
the alteration of reading frame by
insertion or deletion of one or
small number of base pairs.
Insertion - The mutations which
occur due to addition of one or
more extra nucleotides in a triplet
codon is called insertion
mutations.
Deletion - The mutation which is
caused due to loss or deletion of
some portion (single nucleotide
pair) in a triplet codon is called
deletion mutation.

30. References
 Benjamin Lewin. (2008) Genes IX, Jones and Bartlett Publishers Inc.
 Molecular Cell Biology. Lodish, Birk, and Zipursky. Freeman.
Watson James D., Tania Baker, Stephen P. Bell, Alexander Gann, Michael Levine, Richard
Lodwick (2004).
https://microbenotPrescott Harley Klein's Microbiology 7th Edition
es.com/the-wobble-hypothesis/
http://www.biologydiscussion.com/genetics/wobble-hypothesis-with-diagram-genetics/65163
Images taken from internet.