Genetic code is three nucleotides in a row count as a triplet that codes for a single amino acid.

1. Genetic Code
Anup Muni Bajracharya

2. Genetic Code
The genetic code may be defined as the exact sequence
of DNA nucleotides read as three letter words or
codons, that determines the sequence of amino acids
in protein synthesis.
The genetic code links groups of
nucleotides in an mRNA to
amino acids in a protein.

3. Properties of Genetic Code
• Genetic code has some
important properties.
• The genetic code is
• (i) Triplet,
• (ii) Universal,
• (iii) Comma-less,
• (iv) Non-overlapping,
• (v) Non-ambiguous,
• (vi) Redundant/Degenerate,
and
• (vii) Has polarity.

4. 1. The Code is Triplet
• Singlet and doublet codes are not adequate to
code for 20 amino acids; therefore, it was pointed
out that triplet code is the minimum required.
• The triplet code has 64 codons which are
sufficient to code for 20 amino acids and also for
start and stop signals in the synthesis of
polypeptide chain.
• In a triplet code three RNA bases code for one
amino acid.

6. 2. The Code is Universal
• The genetic code is almost
universal.
• Universality of the code means
that the same sequences of 3
bases encode the same amino
acids in all life forms from simple
microorganisms to complex,
multicelled organisms such as
human beings.
• The same codons are assigned to
the same amino acids and to the
same START and STOP signals in
the vast majority of genes in
animals, plants, and
microorganisms.
• However, some exceptions have
been found.

7. 3. The Code is Commaless
• There is no signal to indicate the end of one
codon and the beginning of the next.
• In other words, the codons are continuous
and there are no demarcation lines between
codons.

8. 4. The Code is Non-Overlapping
• A non overlapping code means that the same letter is
not used for two different codons.
• In other words, no single base can take part in the
formation of more than one codon.
• The adjacent codons do not overlap.
• Example: There are Bases : CATGAT
• Non-overlapping Code : 2 that is CAT and GAT;
• Overlapping Code : 4 that is CAT, GAT, ATG and TAT

9. 5. The Code is Non-ambiguous
• The genetic code has 64 codons.
• A particular codon will always code for the same amino acid.
• Out of these, 61 codons code for 20 different amino acids. However,
none of the codons codes for more than one amino acid.
• In other words, each codon codes only for one amino acid.
• the same codon shall not code for two or more different amino acids
(non-ambiguous).
• In case of ambiguous code, one codon should code for more than one
amino acid. In the genetic code there is no ambiguity.

10. 6. The Code is Redundant/Degenerate
• In most of the cases several codons
code for the same amino acid.
• Only two amino acids, viz.
tryptophan and methionine are
codded by one codon each.
• Nine amino acids are coded by two
codons each, one amino acid
[Isoleucine] by three codons, five
amino acids by 4 codons each, and
three amino acids by 6 codons
each .
• This multiple system of coding is
known as degenerate or redundant
code system.

11. 7. The Code Has Polarity
• The genetic code has polarity, that is, the code is
always read in a fixed direction, i.e., in the 5′ → 3′
direction.
• It is apparent that if the code is read in opposite
direction (i.e., 3′ → 5′), it would specify 2 different
proteins, since the codon would have reversed base
sequence.
• This is well known that the message in mRNA is read in
the 5 -3 direction. Thus the polarity of genetic code is
from 5 end to 3 end.

12. A. Chain Initiation Codons
• The start codon is the first codon of a messenger RNA
transcript translated by a ribosome.
• The triplets AUG and GUG play double roles in E. coli.
• When they occur in between the two ends of a cistron
(intermediate position), they code for the amino acids
methionine and valine, respectively in an intermediate
position in the protein molecule.
• But when they occur immediately after a terminator
codon, they act as “chain initiation” signals or “starter
codons” for the synthesis of a polypeptide chain.

13. B. Chain Termination Codons
• 3 triplets UAA, UAG, UGA do not code for
any amino acid and are also called as stop
codons.
• The three stop codons have been named as
amber[ UAG ] , opal or umber [ UGA ] and
ochre [ UAA].
• They are also called chain termination
codons because these codons are used by
the cell to signal the natural end of
translation of a particular peptide chain.
• However, their inclusion in any mRNA results
in the abrupt termination of the message at
the point of their location even though the
polypeptide chain has not been completed.

14. C. Sense Codons and Non-Sense
Codons
• 61 codons, which code for particular amino acids
are termed as sense codons.
• Remaining three codons UAA, UAG, UGA do not
code for any amino acid were called as non-sense
codons.
• However, the so-called non-sense codons have
now been found to be of “special sense”.
• These special-sense codons perform the function
of punctuating genetic message like a full stop at
the end of a sentence.