3. 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.
or
The genetic code is a set of three-letter
combinations of nucleotides called codons, each of
which corresponds to a specific amino acid or stop
signal.
4. GENETIC CODE…
There are 64 possible combinations, of three-letter
nucleotide sequences that can be made from the
four nucleotides.
With three exceptions, each codon encodes for one
of the 20 amino acids used in the synthesis of
proteins.
The codons are of two types:
(1) Sense codons
(2) Signal codons.
5. GENETIC CODE
Sense Codon:
Those codons that code for amino acids are called
sense codons. There are 61 sense codons in the
genetic code which code for 20 amino acids.
Signal Codons:
Those codons that code for signals during protein
synthesis are known as signal codons. There are
four codons which code for signal. These are AUG,
UAA, UAG and UGA.
6. GENETIC CODE
History
The concept of codons was first described by
Francis Crick and his colleagues in 1961.
Marshall Nirenberg and Heinrich Matthaei
performed experiments for deciphering the genetic
code in 1961.
Later on, Nirenberg, Philip Leder, and Gobind
Khorana identified the rest of the genetic code and
fully described each three-letter codon and its
corresponding amino acid.
8. PROPERTIES OF GENETIC CODE
Some important properties genetic code are.
The genetic code is:
(i) Triplet
(ii) Universal
(iii) Comma-less
(iv) Non-overlapping
(v) Non-ambiguous
(vi) Redundant
(vii) Has polarity
10. TRIPLET
The genetic code is triplet. 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.
Example ..UUU
11. UNIVERSAL
The universality of genetic code was explained by
Marshall, Caskey and Nirenberg (1967) who found
that E. coli (Bacterium), Xenopus laevis
(Amphibian) and guinea pig (mammal) amino acyl-
tRNA use almost the same code.
13. COMMA LESS
Genetic code is comma less means that no punctuation or
comma is used between the codons. It is continuous no
letter is used in between.
14. NON OVERLAPPING
Non overlapping codes means that the base in m-
RNA is not used for two different codes.
it has been shown that in the bacteriophage ɸ ×
l74 there is a possibility of overlapping the genes
and codons (Barrel and coworkers, 1976;
Sanger, et al., 1977).
15. UNAMBIGUOUS AND DEGENERATE
Unambiguous nature of the code means that same
codon will code for only one amino acid where
ever it is present.
Degenerate nature of genetic code means that
there is more than one codon for same amino acid
but it will always code for the same amino acid.
