Brief description about genetic codon and its characteristics. Degeneracy of genetic code, Open reading frame, Reading Frame etc

1. Genetic Code
By,
Dr. Kayeen Vadakkan
Assistant Professor,
Department of Biotechnology
St. Mary's College Thrissur,
Kerala , India

2. Topics Covered
• Genetic Code
• Discovery of Genetic Code
• Characteristics of Genetic code
Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

3. Genetic Code
Definition
The genetic code is a triplet of nucleotides that codes for a
specific amino acid.
A U A C G A G U C
Triplet Codon
Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

4. Discovery of genetic code
The discovery of genetic code and translation was based on the
experiments and observations made by three independent group
of scientists.
Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

5. Discovery 1
 In 1950, Paul Zamecnik and
his team discovered that the
site of protein synthesis is
ribosomes
Discovery 2
 Mahlon Hoagland and Paul
Zamecnik discovered that
the amino acids are activated
when incubated with ATPs
Discovery 3
 Francis Crik discovered that
how genetic informations are
coded in 4 letter language
(ATGC)
 A nucleic acid can function
as adaptor where in one part
bind with amino acid and
one with mRNA.
Genetic Code
Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

6. Characteristics of genetic code
1. Translation occurs when genetic code is read in successive ,
non overlapping way
 There is a possibility that the genetic code may be read in either over lapping manner or
non over lapping manner
A U A C G A
1
2
3
Overlapping Code
A U A C G A
1 2
Non- Overlapping Code
 In overlapping code same nucleotide in mRNA will be shared by different codon, however in
non-overlapping codon nucleotides are not shared by codon.
 In all living organisms the genetic code is non-overlapping
Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

7. Characteristics of genetic code
(Continues)2. Reading frame of Genetic Code
 It is the specific first codon in a mRNA sequence.
 A new codon begins at each three nucleotide residue
 There is no punctuation between these triplet codons
 The amino acid sequence is determined by the linear sequence of continuous triplets.
There is a possibility that in a single mRNA shall have various reading frame
U U C U C G G A C
U U C U C G G A C
U U C U C G G A C
Reading Frame I
Reading Frame II
Reading Frame III
Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

8. Characteristics of genetic code
(Continues)3. 64 Triplet Codons
 There are total 64 triplet codons present in mRNA
 Termination Codon : The termination codon UAA, UAG, UGA are also called as the stop
codons/Non-sense codon.
They stop the synthesis of amino acids and does not code for any known amino acids.
In general in a reading frame one in every 20 codons will be a termination codon
 Initiation Codon : The initiation codon (AUG) are the common signal for the beginning of
protein synthesis.
 Open Reading Frame : A reading frame without a termination codon for fifty or more
codons are referred as open reading frame
Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

9. Characteristics of genetic code
(Continues)4. Degeneracy of Genetic Code
Definition
 The phenomenon through which a single amino acid may be specified by more than one
codon is called the degeneracy of genetic code.
 The degeneracy of genetic code is not uniform
Example : there are 6 different codons present in mRNA that codes for Arginine, three
codons for Isoleucine and only one codon for Methionine
UC G
CC G
AC G
GC G
GA A
GGA
Arginine
Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

10. Further reading
1. Lehninger Principles of Biochemistry by Michael M Cox and David L Nelson
2. Molecular Biology of the Genes – J.D.Watson, N.H.Hopkins
3. Moleculary Cell Biology, J.Darnell, H.Lodish
4. Gene VIII, Benjamin Lewin
5. Genomes, T.S.Brown
6. Molecular Cloning: a Laboratory Manual, J.Sambrook.
Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur