1) The document discusses the genetic code, which determines how DNA and mRNA sequences are translated into proteins. 2) Marshall Nirenberg and others were the first to elucidate the nature of codons in 1961 and determine that codons consist of three DNA bases. 3) The genetic code is universal, uses triplets of nucleotides, has no commas, does not overlap, is not ambiguous, but is degenerate meaning several codons can code for the same amino acid.

1. WEL - COME
Mr. Balaji S. Thorat
Regd. No: ADPM/15/ 2418
Discipline: Genetics & Plant breeding.
Dept. : Agrl. Botany, College of Agriculture, Dapoli.

2. GENETIC CODE
The genetic code is the set of rules by which
information encoded within genetic material
(DNA or mRNA sequences) is translated into
proteins by living cells.
The ‘genetic code’ determines a protein's amino acid
sequence.
The set of all the codons that specify the 20 amino
acids is termed as the genetic code

3. DISCOVERY
 A code of 3 nucleotides could code for a maximum
of 43 = 64 amino acids
 The Crick, Brenner et al. experiment first demonstrated
that codons consist of three DNA bases.
 Marshall Nirenberg and Heinrich J. Matthaei were the
first to elucidate the nature of a codon in 1961 at
the National Institutes of Health.
 Har Gobind Khorana identified the rest of the genetic
code.
 Robert W. Holley determined the structure of transfer
RNA (tRNA), the adapter molecule that facilitates the
process of translating RNA into protein.

4.  Nirenberg and Philip Leder revealed the triplet nature of
the genetic code and deciphered the codons of the
standard genetic code.
 Leder and Nirenberg were able to determine the
sequences of 54 out of 64 codons in their experiments.
 In 1968, Khorana, Holley and Nirenberg received the
Nobel Prize in Physiology or Medicine for their work.

5. CODON
 The code defines how sequences of nucleotide
triplets, called codons, specify which amino acid will
be added next during protein synthesis.
 The number and the sequence of bases in mRNA
specifying an amino acid is known as a codon.
 The set of bases in tRNA which base pair with a
codon of mRNA is known as anti-codon.
 Codons are written in 5’ 3’ direction, where as
anticodons are usually written in 3’ 5’ direction.

6. START CODON
 Translation starts with a chain initiation codon or start
codon.
 The most common start codon is AUG, which is read
as methionine or, in bacteria, as N formyl methionine.
 AUG is known as start codon or initiation codon as it
starts the synthesis of polypeptide chain.

7. Stop Codon
 Stop codons are also called ‘termination’ or ‘nonsense’
codons.
 They do not code for any amino acid and hence they are
called nonsense codons.
 The three stop codons have been given names: UAG
is amber, UGA is opal (sometimes also called umber),
and UAA is ochre.
 ‘Amber’ was named by discoverers Richard Epstein and
Charles Steinberg after their friend Harris Bernstein,
whose last name means ‘amber’ in German.
 The other two stop codons were named ‘ochre’ and
‘opal’ in order to keep the ‘color names’ theme.

8. PROPERTIES OF GENETIC CODE
1. The code is triplet.
 first suggested by Gammow in 1954.
 Three RNA bases code for one amino acid.
2. The code is universal.
 The same genetic code is applicable to all forms
of organisms from microbes to human beings.

9. 3. The code is comma less.
 The genetic code is without a comma or break.
 A change or deletion of a single base in the code will
alter the entire sequence of amino acid to be
synthesized.
4. The code is non-overlapping.
 Three nucleotides or bases code for one amino acid
and six bases will code for two amino acids.
 In a non – overlapping code, one base or letter is read
only once.

10. 5. The code is non - ambiguous.
 Out of the 64 codons, 61 code for 20 different aminoacid,
while 3 are nonsense codons
 None of the codons code for more than one amino acid.
6. The code is degenerate.
 several codons code for the same aminoacid. Only two
amino acids, viz., tryptophan and methionine are coded
by one codon each.

11.  Nine amino acids are coded by two codons each, one
amino acid (isoleucine) by three codons each, five amino
acids by four codons each and three amino acids by six
codons each. This type of redundancy of genetic code is
called degeneracy of genetic code.
 Such a system provides protection to the organisms
against many harmful mutations.
 If one base of codon is mutated, there are other codons,
which will code for the same amino acid and thus there
will be no alteration in polypeptide chain.

12. 7. The code has polarity.
 The code has a definite direction for reading of
message, which is referred to as polarity.
 Reading of code in opposite direction will naturally
specify for another amino acid.
 e.g. GUC codes for valine, if reversed, CUG codes for
Leucine.

13. WOBBLE HYPOTHESIS
 Wobble hypothesis:
 The first two bases on the codon are usually the
important ones, the third base may be in some
cases one of the four bases and the triplet would
still code for the same amino acid as long as the
first two bases are the same and have the same
sequence. This concept is known as the Wobble
hypothesis.

14. THANK YOU