3. Introduction
• A codon is the Nucleotides sequence in mRNA
which codes for a perticular amino acid.
• Genetic code is the sequence of nitrogenous bases
in mRNA molecules which incloses information for
the synthesis of protein molecules.
4. Genetic properties
1)The code is a triplet code.
2)The code is non overlapping code.
3)The code is commaless.
4)The code is non ambiguous.
5)The code is polarity.
6)The code is degenerate.
7)Some codes as act start code.
8)Some codes as act stop code.
9)The code is universal code.
5. 1)The code is a triplet code
• The Nucleotides of mRNA are arranged as a linear
sequence of codons each codon consisting of
three successive nitrogenous bases .the code is a
triplet codon.
• 2 types of point mutation
• 1)frame shift mutations
• 2) base substitution
6. 2)The code is non overlapping
• In translating mRNA molecules the codons do not
overlap but are read sequentially.
• A non overlapping code means that a base in
mRNA is not used for different CODONS.
• In case of in overlapping code a single change in
the base sequence will be refected in substitution
of more than one amino acid in corresponding
protein.
7. 3) The code is commaless
• The genetic code is commaless which
means that no codon is reserved
punctuation.
• It means that offer one amino acid is coded
the second amino acid will be automatically
coded by the next three latter's and there
no latter's are wasted as the punctuation
marks.
8. 4)The code is non ambiguous
• Non ambiguous code means that a
perticular codon will always code for the
same amino acid .
• Incase of ambiguous code the same codon
could have different meaning. The same
codon could code two or more then two
amino acid.
9. • The code is always read in fixed direction.
• In other words the codon has a polarity .
• Reading from left to right & right to left will
specyfy for different amino acids.
5) The code is polarity
10. 6)The code is degenerate
• More then one codon may specify the same
amino acid this is called degenerecy of the
code.
• Two type degenerecy code:- pertial and
complete.
11. • Pertial degenrecy occurs when first two
nucleotide of the degenerate codons
deffers.
• Complete degenracy occurs when any of the
the four bases can take third position and
still code for the same amino acid.
12. 7)Some codes act as start codons
• In most organisms AUG codon is the start or
initiation codons the polypeptide chain
starts either with mithionine .
• Normally GUG codes for valine but when
normal AUG codon is lost by deletion only
then GUG is use as initiation codon.
13. 8)Some codes act as stop codons
• Three codons UAG,UAA,UGA are the chain
stop or termination codons.
• They do not code for any amino acid.
• The UAG was the first termination codon to
be discovered by sindey benner(1969)
14. 9)The code is universal
• Same genetic code is found valid for all
organisms ranging from bacteria to man.
• Nirenberg has also stated that the genetic
code may have developed 3 billion years
ago with the first bacteria and it has
changed very little throughout the
evolution of living organisms.
15. Deciphering genetic code
• The genetic code has been cracked or deciphered
by following kinds of approaches.
• Theoretical Approach
• The in vitro codon Assignment
• The in vivo codon Assignment
16. 1) Theoretical Approach
• The physicist George gamow proposed the
diamond code (1954) and the triangle code
(1955) an suggested and exhauslive
theoritical frame work to the different
aspect of the genetic code.
17. Following properties code
• A triplet codon corresponding to one amino
acid by the polypeptide chain.
• Direct template translation by codon amino
acid pairing
• Translation of the code in an overlapping
manner.
18. 2) The In-vitro codon Assignment
1. Discovery and use of polynucleotide
phosphrylase enzyme.
2. Codon assignment with unknown
sequence.
3. Assignment of codons with known
sequence.
19. Discovery and uses of polynucletide
phosphorylase enzyme.
• Marainne graun berg -mango and
severochoa isolated and enzyme from the
bacteria that cataylas the break down of
mRNA in bacterial cells.
• This enzyme is called polynucleotide
phosphoraylase.
20. Codon Assignment with unknown
sequence.
1) Codon assignment by homopolymer.
• First cut to clue to codon assignment was provided
by marshall nirenberg and Heinrich mathali(1961).
• When they used In-vitro system for the synthesis
of apolypeptide using an artifically sinthesized
mRNA molecules containing only one type of
Nucleotides is known as homopolymer.
21. Codon assignment by heteropolymers
2) Copolymers with random sequences.
• They used In-vitro system for the synthesis of
polypeptide using an artifically synthesized mRNA
molecules containing more than one type of
Nucleotides as known as heteropolymers.
• The use of mixed polymers are known as copolers.
22. Codon assignment with known sequences
3) Use of trinucleotides of minimessengers in
filter binding :-
• Ribosome technique of nirenberg and leder.
(1964) made use of finding that amino acid -tRNA
molecules specifically bind to ribosome mRNA
complex.
• In random or defined sequences and the use of
trinucleotides as called mini messanger in
ribosome binding or filter binding.
23. 3)The in vivo codon Assignment
• The cell free protein synthetic systems through
have proved of great significance in deciphermemt
of the genetic code but they could not tell us
wether the genetic code so deciphered is used in
the living system of all organsims also.
