GENETIC CODE

1. GENETIC CODE
SHUBHANGI JAIN
PGT BIOLOGY
KV NO. 2 UDHAMPUR

2. WHAT IS GENETIC CODE ?
The sequence of three nucleotides of RNA that
determines the amino acid sequence of proteins.

3. WHAT LED TO PROPOSITION OF GENETIC
CODE?
• No complementarity exists between Nucleotides and polymer of
amino acids.
• But ample evidences suggested that change in nucleotides were
responsible for change in amino acids.
• Led to proposition that a genetic code could direct the
sequence of amino acids during synthesis of proteins.

4. HOW GENETIC CODE WAS DECIPHERED?
•A single base cannot code for a single amino acid
because then only four amino acids would be coded.
•If a combination of two bases could code for each
amino acid, then only 16 amino acids could be coded.

5. HOW GENETIC CODE WAS DECIPHERED?
•The next option was the combination of three bases
that code for one amino acid, by which 64
combinations were possible that were much more
than the required 20 amino acids. This was
proposed by GEORGE GAMOW , a physicist.

6. Har Gobind Khorana developed a chemical method
which was instrumental in synthesizing RNA molecules
with defined combination of bases.
Finally, Marshall Nirenberg’s cell free system for protein
synthesis helped the code to be deciphered. Marshall
Nirenberg’s is known as the Father of the Genetic Code.
PROOF THAT CODON WAS A TRIPLET

7. FEATURES OF GENETIC CODE
•Codons are triplet: Each genetic code is a triplet
formed of three nucleotide bases that code for a
particular amino acid. There are 64 codons that code
for 20 amino acids which means that there exists
more than one code for a single amino acid.

8. FEATURES OF GENETIC CODE
•Codons are non-overlapping: The genetic codes
are non-overlapping, i.e., the adjacent codons do not
overlap. It means that the same letter is not used for
two different codons, or a single base cannot take
part in the formation of more than one codon while
reading the sequence of bases from the starting of
mRNA in blocks of three.

9. FEATURES OF GENETIC CODE
•Codons are nearly universal: The genetic code is
universal means the same codons code for the
same amino acids in all forms of life that exists
today.
•Some exceptions have been found in mitochondrial
codons and some protozoans.

10. FEATURES OF GENETIC CODE
•Codons are read in m RNA in contiguous
fashion. There are no punctuations.

11. FEATURES OF GENETIC CODE
•Codons are non-ambiguous: The genetic code is
non-ambiguous and specific because each code
codes for only one amino acid.

12. FEATURES OF GENETIC CODE
•Codons are degenerate: Some amino acids are
coded by more than one codon.

13. FEATURES OF GENETIC CODE
•Codons have polarity: The genetic code has a
definite direction for the reading of information which
is called polarity. The message in the mRNA is read
in the 5’ to 3’ direction, and thus, the polarity of the
genetic code is from 5’ end to 3’ end.

14. TYPES OF GENETIC CODE
• Sense Codons: These are defined as those codons that
code for amino acids. There are 61 sense codons in the
genetic code that code for 20 amino acids.
• Start codons: These are those codons that start the
translation process. They are also known as initiation codons
as they initiate the protein synthesis process. For example-
AUG which codes for the amino acid methionine, act as the
initiating or the start codon in eukaryotes.

15. TYPES OF GENETIC CODE
• Stop codons: These are those codons that provide a signal for the
termination of polypeptide chain synthesis. They are also known as
termination codons as they stop the formation of polypeptide chains.
For example- UAA (ochre), UAG (amber) and UGA (opal) are known
as termination codons or stop codons. They do not code for any
amino acids, so they are also known as non-sense codons.

17. THANKS