Genetic code and its characteristics...

1. SHALINI BARA
ASSISTANT PROFESSOR
ST. XAVIER’S COLLEGE, MAHUADANR,
LATEHAR
NILAMBER PITAMBER UNIVERSITY

2.  The four nucleotide bases [Adenine (A), Guanine (G), Cytosine
(C), and Uracil (U); in case of RNA only] are used to form the three-
base codons.
 The collection of codons is called genetic code.
 There are 20 amino acids, which occur naturally.
 Each codon consists of 3 bases (known as triplet) to impart
specificity to each of the amino acid for a specific codon.
 So, for 1 nucleotide- 4 combinations
2 nucleotide- 16 combinations
3 nucleotide- 64 combinations (for 20 amino acids).
 Out of 64 codons, 61 codons code for 20 amino acids found in
protien and 3 codons do not code for any amino acid (i.e. They are
stop codons).

3. The nitrogenous bases

4. The amino acids

5.  The genetic code is composed of words formed by a sequence of
nucleotide bases and a sequence of amino acids. Each word in the code is
composed of three nucleotide bases. These genetic words are called codons.
 It is a dictionary that corresponds with sequence of nucleotides and
sequence of amino acids.
 It is a set of rules by which information encoded in genetic material (i.e.
DNA or RNA) is translated into proteins by living cells.
 The term ‘Genetic Code’ was given by George Gamow.

6. THE GENETIC CODE

7. Mainly two types:
1. Sense codons
2. Signal codons
It is of two types:
a) Start codons/ initiation codons
b) Stop codons/ termination codons

8. Cont.....
1. Sense codons: The codon that code for amino acid.
2. Signal codon: Those codon that code for signal during protein
synthesis. For e.g., AUG, UAA, UAG & UGA.
a) START CODON: It is also known as initiating codon.
For e.g., AUG is the initiation codon. It codes for the first amino acid in all
proties.
 In prokaryptes, at the starting point it codes for amino acid Methionine
and in eukaryotes, codes for formyl methionine.
b) STOP CODON: it is also known as terminating codon or nonsense
codon.
For e.g., UAA, UAG & UGA and are often referred to as amber, ochre &
opal codons.

9. The base sequence of t RNA which pairs with codon of mRNA during
translation is called anticodon.
Fig: Codon and aticodon sequence

10. Codon Anticodon
It can present in both
DNA & RNA
Always present in RNA &
never in DNA
Located on mRNA
molecule
Located on tRNA molecule
Codon are written in 5’ to
3’ direction
Anticodons are written in 3’
to 5’ direction
Sequentially present on
mRNA
Individually present on tRNA
Complementary to the
nucleotide (triplet) DNA
Complementary to the codon
Determines position of
amino acid
It brings the specified amino
acid at its proper position
during translation

11. 1. Triplet code
2. Commaless code
3. Universality of code
4. Non- overlapping code
5. Non- ambigous code
6. Degenerating properties of code
7. Wobble hypothesis
8. The code has polarity
9. Chain initiation code
10. Chain termination code

12. The genetic code is triplet. There are 64 codons. The nucleotides of mRNA
are arranged as a linear sequence of codons, each codon consisting of
three successive nitrogenous bases, i.e., the code is a triplet codon.

13. In all the living organisms the genetic code is the same. So,the code is
universaal.
AUG is the codon for methionine in mitochondria. The same codon (AUG)
codes for isoleucine in cytoplasm.
However, with some exceptions the genetic code is universally accepted.
Non- ambigous code
The code is non- ambigous.
Thus, one codon can not specify more than one amino acid.
e.g.,

14. One base cannot participate in the formation of more than one codon.
Therefore, the code is non- overlapping.
Code is commaless
The gene is transcribed and translated continously from a fixed staring point
to a fixed stop point.
Therfore, punctuations are not present between the codons.

15. Degeneracy
 Three codons (UAA, UAG, UGA) do not code for specific amino acids, these
have been termed as nonsense codons/ stop codons.
At least 2 of these nonsense codons are utilized in the cell as termination
signals, they specify where the polymerization of amino acids into a protein
molecule is to stop.
 The remaining 61 codons code for 20 amino acids. So there must be
degeneracy in the genetic code.
 Every amino acid except methionine is represented by several codons.

Codons that represent the same amino acid are called synonyms.

16. Wobble hypothesis
 It was given by ‘Crick’.
 It explains the degeneracy of the genetic code i.e. Existence of
multiple codon for a single amino acid.
 It states that, the first two bases of a codon pair according to the
normal base pairing rules with the last two bases of the anticodon.
But the base- pairing at the third position of codon is wobble.

17. Polarity
 The genetic code has polarity, i.e., the code is always read in a fixed
direction, i.e., in the 5′ → 3′ direction.
 It is apparent that if the code is read in opposite direction (i.e., 3′ → 5′), it
would specify 2 different proteins, since the codon would have reversed
base sequence.
For e.g. UUG stands for leucine in 5ʹ- 3ʹ́ and from 3ʹ- 5ʹ GUU which stand
for valine.5’
Fig: 5ʹ - 3ʹ direction of codon

18. Initiation or Start Codon
 In most organisms, AUG codon is the start or initiation codon, i.e., the
polypeptide chain starts either with methionine (eukaryotes) or N-
formylmethionine (prokaryotes).
 Methionyl or N-formylmethionyl-tRNA specifically binds to the initiation
site of mRNA containing the AUG initiation codon.
 In rare cases, GUG also serves as the initiation codon, e.g., bacterial
protein synthesis. Normally, GUG codes for valine, but when normal AUG
codon is lost by deletion, only then GUG is used as initiation codon.

19. Termination (Stop or Nonsense) Codons
Three codons (UAG, UGA, and UAA) do not code for amino acids and hence,
termed as termination codons.
If one of these codons appears in an mRNA sequence, it signals the complete
synthesis of the peptide chain coded for by that mRNA.

20. Mutations can be well explained using the genetic code.
For e.g.
1. Point mutation
a) silent
b) Missense
c) Nonsense
2. Frame shift mutation

21. Silent mutation
 The change in one codon for an amino acid into another codon for that
same amino acid. Such as A to G.
 Same amino acid is incorporated.
 Also known as synonymous mutation.

22. Missense mutation
 The codon for one amino acid is change into a codon for another amino
acid.
 Single nucleotide change such as A to C.
 Different amino acid incorporated.
 Loss of functional capacity of protein.

23. Non sense mutation
 The codon for one amino acid is changed into a translation termination
(stop) cogon.
 Single nucleotide change from C to T
 Stop codon is generated (in mRNA represented by UAG)
 Premature termination of polypeptide chain.

24. Frame shift mutation
 Insertion or removal of a bases an alter the reading frame with the
resultant incorporation of different amino acids.

25. THANK YOU......!