Characteristics of Genetic code

Characteristics of Genetic code

Triplet code

A single base cannot be codes because there are 20aminoacids and only 4 bases. Pairs of bases also cannot serve  as codons because there are only  42 = 16 possible pairs  of 4 bases .Triplets of bases are possible because there are 43  =  64. triplets which is more than adequate .

Ø  Triplet code is minimum required to code for 20 amino acid . There are certain trends for patters of genetic codes .

Ø  Aspartic acid codons are similar ( GAU , GAC ) to  glutamic acid codons(GAA,GAG) the difference only exhibited in the third base .

Ø  Codes for aromatic acids phenylalanine ( UUU , UU C ) and tyrosine (UAU,UAC) and tryptophan UGG   all begins with uracil

First two bases are assigned to 5 amino acids

GC –Alanine

GG- Glycine

CC-Proline

AC-Threonine

GU- Valine

All codons with A in the second position specify the changed amino acid except arginine.

All the acidic and basic amino acids have A or G as the second base.

Degeneracy

It means that same amino acid is coded by more than one base triplet. Particular amino acid can be directed to its place in the peptide by more than one triplets. The code degeneracy is of 2 types

·         Partial

·         Complete

·         In partial, the first two nucleotide are identical but the third one is different.

CUU

CUC  both codes for leucine.

 Complete Degeneracy

When any of the 4 bases can take third position and still code the same amino acid.

Eg;

UCU

UCC                                 Codes for serine

UCA

UCG                                              

Advantages of Degeneracy

·         It permits essentially the same complement of enzymes and other proteins  to be specified by microorganisms.

·         Provides a mechanism of minimizing mutational lethality  .

Universality

It means that the same sequences of 3  bases encode the same amino acidss in all life forms from simple micro organism to complete multi celled organisms such as human beings .

Exception is seen in  mitochondria of  some  mammals and in   Tetrahymena  protozoa.

Redundancy

Genetic code is high redundant since there are only 20 amino acid , there are many cases in which several codons direct the insertion of the same amino acids into protein chain .Redundancy is the rule that there  are multiple codons except for tryptophan and methionine.

GGU

GGC                           CODE FOR GLYCINE

GGA

GGG

Significance of Redundancy is that it minimize the delirious effect of mutations

Genetic code  is continuous  and comma less

·         no codon is reserved for punctuation or the code is without spacers .

Non ambiguity

·        A particular codon  will always code for same amino acids .

Polarity

·         The code is always read in fixed directions i.e; in the 5’ -3’direction

·         The code is read in opposite direction it would specify  different proteins .

The codon   has start and stop signals .

Ø   In both prokaryotes  and eukaryotes AUG  -  methionine is start codon for protein synthesis .

Ø  Only 61 of 64codons specify amino acids these codons are called sense  codons .

Ø  The other three UGA , UAA, UAG are known as STOP  CODONS   or non sense codons or chain terminating codons

Wobble occurs in the anti codon  region .

Ø   Since 61sense codon specify amino acids in mRNA  a total of 61 tRNA molecules have appropriate anti codon ,  but the complete sense of 61 sense codon is  read by fewer than 61 tRNAs because of pairing properties of the base in anti codon

 



 

 


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