Genetic Code

If genes are segments of DNA and if a strand DNA is just a string of nucleotides, then the sequence of nucleotides must somehow dictate the sequence of amino acids in proteins.

How does the DNA sequence dictate the protein sequence?

A  combination  of  3  nucleotides  in  DNA/mRNA,  which  specify  a  particular  amino  acid  during  protein  synthesis ,  is  called  genetic  code .  The  genetic  information  of  an  organism  is  carried  on  its  DNA  in  the  base  sequence  and  is  organized   as  a  series  of  genes.  The  DNA  sequence  of  a  gene  is  divided  in  to  a  series  of  units  of  three  bases, i.e.  Genetic codes.

CHARACTERISTICS 

  1. TRIPLET CODE

There  are  only  four  kinds  of  bases  and  twenty  kinds  of  amino  acids.  If  only  one  base  served  as  a  code  for  amino  acid  we  could  have  only  four  codes (4)1 =4.  If  two  bases  served  as  a  code  then  we  would  have  16  codes (4)2 = 16 .  These codes  were  not  enough  for  the  20  amino  acids.  The  minimum  number  of  bases  that  could  be  involved  in  amino  acid  selection  seemed  to  be  three .  this  number  gave  a  possibility  of  selection  of  64  amino  acids (4)3 = 64. So  the  code  for  specifying  amino  acids  does  consist  of  3-bases .  This is called the triplet code.

Most  of  the  amino  acids  are  specified  by  more  than  one  code .  The  reading  occurs  continuously  without  punctuation  between  the  three  nucleotide  units .

After  Cricks  initial  experiments ,  Marshall  Nirenberg ,  Philip  Leader  and  Har  Gobind  Khorana  tested  all  the  64  codons  by  making  artificial  mRNAs  and  triplet  codons  and  using  them to  synthesize  a  protein  or  aminoacyl-tRNA  complexes  in  cell  free  systems.  The full genetic code was determined during mid 1960s.

Consider the following example in which three-letter English words are used to represent the codons:

The insertion suppresses the effect of the deletion by restoring most of the sense of the sentence. By itself, however,the insertion also disrupts the sentence:

THE FAT CAT AAT ETH EBI GRA T

 

  1. INITIATION CODON

Every  gene  starts  with  codon  AUG , which  normally  encodes  the  amino  acid

methionine .

  1. NON-SENSE OR STOP CODONS

Out  of  64  codons ,  three  codons  UAA  (Ochre ) ,  UGA  (Amber ) ,  and  UGA  (Opal) do  not  code  for  any  amino  acid ,  and  hence  are  known  as  non-sense  codon .  These  codons  are  usually  present  at  the  end  of  gene ,  and  hence  they  are  also  called  stop  codons .

  1. DEGENERACY

The code is degenerate which means that the same amino acid is coded by more than one codes. For example, the three amino acids arginine, alanine and leucine each have six synonymous codons.

  1. NONOVERLAPPING-CODE

The genetic code is non-overlapping, i.e. the adjacent codons do not overlap. A nonoverlapping code means that the same letter is not used for two different codons. In other words, no single base can take part in the formation of more than one codon.

Figure 1 Comparison of an overlapping and a nonoverlapping genetic code. The example uses a codon with three nucleotides in the RNA (a triplet code). In an overlapping code, single nucleotides occupy positions in multiple codons. In this figure, the third nucleotide in the RNA, U, is found in three codons. In a nonoverlapping code, a protein is translated by reading nucleotides sequentially in sets of three. A nucleotide is found in only one codon. In this example, the third U in the RNA is only in the first codon.

  1. CODE IS COMMALESS

there is no signal to indicate the end of one codon and the beginning of the next. The genetic code is commaless (or comma-free).

Deletion of a single base in a commaless code alters the entire sequence of amino acids after the point of deletion as given below.

RNA bases:                           UUU   CUC   GUA   UCC   ACC

Amino acids:                         Phe     Leu     Val      Ser      Thr

The deletion of base C from leucine will change the genetic message in the following manner:

RNA bases:                           UUU   UCG   UAU   CCA   CC

Amino Acids:                        Phe     Ser      Tyr       Pro      —

  1. CODE IS NON-AMBIGUIS

A particular codon will always code for the same amino acid. While the same amino acid can be coded by more than one codon (the code is degenerate),

the same codon shall not code for two or more different amino acids (non-ambiguous).

  1. POLARITY OF CODE

The genetic code has polarity, that is, the code is always read in a fixed direction, i.e., in the 5′ → 3′ direction.

  1. GENETIC CODE IS UNIVERSAL EXCEPT FOR MITOCHONDRIA

The genetic code is almost universal. The same codons are assigned to the same amino acids and to the same START and STOP signals in the vast majority of genes in animals, plants, and microorganisms. However, some exceptions have been found.

Most of these involve assigning one or two of the three STOP codons to an amino acid instead. Some exceptions have been reported for mitochondrial genome and in unicellular eukaryotes for synthesis of nonstandard proteins such as selenocysteine and pyrolysine.

Fig: 2 The genetic cod