Genetic Code : Definition, Nature & Characteristics, genetic code table and genetic bias


  Central dogma of molecular biology describes the two step process by which information in genes flow into proteins.
  DNA ➞ RNA ➞ Protein
DNA to RNA by Transcription and RNA to Protein by Translation.

As the language of nucleotide sequence on mRNA is translated to language of an amino acid sequence.
  Translation requires a genetic code through which information contained in nucleic acid is expressed in specific sequence of amino acid and this collection of codons as we known as Genetic codon.

The letters A,G,T,C correspond to nucleotides in DNA they are organised into codons.
For 20 Amino acid (standard) requires at least 20 codons.
• If 1 nucleotide act as a codon there will be 4 combinations.
• If 2 nucleotide act as a codon there will be (4)² = 16 combination.
• If 3 nucleotide act as a codon there will be (4)³ = 64 combination.

George Gamow postulated that 3 letter codon must be employed to encode 20 standard amino acid used by living cells for protein synthesis.


Definition :

Genetic code is a set of rules (defined by 64 triplet codons) by which information encoded in genetic material (DNA or mRNA sequences) is translocated into protein by living cells.
Codon is a set of 3 letters combination of nucleotide bases(A,G,C,T).
  Genetic code defines how codons specify which amino acid will be added next during protein synthesis.


Types of Codons :

There are two type of Codons

1) Sense codons : the codon that codes for amino acid.

2) Signal codons : Those codons that code for signal during protein synthesis are called signal codons.

Signal codons has two types

a). Initiation Codon :

   This codon also called start codon. AUG is the initiation codon. It is the first codon of mRNA transcript that is translated and codes for first amino acid in all proteins.
AUG codon - it code for methionine in eukaryotes, formyl methionine in prokaryotes and ubiqutinated lysine in protists, bacteria, mitochondria and plastids.
• The most common start codon is AUG. In few mRNA GUG and UUG also act as initiation codon as AUG codon is unavailable (arches & bacteria).
• In E.coli AUG and UUG is read as formyl-methionine if it is used as start codon, when used with in coding region it is read as methionine.
  Similarly GUG as formyl-methionine, if act as start codon. GUG act as valine if read in between coding region.
• In eukaryotes initiation codon is located between kozak sequence.
• In prokaryotes initiation codon is located after 3-10 nucleotides of shine-Dalgarno sequence.

b). Termination codons :

  This codon also called chain termination codon or stop codon or non sense codon.
Out of 64 codons 3 do not code for any amino acids and terminate translation. At stop codon ribosome pauses and fall off the mRNA
Termination codons are -
UAA - Orche codon
UAG - Amber codon
UGA - Opal codon

Anticodon -

•The base sequence of tRNA which pairs with codon of mRNA during translation is called anticodon
• Codons could be present in both DNA and RNA but anti codon is always present in tRNA.
• Codon written in 5' ➞ 3' direction while anticodon written in 3' ➞ 5' direction.
• Codons are arranged in sequence in nucleic acid strand while anticodons are present discretely in cells with amino acid attached or not.
• Codon defines which anticodons to come next with an amino acid to create the protein strand.
• Anticodons helps in bringing a particular amino acid at its proper position during translation.
• Anticodons of some tRNA molecules have to pair with more than one codon.


Genetic code table :


Ala - Alanine 
Arg - Arginine 
Asn - Asparagine 
Asp - Aspartic acid  
Cys - Cysteine 
Gln -Glutamine  
Glu - Glutamic acid
Gly - Glycine 
His - Histidine 
Ile - Isoleucine 
Leu - Leucine 
Lys - Lysine 
Met - Methionine 
Phe - Phenylalanine 
Pro - Proline 
Ser - Serine 
Thr - Threonine 
Trp - Typtophan 
Tyr - Tyrosine 
Val - Valine


Characteristics & Nature of genetic code :

1) Triplet code -

Genetic code is triplet code consisting of 64 codons. Codon is a set of 3-latter combination of nucleotide.

2) comma less or continuous translation -

The gene is transcribed and translated from a fixed starting point to fixed stop point. Punctuations are not present between the codons.

3) Non overlapping -

Genetic code is non-overlapping, any single ribonucleoprotein at specific location in mRNA is part of only one triplet codon.
Codon is read as a continuous sequence of bases, taken 3 at a time called as reading frame.

4) Universal

  means that the same code is used for all life forms. This is not strictly true, there is few context dependent codons also. in this context the genetic code is nearly universal
E.g. UGA is a stop codon while it codes tryptophan in mycoplasma.
CUG codes for leucine while it codes threonine in mytochondria in yeasts.
UAA,UAG is a stop codon while it codes glycine in paramecium.

5) Unambiguous -

The genetic code is non-ambiguous. Thus one codon can not specify more than one amino acid.

6) Degenerate

  A given amino acid can be specified by more than one triplet codon. This is the code for 18 of the different codons for a given amino acid are said to be synonymous codons.
e.g. UUU and UUC are synonyms for phenylalanine. UCU, UCC, UCA, UCG, AGU, and AGC are synonyms for serine.

7) Polarity of genetic code -

  The code has definate direction for reading of message which is referred to as polarity
  5' ➞ 3' direction
Reading of message from left to right & right to left will specify for different amino acids.
e.g. UUG ➞ leucine
       GUU ➞ valine.

Codon Bias :

Codon bias refers to the fact that not all codons are used equally in the genes of a particular organism.
Codon bias is the probability that a given codon will used to code for an amino acid over a different code which codes for the same amino acid.
e.g. Out of the four valine codons(GTG, GTA, GUG, GUA), human genes use GTG four times more frequently than GTA.