Genetic Code: Features, Codons, Degeneracy & Universal Nature

Botany Sir Himansu

Genetic Code: Features, Codons, Degeneracy and Universal Nature

The genetic code is the set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein. The genetic code is commonly represented using mRNA codons.

What Is a Codon?

A codon is a sequence of three nucleotides in mRNA that specifies an amino acid or a termination signal during protein synthesis.

There are 64 possible codons in the genetic code:

  • 61 codons specify amino acids.
  • 3 codons are stop/termination codons.

Discovery of the Genetic Code

  • The term genetic code was introduced by George Gamow.
  • Important early experimental work on deciphering the genetic code was carried out by scientists including Marshall Nirenberg and J. Heinrich Matthaei.
  • The first codon to be deciphered was UUU, which codes for phenylalanine.

Sense and Nonsense Codons

Sense Codon

A sense codon specifies an amino acid.

Nonsense or Stop Codon

A nonsense codon, also called a termination or stop codon, does not specify an amino acid. Instead, it signals termination of translation.

The three stop codons are:

  • UAA — Ochre
  • UAG — Amber
  • UGA — Opal

Initiation Codons

AUG is the usual initiation codon and specifies methionine. In bacteria, GUG can also function as an alternative initiation codon, although at internal positions GUG normally specifies valine.

When AUG functions as an initiation codon, it establishes the reading frame for translation.

Triplet Nature of Genetic Code

The genetic code is triplet in nature, meaning that three nucleotides form one codon.

There are four different RNA bases: A, U, G and C.

If one base formed a codon:

41 = 4 codons

If two bases formed a codon:

42 = 16 codons

If three bases formed a codon:

43 = 64 codons

Since 64 possible codons are sufficient to specify the 20 amino acids used in protein synthesis, the genetic code is triplet.

Important Features of the Genetic Code

1. Triplet

Each codon consists of three nucleotides.

2. Degenerate

The genetic code is degenerate because more than one codon can specify the same amino acid.

For example, several different codons specify the amino acid leucine.

Non-Degenerate Exceptions

Two amino acids are specified by only one codon each:

  • UGG → Tryptophan
  • AUG → Methionine

3. Non-Ambiguous

The genetic code is generally non-ambiguous: a particular codon specifies only one amino acid.

Important: GUG normally specifies valine, but in some bacteria it can function as an alternative initiation codon and initiate translation with methionine.

4. Nearly Universal

The genetic code is nearly universal, meaning that the same codons generally specify the same amino acids in most organisms.

There are, however, some known exceptions, particularly in mitochondrial and certain other genetic systems.

The near-universality of the genetic code is one reason genetic engineering and recombinant DNA technology are possible across different organisms.

5. Non-Overlapping

The genetic code is non-overlapping. A nucleotide generally belongs to only one codon within a particular reading frame.

6. Commaless

The genetic code is commaless. There are no punctuation marks or gaps between successive codons during translation.

First Deciphered Codon

The first codon to be deciphered was:

UUU → Phenylalanine

This work was an important milestone in understanding how nucleotide sequences specify amino acids.

Polynucleotide Phosphorylase

Polynucleotide phosphorylase can catalyze the template-independent synthesis of RNA from nucleotide diphosphates. It was an important experimental tool in early studies of the genetic code.

Genetic Code: Quick Revision for NEET

  • Genetic code is read in the form of mRNA codons.
  • One codon contains 3 nucleotides.
  • Total possible codons = 64.
  • Sense codons = 61.
  • Stop codons = 3.
  • Stop codons are UAA, UAG and UGA.
  • AUG codes for methionine and usually acts as the initiation codon.
  • UGG is the only codon for tryptophan.
  • The genetic code is triplet, degenerate, non-overlapping and commaless.
  • The genetic code is nearly universal.
  • UUU was the first codon deciphered and codes for phenylalanine.

Frequently Asked Questions

How many codons are present in the genetic code?

There are 64 possible codons, of which 61 specify amino acids and 3 are stop codons.

What is the start codon?

AUG is the usual start codon and codes for methionine.

What are the three stop codons?

The three stop codons are UAA, UAG and UGA.

Why is the genetic code called degenerate?

It is called degenerate because more than one codon can specify the same amino acid.

Which codon codes for tryptophan?

UGG is the only codon that specifies tryptophan.

Which was the first codon deciphered?

UUU, coding for phenylalanine, was the first codon deciphered.