Genetic Code – Definition, Characteristics, Codon Table

Genetic code, the sequence of nucleotides in DNA and RNA that determines the amino acid sequence of proteins.

In this article, we would be knowing all genetic code, its definition, characteristics, codon table, and much more.

Let dive into it.

Introduction to Genetic Code

DNA is the genetic material. It carries genetic information from one cell to the other and from one generation to the next. It also controls all the vital activities by the synthesis and regulation of different kinds of proteins.

Many proteins are found in the living body but at least 20 amino acids take part in their formation.

The information for the sequence of amino acids in a protein is contained in the sequence of bases of the m-RNA which in turn is governed by the nucleotide sequence of the DNA.

However, there are four nucleotide or nitrogen bases in the DNA namely:

  1. Adenine (A)
  2. Thymine (T),
  3. Cytosine (C) and
  4. Guanine (G).

Therefore, a four-letter language of the DNA is transcribed into a twenty-letter language (twenty amino acids of proteins). The language or code of DNA is written by four letters A, T, C, and G.

It is now established that each amino acid is coded for a three letter word of the nucleic acid.

Thus, a sequence of three bases determines to which amino acid is to be inserted into a polypeptide chain during its synthesis on the m-RNA ribosome complex.

Definition of Genetic code: It is that sequence of nitrogen bases, located in DNA molecules, in which information is coded for the synthesis of protein molecules.

The group of nucleotides that contain the code or information for one amino acid is called a codon.

If three nitrogen bases are present in the sequence AAA at a place of DNA, then the sequence of complementary bases in that place in mRNA will be UUU. This sequence will code for the protein, phenylalanine.

Thus, AAA sequence in DNA or UUU sequence in m-RNA represent a codon.

Genetic Code Table
Genetic Code Table

The four nitrogen bases can form 64 possible codons (Given in the table below) but there are only 20 essential amino acids.

It is therefore evident that each amino acid may correspond to more than one codon.

The bases of DNA differ from those RNA in the sense that they have thymine in place of uracil.

The DNA and RNA code words are complementary to each other.

Triplet Genetic Code Table for 20 Amino Acids

Amino acidAbbreviationDNA Codem-RNA Transcription
2. ArginineargGCA, GCT, GCC,
3. AsparagineasnTTA, TTGAAU, AAC
4. Aspartic acidaspCTA, CTGGAU,GAC
5. CysteinecysACA, ACGUGU, UGC
6. GlutamineglnGTT, GTCCAA, CAG
7. Glutamic acidgluCTT, CTCGAA, GAG
9. HistidinehisGTA, GTGCAU, CAC
10. IsoleucineiluTAA, TAGAUU, AUC
11. LeucineleuAAT, AAC, GAA,
12. LysinelysTTT, TTCAAA, AAG
13. MethioninemetTACAUG
14. PhenylalaninepheAAA, AAGUUU, UUC
15. ProlineproGGA, GGG, GGT, GGCCCU, CCC, CCA, CCG
16. SerineserAGA, AGG, AGT,
17. ThreoninethrTGA, TGG, TGT, TGCACU, ACC, ACA, ACG
18. TryptophantryACCUGG
19. TyrosinetyrATA, ATGUAU, UAC

As a result of the brilliant research work of Nirenberg, Crick, Khurana, and their associates it has been possible to identify the various code words for different types of amino acids.

In the above Table, twenty amino acids are represented which can be obtained by the various combinations of three bases.

A glance at the above table indicates that there are two alternative code words for the amino acid phenylalanine-UUU and UUC of m-RNA and AAA and AAG of DNA.

Leucine and arginine have six each, whereas tryptophan and methionine have only one code word each.

Other amino acids have either two three or four codons each. Out of sixty-four, sixty-one codons stand for amino acids, and the rest three (UAA, UAG, and UGA) serve as full stops. They determine the endpoint of a polypeptide chain.

The codon AUG codes for methionine as well as for starting signal.

A DNA segment, which is bounded by a start and stop signal contains enough genetic information for a complete RNA or polypeptide molecule and is called a cistron.

Thus, a cistron codes for a polypeptide chain or m-RNA molecule.

Sometimes more than one adjacent cistrons code for polypeptides which have related functions or which associate together to form a protein. Such a group of functionally related cistrons has been called a gene.

A triplet code of m-RNA is recognized by a definite t-RNA species. Each species of t-RNA has two recognition sites.

One of them recognizes the correct amino acid and the other recognizes the mRNA codon. This second site of a tRNA molecule is called an anticodon.

The amino acids, therefore, are ordered according to the codon sequences of m-RNA and codon-anticodon recognition.

The genetic code is the same in all organisms which suggests that it originated quite early during organic evolution.

General Features of Genetic Code

The genetic dictionary of m-RNA codons shows the following features of triplet codon:

1. Degeneracy

Most of the amino acids are coded by more than one codon. These codons are synonymous.

For example, leucine (an amino acid) is represented by six synonymous codons i.e., UUA, CUU, UUG, CUC, CUA, and CUG.

Therefore, in different species, different codons may be used for a given amino acid.

2. Non-overlapping

The code is non-overlapping. This means that no single nitrogenous base can take part in the formation of more than one codon. For example, A. T.G; C. T. A; G. A. T, etc.

3. Commaless

The code is commaless. It means that no codon is reserved for punctuation.

4. Initiating codon

AUG codon is called chain initiating codon.

5. Non-sense codons

The period at the end of the sentence has to be a codon that codes for no amino acid but brings to an end the elaboration of the polypeptide chain.

It is evident from the recent research that nonsense codons UAA, UGA, and UAG do not specify any aminoacids but serve as alternative chain-terminating codons.

6. Universality

Genetic code appears as universal in its application i.e., same code applicable for all species.

7. Ambiguity:

The code is ambiguous because same codon may specify more than one amino acid.

Keywords on Genetic Code

  • Amino acid: An organic compound containing both amino group (NH2– group) and carboxyl (acidic) group (COOH group). Amino acids are the building blocks of protein.
  • Anticodon: The specific nucleotide triplet of transfer RNA which is complementary to the triplet codon of messenger RNA with which the former (t-RNA) remains associated.
  • Cistron: A length of DNA producing the RNA that in turn produces a specific polypeptide chain is called a cistron.
  • Codon: A specific nucleotide triplet in the genetic material corresponding to RNA which codes for a single amino acid.
  • Genetic Code: It represents the sequence of nitrogenous bases in which information is coded for the synthesis of protein molecules. It is located in the DNA molecules.
  • Peptide bond: A bond linking the successive peptides or amino acids in a protein molecule.

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