Genetic Code: We all are acquainted with the fact that DNA contains all the information required for the formation of a protein molecule or a polypeptide chain, but the proteins are not made directly from DNA. They are made from the mRNA molecule that is synthesised from the DNA. Decoding of information in mRNA is a key step in the expression of genes to build a protein.

The tRNAs do this decoding of mRNAs by reading their nucleotides in groups of three called genetic codons that code for a particular amino acid. This relationship between the codons and amino acids is called the genetic code. The genetic code is almost identical in all the species, with the same codons specifying the same amino acid. Read this article to know more about the genetic code and its characteristics.

What is a Genetic Code?

The genetic code is defined as the sequence of three nucleotides in RNA that determines the sequence of amino acids for the synthesis of proteins. In other words, the genetic code is defined as the set of rules by which information encoded in the DNA or RNA sequences is translated into proteins by the living cells. Marshall Nirenberg is known as the Father of the Genetic Code.

Genetic Code Table

The full set of relationship between the amino acids and the codons is known as genetic code, which is often summarised in a table as given below:

 Fig: Genetic Code Table

Many scientists like Marshall Nirenberg, Heinrich Matthaei, Severo Ochoa, Har Gobind Khorana, Francis Crick and their associates tried to identify various codes for different amino acids. It was clear that a single base cannot code for a single amino acid because then only four amino acids would be coded. The possible combinations would be A, U, G and C only. If a combination of two bases could code for each amino acid, then only 16 amino acids could be coded.

So, the next option was the combination of three bases that code for one amino acid, by which 64 combinations were possible that were much more than the required 20 amino acids.

Properties of Genetic Code

The properties of genetic code are as follows:

1. Codons are triplet: Each genetic code is a triplet formed of three nucleotide bases that code for a particular amino acid. There are 64 codons that code for 20 amino acids which means that there exists more than one code for a single amino acid.

2. Codons are non-overlapping: The genetic codes are non-overlapping, i.e., the adjacent codons do not overlap. It means that the same letter is not used for two different codons, or a single base cannot take part in the formation of more than one codon while reading the sequence of bases from the starting of mRNA in blocks of three. For example- if the bases from the starting are AUG/CCA/AUC, then the sequence of codons will be read as AUG/CCA/AUC only and not AUG/GCC/CAA/AUC.

3. Codons are universal: The genetic code is universal means the same codons code for the same amino acids in all forms of life that exists today.

4. Codons are commaless: The genetic code is commaless, which means codons are read continuously. The deletion of a single base in a comma less code alters the entire sequence of amino acids.

5. Codons are non-ambiguous: The genetic code is non-ambiguous and specific because each code codes for only one amino acid.

6. Codons are degenerate: Several codons code for the same amino acid except for tryptophan and methionine, which are coded by one codon each. These multiple systems of coding for the same amino acid are known as degenerate coding systems. It provides protection to the organisms against many harmful mutations.

7. Codons have polarity: The genetic code has a definite direction for the reading of information which is called polarity. If the codon is read in the opposite direction, then it would specify another amino acid because of the alteration in the base sequences in the code. The message in the mRNA is read in the 5’ to 3’ direction, and thus, the polarity of the genetic code is from 5’ end to 3’ end.

Types of Genetic Code

The genetic codons are mainly divided into two types as follows:

1. Sense Codons: These are defined as those codons that code for amino acids. There are 61 sense codons in the genetic code that code for 20 amino acids.

2. Signal Codons: These are defined as those codons that code for the amino acids that act as signals during protein synthesis. There are four codons that act as signals, namely, AUG, UAA, UAG and UGA.

Signal codons are again divided into two types:

i) Start codons: These are those codons that start the translation process. They are also known as initiation codons as they initiate the protein synthesis process. For example-AUG which codes for the amino acid methionine, act as the initiating or the start codon in eukaryotes.

ii) Stop codons: These are those codons that provide a signal for the termination of polypeptide chain synthesis. They are also known as termination codons as they stop the formation of polypeptide chains. For example- UAA (ochre), UAG (amber) and UGA (opal) are known as termination codons or stop codons. They do not code for any amino acids, so they are also known as non-sense codons.

Exceptions to Genetic Code

The universality of genetic code conveys that similar codons are assigned to identical amino acids, along with similar start and stop signals in the genes of most microorganisms and plants. Out of the few exceptions that were discovered, most of them include assigning one or two stop codons to an amino acid.

Another exception includes the two codons GUG and AUG. Both of their codes for the amino acid methionine acts as a starting codon, although GUG is meant for valine. This exception deviates from one of the properties of the genetic code, i.e., genetic codes are non-ambiguous. Thus, it is confirmed that few codes often differ from the property of non-ambiguous and universality.

  Fig: Exceptions to the Genetic Code

Summary

Thus, genetic code is defined as the sequence of three nucleotides in RNA that determines the sequence of amino acids for the synthesis of proteins. In other words, the genetic code is defined as the set of rules by which information encoded in the DNA or RNA sequences is translated into proteins by the living cells.

Marshall Nirenberg is known as the Father of the Genetic Code. There are, in total, 64 triplets called codons. Except for three codons, the remaining 61 codons code for one of the 20 amino acids which are used in the synthesis of proteins.  Several codons code for the same amino acid except for tryptophan and methionine, which are coded by one codon each. The genetic code is mainly divided into two types- sense codons and signal codons.

Frequently Asked Questions (FAQs) on Genetic Code

Q.1. What is a genetic code?
Ans: A genetic code is defined as the sequence of three nucleotides in RNA that determines the sequence of amino acids for the synthesis of proteins.

Q.2. What are the types of genetic code?
Ans: There are two main types of genetic codons:
i) Sense codons
ii) Signal codons

Q.3. Mention the characteristics of the genetic code?
Ans: The characteristics of the genetic code are as follows:
i) Codons are triplets.
ii) Codons are non-ambiguous.
iii) Codons have polarity.
iv) Codons are non-overlapping.
v) Codons are universal.
vi) Codons are degenerate.
vii) Codons are commaless.

Q.4. Who is known as the Father of genetic codons?
Ans: Marshall Nirenberg is known as the Father of genetic code.

Q.5. What are the important rules of genetic code?
Ans: The important rules of genetic code are as follows:
1. The sequence of bases in the genetic codon must follow the direction of translation.
2. The genetic codon is non-overlapping.
3. The genetic codon is read in a fixed reading frame.

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