Genetic Code

Introduction

DNA or RNA carries all the genetic information. DNA or RNA is formed by the four alphabets A, G, T or U and C. These alphabets for DNA molecules are the nitrogenous bases such as Adenine(A), Guanine(G), Thymine(T) and Cytosine(C). Similarly, the nitrogen bases for RNA are Adenine(A), Guanine(G), Cytosine(C) and Uracil(U). These four bases encode hereditary characters and messages and are hence called code letters or codons. The line arrangement of these codons determines the sequence of amino acids.

The linear sequence of nitrogen bases in a gene is a code for the sequence of amino acids in a protein. This relationship is called genetic code. In other words, the genetic code is defined as the dictionary that helps in translating the language of nucleic acid into the language of protein. George Gamow gave the term genetic code. The studies on genetic code are also made by Marshall Nirenberg, Hargovind Khorana, Francis Crick and many others.

The code in a DNA is carried out by 4 nitrogen bases- A, T, G and C. The genetic information coded in these bases are passed into mRNA by U, A, C and G and then to protein. The genetic code is triplet which was suggested by George Gamow in 1954 A.D. Triplet code or codon or code word is the smallest unit of three nitrogen bases that specifies specific amino acid in a protein or polypeptide chain. According to triplet code, three bases specify one amino acid and for 4 bases in an mRNA, there are 64 (4X4X4) possible triplets or codons. There are however only 20 different amino acids which are needed to be coded for.

Genetic code may be of following types-

1)Singlet code (single base) :A singlet code is a single set of the bases. It would give only four codes. It is shown in table below:

2)Doublet code (two bases) :A doublet code is a set of two bases. It would give 16(4X4) codon combination as shown in below table.

3)Triplet code (three bases):A triplet code is a set of three bases. It would give 64 (4X4X4) amino acids. The minimum requirement to code an amino acid is triplet code. It is shown below:

AMINO ACIDS ABBREVIATION:

The full forms of the amino acids above are given below:

• STOPindicates termination codon.

Characteristics of Genetic code:

1) The genetic code is triplet -A genetic code consists of three nitrogen bases and the mRNA has 4 nitrogen bases. In this regard, there are 64 possible codons which can be formed as triplets for coding 20 different amino acids,

2) The genetic code is commaless -There is no comma (,) between the codons. Example: UUU, UAC, UUA e.t.c.

3) The genetic code is universal -It means that the codons for particular DNA or mRNA are the same to specify the amino acid in all the organisms including the virus. Example AUG codes methionine.

4) The genetic code is non-overlapping -It means that the same nitrogen base is not used for two different codons. Example: A nucleotide sequence CAT GAT which represents only two codons CAT and GAT in the case of not overlapping.

CAT GAT→ CAT and GAT (non-overlapping)

CAT GAT→ CAT ATG TGA GAT (overlapping)

5) The genetic code is degenerate -It means that more than one genetic code may be used for the same amino acid. These multiple systems of coding are called degenerate code. For example, Two codons UUU and UUC code the single amino acid phenylalanine.

6) The genetic code is non-ambiguous -One codon specifies only one amino acid but not more than one amino acid. This is called non-ambiguous genetic code. Only GGA is ambiguous code as it codes for glycine and glutamic acid.

7) The genetic code is co-linear - The linear sequence of nitrogen bases in DNA specifies the codon in mRNA. This codes the linear sequence of amino acid in the polypeptide chain.

8) Initiation codon and termination codon -The codon AUG present at the first position of mRNA that indicates the initiation codon and specifies the formation of amino acid methionine. This signals the start of the polypeptide chain formation.

Three codons UAA, UAG and UGA doesn't code for any amino acid and are called termination codons or nonsense codons. These indicate the termination of protein synthesis during translation. ,

Wobbling effect :

If the 3^rdbase of a codon is changed, then there is no effect in reading. For example, the codons of Serine and Arginine.

• Serine - UCU, UCC, UCA, UCG { the third bases (U, C, A and G) do not show any effect in reading. }
• Arginine - CGU, CGC, CGA, CGG { the third bases (U, C, A and G) do not show any effect in reading. }

Gene pool:

Study of genes in a population is known as population genetics. The sum of the total of all alleles of all genes in an interbreeding population at a given time is referred to as populations' gene pool. A gene pool is the total variety of genes and alleles present in sexually reproducing organisms.

Gene or allele frequency:

The number of organisms in a population carrying a particular gene or allele for the character is called gene or allele frequency.

Role of mutation in gene pool :

1) Mutation influences gene frequency in a population.

2) It changes the frequency of genes from generation to generation.

3) It provides new genetic information.

4) It increases genetic variation in the gene pool and provides raw materials for evolution.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram. Principle of biology. 2nd edition. Kathmandu: Asmita, 2068,2069.

Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.