Genetic code

Genetic code

DNA or RNA carriers all the genetics information, DNA or RNA is formed by the four alphabets A, G, C, and T or U, and. These alphabets for DNA molecules are the nitrogenous bases of RNA are adenine (A), guanine (G), cytosine (C) and thymine (T). Similarly, nitrogenous bases of RNA are adenine (A), guanine (G), cytosine (C) and uracil (U). These four alphabets encode hereditary messages, hence called code letters or codons. The linear arrangement of nitrogenous bases in DNA determines the sequence of amino acids.

The three nucleotide (triplet) base sequence in mRNA that acts as code words for amino acids in protein constitutes the genetic code or codon. The genetic code may be regarded as a dictionary of nucleotide base (A,G,C and U) that determines the sequences of amino acids in proteins.

Therefore, the genetic code is defined as “the dictionary that helps in translating the language of the nucleic acid into the language of protein”. The group of nucleotides that specifies or codes for one amino acid known as codon or code word. 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 codons are composed of the four nucleotides bases; namely the adenine (A), guanine (G). cytosine (C) and uracil (U). These four bases from 64 different combinations (4=4³=64) of three base codons. Out of total 64n codons, sixty-one codons code for the 20 amino acids found in proteins. The remaining three codons UAA, UAG, and UGA do not code for amino acids. They act as stop signals in protein synthesis. These three codons are collectively known as termination codons or nonsense codons. However, the codons AUG and sometimes, GUG are the known as initiating codons.

Genetic code may be off-

1. Singlet code (single base): A singlet code is a single base. It would give only four codes.
2. Doublet code (two bases): A doublet code is of two bases. It would give 16(4) codon combinations.
3. Triplet code (three bases): A triplet code is of three bases. It would give 64(4) codon combinations.
4. The singlet and doublet code are not adequate to code for 20 amino acids. The minimum requirement to code an amino acid is the triplet.

Characteristics of genetic code

Following characteristics of the genetic code have been proven by definite experimental evidence:

1. The code is triplet
2. The code is degenerate
3. The code is non-overlapping
4. The code is commaless
5. The code is non-ambiguous
6. The code is universal
7. The code is collinear

The code is triplet: A codon is a triplet in nature. It means that for any amino acid, it is necessary to have triplet codon. A triplet codon codes for a particular amino acid. There are 64(4) triplets are possible which would be enough to code 20 amino acids.

The code is degenerate: Most of the amino acids have more than one triplet codon. The codon is degenerate since there are 61 codons available to code for only 20 amino acids. For instance, glycine has four triplet codons. Similarly, arginine has six triplet codons.

Note: All the amino acids except methionine and tryptophan are specified by more than one triplet codons. The occurrence of more than one code per amino acid is called degeneracy.

The code is non-overlapping: A base is a part of only one codon. The same base letter is not used for two different codons. The genetic code is read from a fixed point as a continuous base sequence. Addition or deletion of one or two bases will radically change the message sequence in mRNA.

For instance, a nucleotide sequence C A T G A T is read as C A T and G A T, it represents only two codons i.e., C A T and G A T when it is not overlapped. But when it is overlapped four codons like CAT, ATG, TGA, and GAT is achieved which is not possible in genetic code.

The code is commaless: The genetic code is continuous and does not possess any pause or gap after the triplets. In other words, we can say that one amino acid is coded, the second amino acid will be automatically coded by next three letters (bases) and thus no base is wasted. If a nucleotide is deleted or added, the whole genetic code will read differently.

The code is non-ambiguous: It means that there is no ambiguity about particular codon. A particular codon will always code for the same amino acid, hence the genetic code is highly specific or unambiguous, eg. UGG is the codon for tryptophan. There is one exception for GGA which codes for glycine as well as glutamate which becomes an ambiguous code.

The code is universal: The genetic code is applicable universally, ie. A codon specifies the same amino acid from a virus to a plant or human being. If a codon codes for particular amino acids, that will be the same in all organisms like prokaryotes and eukaryotes including the virus.

The code is collinear: Genetic code works on the principle of collinearity i.e it explains the specific relationship between DNA, RNA, and polypeptide chain. The linear order of nucleotides in DNA determines the linear order of codons in mRNA which in turn determines the linear order of amino acids in a polypeptide chain.

Wobbling effect:

If the 3^rd base of a codon is charged, then there is no effect in reading. For example, the codons of serine and arginine

• Serine – UCU, UCC, UCA, UCG,
• Arginine – CGU, CGC, CGA, CGG

The 3^rd base U, C, A and G that do not show any effects in reading.

Gene pool

Study of genes in a population is known as population genetics. In a given population there are many possible alleles for all genes. The sum total of 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 a sexually reproducing population. In a given population the composition of gene pool may be constantly changing from generation to generation.

A population whose gene pool shows consistent change from generation to generation indicates an evolutionary change. A static gene pool represents a situation where genetic variation between members of the species is inadequate to bring about evolutionary change. The change in the gene pool may cause evolutionary change in population.

Gene or allele frequency

In a population, there are many genes. For a particular phenotype or character, a gene is a must. The number of organisms in a population carrying a particular gene or allele determines the gene or allele frequency. For example, assume that in a given population the percentage of albinism is 1%, then the gene frequency of albinism will be 1%.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.