In this article we will discuss about:- 1. Subject-Matter of Genetic Code 2. Degeneracy of Genetic Code 3. Universality of the Genetic Code.
Subject-Matter of Genetic Code:
The information stored in DNA is in the form of code. DNA contains 4 bases, A, T, G and C, whereas, proteins are made of 20 different amino acids. Therefore, the genetic code must contain more than one bases so as to specify the 20 different kinds of amino acids.
By the work of a number of scientists, the code and the relationship of amino acids with different codes was determined. It has been established that the code consists of 3 letters (3 bases), i.e., the code is
a “triplet” code. The number of triplet codes constituted from the 4 bases is 43 = 64.
The sequence of bases in DNA specifying an amino acid is called “code”, while its completely base sequence in mRNA is called “codon”. In tRNA, the sequence of bases specifying an amino acid is called “anticodon”. Thus, if one reads in 5′-»3’direction, the code for methionine is 5′ CAT3′-, the codon is 5′ AUG3′ and the anticodon is 5′ CAU3′.
Since the mRNA is directly involved in protein synthesis, “codons” are in common usage instead of “code” (DNA) to specify the amino acids. All the 64 codons with their meanings constitute the “coding dictionary” (Table 4.3). Of them, 61 codons specify amino acids, so they are called “sense” “codons” or “sense words”.
The rest 3 condons, UAA, UAG and UGA do not specify some amino acid and they are called “nonsense codons” or “nonsense words” (in terms of amino acid). But they are very important and necessary codons because they are used to stop or terminate the growing polypeptide chain.
Therefore, these codons are called “stop codons” or “chain termination codons.” The amino acids methionine and trytophane are specified by single codons AUG and UGG, respectively.
The codon AUG is an ambiguous codon because it specifies N-formylmethionine and methionine, both amino acids. Others are “degenerate” or synonymous” codons, i.e., the same amino acid is specified by more than one codons, (Table 4.3).
Degeneracy of Genetic Code:
There are two methods by which the same amino acid is specified by two or more codons:
1. The tRNAs accepting the same amino acid are different for different synonymous codons. Such tRNAs are called “isoacceptortRNAs” and they differ in anticodons. For example, one of the tRNAs carrying leucine is tRNA1leu with anticodon 3′ GAC5′, while the other is tRNA2leu with anticodon 3′ GAG5′.
2. A single type of tRNA pairs with two or more synonymous codons. For example, tRNA. accepting the amino acid alanine in yeast (tRNAaIa) bears the anticodon 3′ CGI5′ that can pair with the codons 5′ GCU3′, 5′ GCC3 and 5′ GCA3′ on mRNA Crick in 1966 proposed the “wobble hypothesis” to explain the pairing of a single type anticodon with synonymous codons.
According to the Wobble hypothesis, the base position at the 5′-end of anticodon is the “wobble position”. Two bases of anticodon from 3′-end are complementary to the two bases of the codon (in mRNA). The base at the wobble position can pair with different bases. For example, a single type of tRNAgly with the anticodon 3′ CCI5′ can pair with the codons 5’GGU3′, 5’GGC3′ and 5’GGA3′ specifying the amino acid glycine.
Thus inosine (I) at the wobble position can pair, with U, C and A in the codon. Similarly, U can pair with A and G, while G at the wobble position can pair with C and/U.
Universality of the Genetic Code:
The meaning of the universality of genetic code is that the same genetic code is utilized by all the organisms. For example, the lac+ gene producing the enzyme P-galactosidase in E. coli functions to produce the same enzyme in human fibroblast tissue culture cells deficient in this enzyme.
When the hemoglobin mRNA molecules are injected into the Xenopus eggs, protein synthesis occurs and the α and β polypeptide chains are produced. However, variation in the genetic code has been observed in mitochondria where some of the condons are differently translated.
UGA (termination codon in universal code) specifies tryptophane, while AUA (for isoleucine in universal code) specifies methionine in mitochondria. To some extent, the mitochondria of different organisms also differ in genetic code. For example, CUA is a codon for threonine in yeast mitochondria, while it specifies leucine in Drosophila and mammalian mitochondria.
So, it may be said that the genetic code is not entirely universal.