In this article we will discuss about: 1. Meaning of Genetic Code 2. Patterns to Genetic Code 3. Properties.
Meaning of Genetic Code:
It has became obvious that nucleic acids are the genetic material. The nucleic acids being polynucleotide, function to store genetic information’s and to replicate. The genetic information flow from polynucleotide to polypeptide.
It is surprising to note that at the origin of life any polynucleotide that helped to guide the synthesis of a useful polypeptide in its environment would have had a great advantage in the evolutionary struggle for survival.
A long chain of a DN A molecule consists of three components, nitrogen bases, deoxyribose sugar and phosphoric acid. Except nitrogen base, the chemical configuration does not change. The nitrogen bases are of four types, adenine, guanine, thymine, cytosine. Therefore, it is likely that the sequence of these bases on a segment of DNA molecule changes.
Obviously, the above three components (nucleotides) are involved in restoring the genetic information. Thus, there are four alphabets (A, G, T, C) of DNA. We know that there are 20 different amino acids that constitute a protein as the nitrogen bases constitute nucleotides.
Hence, there are 20 alphabets of the language of a protein. From DNA to protein the information pass through an mRNA. One mRNA carries the genetic information of one protein. The synthesis of specific protein under the guidance of mRNA required the evolution of a code by which the polynucleotide sequence specifies the amino acid sequence that makes up the protein.
This code is called “genetic code” which is spelled out in a dictionary of words. This code is virtually the same in all living organisms.
Patterns to Genetic Code:
After going through Table 7.4 a remarkable pattern of genetic code emerged.
Following are some of the important features of genetic code:
(i) Sixty one codons correspond to amino acids.
(ii) Four codons are the signals. There are three stop codons (UAA, UAG, and UGA) and one start codon (AUG). Rarely, GUG also acts as start codon.
(iii) Amino acids with similar structural property consist of related codons; therefore, the aspartic acid codons (GAU and GAC) are related to glutamic acid codons (GAA and GAG). Similarly, the codons of phenylalanine (UUU, UUC), tyrosine (UAU, UAC) and tryptophan (UGG) start with uracil. This characteristic of codons facilitates to minimize the effect of mistakes arising during translation or mutagenic base substitution.
(iv) For many synonym codons specifying the same amino acid the first two bases of the triplet are constant, while the third base varies. For example, all codons starting with CC (e.g. CCU, CCC, and CCG) specify proline, and all codons starting with AC (ACU, ACC, ACA, and ACG) specify threonine. The flexibility in third codon may be to minimize errors.
Properties of the Genetic Code:
Through the experiments it has been proved that the mRNA codons of the genetic code have the following properties:
(i) The Code is a Triplet:
Singlet and doublets are not adequate to code for 20 amino acid; therefore, the triplet codes that consist of 43 = 64 codons may code for 20 essential amino acids. The triple code of mRNA has been accepted.
(ii) The Code is Degenerate:
There are 64 codons in the genetic code for 20 amino acids of which 4 codons are the signals. Therefore, 60 codons are to code for amino acids. It means that more than one codons may be coding for individual amino acid.
The number of codons coding for different amino acids are as below:
The codons that code for more than one amino acid are called degenerate. For example, a codons starting with CC specify proline (CCU, CCC, CCA, and CCG) and all codons starting with AC specify threonine (ACU, ACC, ACA, and ACG). Unequal distribution of amino acids in protein may be due to this variability in number of codons for amino acids.
(iii) The Code is Non-Overlapping:
The genetic code is non-overlapping which means that the same letter does not take part in the formation of more than one codon.
The overlapping and non-overlapping codes are given below:
In addition, it has been shown that the bacteriophage Ø×174 consists of overlapping genes. Two genes besides separately coding for its own protein, also take part in coding for the third protein with different amino acid sequences. This is done by a frameshift mechanism (i.e. overlapping code). The entire nucleotide sequence of Ø×174 has been given by Sanger (1977). For detail see ‘overlapping genes’.
(iv) The Code is Non-Ambiguous:
The non-ambiguous means that a particular codon will always code for the same amino acid. It may also be that the same amino acid may be coded by two different codons (degenerate). However, when one codon codes for two amino acids, it is called ambiguous. For example, UUU codon codes for phenylalanine, but in the presence of streptomycin it may code for isoleucine, leucine or serine.
(v) The Code is Commaless:
The genetic code is without comma i.e. no punctuations are required between the two codons. There are no de-markating signals between the two codons. This results in continuous coding of amino acid without interruptions. No codons are left un-coded.
The commaless codons may be written as below:
UUUCUCGUAUCC – Bases
Phe-Leu-Val-Ser – amino acids
The genetic code, however, with comma may be represented as below:
Due to deletion of a base (e.g. C), a drastic change in coding of amino acids occurs.
UUUUCGUAUCC – Bases
Phe-Ser-Tyr…. – amino acids
However, if the introns are present, the coding process is interrupted. See introns i.e. split genes in preceeding section.
(vi) The Code has Polarity:
The code has polarity i.e. it is read between the fixed start and stop codons. The start codon is also known as initiation codon, and stop codon as termination codon. The message of mRNA is read in 5’→ 3’ direction. The polypeptide chain is synthesized from the amino (-NH2) end to the carboxyl (-COOH) end i.e N→ C.
However, if the code is read in opposite directions the message will be reversed due to change in base sequence. Therefore, two different proteins will be synthesized. For example, if 5′- AUCGUCUCGUUG ACA-3′ is read from left to write it will specify: Ile-Val-Ser-Leu-Thr-, and if it is read from right to left it will specify: Thr-Val-Ala-Leu-Leu.
The codon, AUG is the initiation codon. Rarely GUG also acts as initiation codon in bacterial protein synthesis, when AUG is lost by deletion or it is non-functional. In the phage MS2, GUG acts as initiation codon for A protein. However, generally GUG codes for the amino acid valine and AUG codes for methionine.
Moreover, three of 64 codons are called as non-sense codons because they do not specify any tRNA. These codons are amber (UAG), Ochre (UAA) and Opal or amber (UGA). These also bring about termination of polypeptide chain; therefore, they are also called termination codons.
For the first time the codon UAG was investigated by a graduate student who belonged to the Bernstein family. Bernstein means ‘amber’ in German. He helped in discovery of a class of mutation. The other two termination codons were also named after the colours just to give the uniformity.
(vii) The Code is Universal:
Though the genetic code has been worked out by using in vitro systems of microorganisms, yet there is no doubt of being its universal for all groups of microorganisms. In 1967, Nirenberg and his associates demonstrated the universality of the code.
They found that aminoacyl tRNAs of E. coll (bacterium), Xenopus laevis (amphibian) and guinea pig (mammal) use almost the same codon. It has also been shown that when purified mRNAs from rabbit reticulocytes specifying the synthesis of haemoglobin are injected into frog oocytes, these synthesise rabbit haemoglobin by using the translation machinery of frog.