The process of synthesis of proteins involves one of the central dogma of molecular biology, according to which genetic information flows from nucleic acids to proteins. It was first proposed by Crick in the year 1958. The first step of this central dogma is the synthesis of RNA from DNA. This is known as transcription. The second step involves a change of code from nucleotide sequences to amino acid sequences and is called translation.
It can be illustrated as follows:
The DNA found in organisms has two main functions – replication and phenogenesis. Phenogenesis is a mechanism by which the phenotype of an organism is produced under the control of DNA in a given environment. The environment includes external factors such as temperature, quality and quantity of light, and internal factors such as hormones and enzymes.
The phenotype of an organism is the result of various embryological and biochemical activities of its cells from the zygotic to the adult stage. All these activities involve the action of a variety of structural and functional enzymes. The enzymes perform catalytic functions causing the splitting or union of various cellular molecules. Each reaction occurs in a stepwise manner involving the conversion of one substance to another.
The various steps involve the transformation of a precursor substance to its end product which ultimately is a structural or functional phenotypic trait. The various steps constitute a biosynthetic pathway. Each step of the pathway is catalysed by a specific enzyme, which in turn is produced by a specific gene.
DNA however, is not involved directly in the biosynthetic pathway. An intermediate molecule called mRNA is involved in the assemblage of amino acids to form enzymes. Thus, to produce a particular phenotypic trait, DNA transcribes mRNA which translates into either an enzymatic or structural protein. The groundwork for a functional relationship between genes and enzymes was laid in 1902 when Bateson reported a rare human defect known as alkaptonuria, which is inherited as a recessive trait.
Later in the year 1909, an English physician, Archibald Garrod known popularly as the father of biochemical genetics published his work in his book ‘Inborn errors of metabolism’ suggesting a relationship between genes and specific chemical reactions. But his work remained unnoticed until Beadle, Tatum and other geneticists worked on Neurospora to get a better understanding of gene action. They found that mutational change of genes can cause loss of specific enzymes. This concept was widely known as the ‘One gene one enzyme hypothesis’.
The concept of ‘one gene one enzyme’ one phenotypic effect relationship is exemplified by Neurospora crassa. The wild type or prototroph of Neurospora can live in a simple medium containing inorganic salts, a source of organic carbon and vitamin biotin. This simple medium is known as minimal medium.
Thus, the fungus has an innate capacity to synthesize all the other vitamins, amino acids and nitrogenous bases required for normal development. According to Beadle and Tatum, a gene controls a structural or functional trait through controlling the synthesis of a specific enzyme formed by the latter.
They treated the conidia of Neurospora to X rays or UV rays and obtained number of mutants called auxotrophs. These are nutritional mutants, which cannot grow on normal or minimal medium. Beadle and Tatum selected three mutants for their study.
They are as follows:
a. Ornithine Requiring Auxotrophs:
These are unable to synthesise citrulline and arginine. They were however able to synthesise arginine if supplemented with ornithine.
b. Citrulline Requiring Auxotrophs:
These synthesise ornithine, but could not make arginine.
c. Arginine Requiring Auxotrophs:
These could synthesise both citrulline and ornithine. These would grow normally if supplemented with arginine.
The existence of these three types of mutants indicates a sequence of reactions involved in the synthesis of arginine. The work of Beadle and Tatum have been summarised in Fig. 8.
Precursor ————-> Ornithine —————> Citrulline ————> Arginine
They reasoned that mutations caused defects in enzymes. One mutation produces only one defective enzyme. Beadle and Tatum were awarded the Nobel Prize for their contribution in 1958.
But ‘one gene one enzyme’ hypothesis has some limitations which are as follows:
a. All genes do not produce enzymes or their components. Some of them control other genes.
b. All proteins are not enzymes. Many proteins are made up of subunit called polypeptides, with each distinct polypeptide under the control of a gene. For example, the enzyme tryptophan synthetase of bacterium Escherichia coli consists of two separate polypeptides, A and B.
Polypeptide A is of α-type while polypeptide B is β-type. A change in any of the two genes causes inactivation of tryptophan synthetase through non-synthesis of A and B-type. Inactivation of enzyme stops the synthesis of tryptophan. Similarly, adult human haemoglobin consists of four polypeptide chains, 2a and 2b.
Each polypeptide chain is coded by a separate gene. Therefore, one gene one enzyme hypothesis was changed to ‘one gene one polypeptide’ hypothesis. According to which, a structural gene specifies the synthesis of a single polypeptide. Since the segment of DNA that codes for a polypeptide is termed as Cistron, the hypothesis is also named as one cistron one polypeptide hypothesis.
In 1970, H M Temin and D Baltimore reported the existence of an enzyme ‘RNA dependent DNA polymerase’ in certain RNA containing viruses. This enzyme could synthesise DNA from a single stranded RNA template. This process is also known as reverse transcription or teminism. The newly synthesised DNA then synthesises mRNA by transcription which in turn produces polypeptide by translation.
This enzyme gave rise to the concept of ‘central dogma reverse’ according to which the sequence of information is not necessarily from DNA to RNA to protein but can also take place from RNA to DNA.
The central dogma reverse can be illustrated as follows: