Why did the original “one-gene, one-enzyme” hypothesis have to be modified?

a. Wobble allows more than one gene to code for an enzyme.

b. Not all proteins are coded by genes.

c. Some enzymes are made up of more than one polypeptide.

d. All genes code for multiple enzymes.

e. Not all enzymes are coded for by genes.

Diagram of the experiments of Beadle and Tatum
Diagram of the experiments of Beadle and Tatum

The correct answer is c. Some enzymes are made up of more than one polypeptide

The one-gene one-enzyme hypothesis was proposed in 1941 by George Beadle. Working with other researchers on Drosophila started the investigation into the link between genes and enzymes.

Later work by Beadle and Tatum on the mold Neurospora gave clear evidence that genes were needed to make enzymes and that a lack of these enzymes caused failure of the mold to grow.

Tatum postulated that a single gene gave rise to only one enzyme; this was an idea that won him and Tatum the Nobel Prize in later years. However, in later years researchers discovered that the true picture of how enzymes form from genes is actually much more complex than this.

Scientists discovered, for instance, that a gene does not always code for an enzyme, and that an enzyme sometimes is made from two or more genes.

The other fact that was discovered was that a gene codes for a polypeptide chain and not a final protein, which involves a complex process of bonding and folding.

Enzymes are formed by protein synthesis which occurs in all cells. The process in eukaryotes involves transcription in which the code on the DNA is copied onto mRNA.

The mRNA then carries the code to the ribosome where translation involving tRNA takes place to form a polypeptide chain.

The one-gene one-enzyme hypothesis

This hypothesis was originally put forward by George Beadle in 1941, and it proposed that one gene resulted in the synthesis of only one enzyme.  Some of the ideas about the link between genes and enzymes came from the research that was conducted on the fruit fly, Drosophila.

The theory was formulated after Beadle, working with another scientist named Edward Tatum, conducted research on Neurospora, a type of mold. They created mutant forms and varied environmental conditions to see the effect this had on the growth of the mold.

The researchers were able to show that in each case the mutant individuals lacked the gene that made the enzymes that were needed for the reactions of the cell since these mutants were unable to grow when compared with normal wild-type non-mutant mold cells.

Beadle and Tatum, therefore, did demonstrate that a gene was important in regulating a biochemical pathway in a living cell since it made necessary substances that the cell required, and they subsequently won the Nobel Prize in Physiology or Medicine for their work.

Later research

In later years, scientists demonstrated that certain genes specified a chain of polypeptides and not the final protein that is formed in the cell, and one gene was not always sufficient for forming an enzyme.

This meant that the one-gene one-enzyme hypothesis did not always hold true, but it was true that genes were involved in the formation of enzymes.

Furthermore, researchers discovered that an enzyme is sometimes made from the action of two or more genes, not a single gene.

It is also a fact that not every gene even codes for an enzyme and non-enzyme proteins are also coded for genetically, but the discovery of Beadle and Tatum did start the process of investigation into how genes and enzyme formation is connected.

Protein synthesis: the connection between genes and enzymes

It is important to understand that all enzymes are proteins but not all proteins are enzymes. This means that enzymes are formed by the process of protein synthesis that takes place in cells.

This is how genetic information is actually expressed, through the two stages of protein synthesis, transcription, and translation. The end result will be a polypeptide chain, which in some cases is an enzyme or part of an enzyme.

Transcribing the code

Transcription is the first stage of protein synthesis and this occurs in the nucleus of eukaryotic cells. In this stage, the DNA code is copied and a messenger RNA (mRNA) molecule is formed.

The DNA double helix unwinds and hydrogen bonds between corresponding bases of the two strands break. There are free RNA nucleotides that then pair up with the bases on the coding strand of the DNA molecule.

An enzyme known as RNA polymerase helps catalyze this reaction in which a strand of mRNA is formed from the DNA template. It is important to remember that the sequence of the DNA bases is the genetic code.

It is this code that is copied (transcribed) when mRNA is formed during transcription. After the code has been copied and the mRNA is formed it undergoes a process known as RNA splicing.

Splicing is when an enzyme is used to remove all the non-coding sections of the nucleic acid; these regions are known as introns. The end result of this modification is that only the coding regions, the exons remain in the final mRNA transcript.

Translating the code

Translation occurs at the ribosomes in the cytoplasm of the cell. The ribosome is made of ribosomal RNA (rRNA); an additional RNA molecule. Transfer RNA (tRNA) also becomes involved in translating the code.

The mRNA transcript lines up in the ribosome at a special active site. The tRNA molecules each have three nucleotide bases on one section of their molecular structure known as an anti-codon.

This sequence codes for a specific amino acid which the tRNA attaches to. An enzyme is involved in helping the amino acid bind to the tRNA molecule.

The code is read every three bases, and the tRNA with an amino acid attached aligns opposite the corresponding three bases of the mRNA that is at the ribosome. The three nucleotide bases on the mRNA are known as a codon.

Eventually, all the tRNA molecules with the amino acids are aligned at the ribosome in the correct order as the code present on the mRNA strand. A series of enzyme reactions occur in this process and finally, all these amino acids become linked together to form a polypeptide chain.

In some cases, this polypeptide chain can form the basis for the formation of an enzyme or part of an enzyme molecule.

References

  1. H Lodish, A Berk, Sl Zipurksy, et al. (2000). Molecular Cell Biology. USA: New York, W.H. Freeman Publishers.
  2. PH Raven, RF Evert, SE Eichhorn (1987). Biology of Plants. Worth Publishers.
  3. Editors of Encyclopedia Britannica (2019). One gene-one enzyme hypothesis. Retrieved from Encyclopedia Britannica.
  4. RL Dorit, WF Walker, RD Barnes (1991).  Zoology. Philadelphia: USA, Saunders College Publishing.
  5. D Chhetri (2014). George Beadles one-gene one-enzyme hypothesis. Retrieved from asu.edu.

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