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Course: Biology library > Unit 18

Lesson 1: Central dogma and the genetic code
Science
Biology library
Central dogma (DNA to RNA to protein)
Central dogma and the genetic code
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One gene, one enzyme

Google Classroom
Historical experiments: Garrod's studies of alkaptonuria,  Beadle and Tatum's work with Neurospora mutants.

Key points:

  • The one gene, one enzyme hypothesis is the idea that each gene encodes a single enzyme. Today, we know that this idea is generally (but not exactly) correct.
  • Sir Archibald Garrod, a British medical doctor, was the first to suggest that genes were connected to enzymes.
  • Beadle and Tatum confirmed Garrod's hypothesis using genetic and biochemical studies of the bread mold Neurospora.
  • Beadle and Tatum identified bread mold mutants that were unable to make specific amino acids. In each one, a mutation had "broken" an enzyme needed to build a certain amino acid.

Introduction

Today, we know that a typical gene provides instructions for building a protein, which in turn determines the observable features of an organism. For instance, we now know that Gregor Mendel's flower color gene specifies a protein that helps make pigment molecules, giving flowers a purple color when it works correctly.
Mendel, however, did not know that genes (which he called "heritable factors") specified proteins and other functional molecules. In fact, he didn't even speculate about how genes affected the observable features of living organisms. Who, then, first made the connection between genes and proteins?

Garrod’s "inborn errors of metabolism"

We often see cases where basic biology breakthroughs happen in the lab. However, they can also happen at the bedside! Sir Archibald Garrod, an English medical doctor working at the turn of the 20th century, was the first to draw a connection between genes and biochemistry in the human body.
Portrait of Sir Archibald Garrod. The photograph is black-and-white and depicts Garrod looking down at a stack of papers, glasses in hand.
_Image modified from "Archibald Edward Garrod." Original image by Frederick Gowland Hopkins (CC BY 4.0)._
Garrod worked with patients who had metabolic diseases and saw that these diseases often ran in families. He focused on patients with what we today call alkaptonuria. This is a non-fatal disorder where a person’s urine turns black because they cannot break down a molecule called alkapton (which, in normal people without the disorder, gets broken down into other, colorless molecules).1
By looking at family trees of people with the disorder, Garrod realized that alkaptonuria followed a recessive pattern of inheritance, like some of the traits Mendel had studied in his pea plants. Garrod came up with the idea that alkaptonuria patients might have a metabolic defect in breaking down alkapton, and that the defect might be caused by the recessive form of one of Mendel's hereditary factors (i.e., a recessive allele of a gene).2
Garrod referred to this as an “inborn error of metabolism,” and he found other diseases that followed similar patterns. Although the nature of a gene was not fully understood at the time, by Garrod or anyone else, Garrod is now considered "the father of chemical genetics" – that is, the first to have linked genes with the enzymes that carry out metabolic reactions.3

Beadle and Tatum: Connecting genes to enzymes

Regrettably, Garrod's ideas went largely unnoticed in his own time. In fact, it was only after two other researchers, George Beadle and Edward Tatum, carried out a series of groundbreaking experiments in the 1940s that Garrod's work was rediscovered and appreciated.4
Beadle and Tatum worked with a simple organism: common bread mold, or Neurospora crassa. Using Neurospora, they were able to show a clear connection between genes and metabolic enzymes.

Why bread mold is great for experiments

You may be wondering: why did Beadle and Tatum choose to study something as icky (and seemingly unimportant) as bread mold?
Well...at first, Beadle had planned to work with the fruit fly Drosophila (also a bit gross, but a much more common organism for experiments at the time). However, as he got more and more interested in the connection between genes and metabolism, he realized that Neurospora might give him a better way to answer the questions he was curious about. For one thing, Neurospora had a fast and convenient life cycle, one with both haploid and diploid phases that made it easy to do genetic experiments.5
Minimal medium: contains sugars, salts, and biotin
Complete medium: contains sugar, salts, amino acids, and many vitamins
Perhaps most importantly, Neurospora cells could be grown in the lab on simple medium (nutrient broth or gel) whose chemical composition was 100% known and controlled by the experimenter. In fact, the cells could grow on minimal medium, a nutrient source with just sugar, salts, and one vitamin (biotin). Neurospora cells can survive on this medium, while many other organisms (such as humans!) cannot. That's because Neurospora has biochemical pathways that turn sugar, salts, and biotin into all the other building blocks needed by cells (such as amino acids and vitamins).6
Neurospora cells will also grow happily on complete medium, which contains a full set of amino acids and vitamins. They just don't need complete medium in order to live.

Let's make some mutants!

If genes were connected to biochemical enzymes, Beadle and Tatum reasoned that it should be possible to induce mutations, or changes in genes, that "broke" specific enzymes (and thus, specific pathways) needed for growth on minimal medium. A Neurospora line with such mutation would grow normally on complete medium, but would lose the ability to survive on minimal medium.7
  1. Obtain Neurospora spores.
  2. Expose spores to X-rays. Some spores now have random mutations.
  3. Cross spores to normal (non-irradiated spores) and collect the progeny spores.
  4. Transfer each progeny spore individually to its own tube of complete medium, so that it makes a colony.
  5. Transfer part of each colony to its own tube of minimal medium.
  6. Nutritional mutants may be identified as colonies that grew on complete medium, but did not grow when transferred to minimal medium.
Diagram based on similar diagram in Griffiths et al. 8.
To look for mutants like this, Beadle and Tatum exposed Neurospora spores to radiation (x-ray, UV, or neutron) to make new mutations. After a few genetic cleanup steps, they took descendants of the irradiated spores and grew them individually in test tubes containing complete medium. Once each spore had established a growing colony, a small piece of the colony was transferred into another tube containing minimal medium.
Most colonies grew on either complete or minimal medium. However, a few colonies grew normally on complete medium, but couldn't grow at all on minimal medium. These were the nutritional mutants that Beadle and Tatum had been hoping to find. On minimal medium, each mutant would die because it could not make an particular essential molecule out of the minimal nutrients. Complete medium would "rescue" the mutant (allow it to live) by providing the missing molecule, along with a variety of others.9

Pinpointing the broken pathway

To figure out which metabolic pathway was "broken" in each mutant, Beadle and Tatum performed a clever, two-step experiment.
First, they grew each mutant on minimal medium supplemented with either the full set of amino acids or the full set of vitamins (or sugars, though we won't examine that case here).8,10
  • If a mutant grew on minimal medium with amino acids (but not vitamins), it must be unable to make one or more amino acids.
  • If a mutant grew on the vitamin medium but not the amino acid medium, it must be unable to make one or more vitamins.
  1. Start with a nutritional mutant. By definition, the nutritional mutant can grow on complete medium, but not on minimal medium.
  2. Now, we are going to find out what in the complete medium it is that the nutritional mutant needs to grown. To do so, we transfer a little bit of the colony into each of two tubes: one with minimal medium + full set of vitamins, the other with minimal medium + all 20 amino acids.
  3. In this example, the mutant is rescued by the mixture of all 20 amino acids, but not by the set of vitamins. This indicates that the mutation makes the mutant unable to synthesize one or more amino acids.
  4. Since the mutant is rescued by the amino acid mix, the next question becomes: what amino acid(s) is it unable to make? To answer this question, we transfer a bit of the mutant colony into each of 20 tubes. Each tube contains minimal medium plus one of the 20 amino acids.
  5. In this example, the mutant can grow in the tube containing minimal medium + arginine, but not in any of the other 19 tubes. (I.e., the mutant is rescued by arginine). This indicates that the mutation in the mutant must disrupt arginine biosynthesis.
Diagram based on similar diagram in Griffiths et al. 8.
Beadle and Tatum further pinpointed the "broken" pathway in each mutant through a second round of tests. For instance, if a mutant grew on minimal medium containing all 20 amino acids, they might next test it in 20 different vials, each containing minimal medium plus just one of the 20 amino acids. If the mutant grew in one of these vials, Beadle and Tatum knew that the amino acid in that vial must be the end product of the pathway disrupted in the mutant.8
In this way, Beadle and Tatum linked many nutritional mutants to specific amino acid and vitamin biosynthetic pathways. Their work produced a revolution in the study of genetics and showed that individual genes were indeed connected to specific enzymes.11

"One gene-one enzyme" today

The initially discovered link between genes and enzymes was called the “one gene-one enzyme” hypothesis. This hypothesis has undergone some important updates since Beadle and Tatum12,13:
  • Some genes encode proteins that are not enzymes. Enzymes are just one category of protein. There are many non-enzyme proteins in cells, and these proteins are also encoded by genes.
  • Some genes encode a subunit of a protein, not a whole protein. In general, a gene encodes one polypeptide, meaning one chain of amino acids. Some proteins consist of several polypeptides from different genes.
  • Some genes don't encode polypeptides. Some genes actually encode functional RNA molecules rather than polypeptides!
Although the "one gene-one enzyme" concept is not perfectly accurate, its core idea – that a gene typically specifies a protein in a one-to-one relationship – remains helpful to geneticists today.

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