Beadle and Tatum conducted experiments with the fungus Neurospora crassa to test the connection between genes and enzymes. They induced mutations in Neurospora and found mutants that could grow in nutrient-rich medium but not minimal medium, showing the mutation disrupted a metabolic pathway. Through further testing of growth in supplemented mediums, they were able to link specific mutations to disrupted amino acid or vitamin biosynthesis pathways. Their work supported the "one gene-one enzyme" hypothesis that individual genes control single enzymes.

1. Induction of mutation in
Neurospora crassa

2. Why bread mold is great for
experiments
 Neurospora crassa was one of the first eukaryotic microbes to be
adopted by geneticists as a model organism.
 Beadle had planned to work with (fruit fly) Drosophila . However, as
he got 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.
 why neurospora bcz Neurospora had a fast and convenient life
cycle, one with both haploid and diploid phases that made it easy to
do genetic experiments.

3. Mutation analysis
 Garrod’s "inborn errors of metabolism’’ gAEruhd
 Sir Archibald Garrod, an English medical doctor working during 20th
century, and was the first to draw a connection between genes and
biochemistry in the human body.
 Garrod worked with patients who had metabolic diseases and saw that
these diseases often ran in families. So He focused on patients who
has 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).
 the nature of a gene was not fully understood at the time, by Garrod or
anyone else . He is considered as "the father of chemical genetics" and
he was , the first to have linked genes with the enzymes that carry out
metabolic reactions.

4. Beadle and Tatum: Connecting genes to enzymes
 Garrod's ideas went largely unnoticed in his own time . George Beadle
and Edward Tatum, carried out a series of experiments on Garrod's
work .
 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. There are 2
medium which they have used – minimal medium and complete medium
 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).

5.  Neurospora cells will also grow on complete medium, which contains
a full set of amino acids and vitamins.
 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.
 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 steps, they took descendants of the irradiated
spores and grew them individually in test tubes containing complete
medium.

6.  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.
 On minimal medium, each mutant would die because
it could not make an particular essential molecule out
of the minimal nutrients.
 Complete medium would allow the mutant it to live
by providing the missing molecule, along with a variety
of others.

7.  To figure out which metabolic pathway was "broken" in each mutant,
Beadle and Tatum performed , 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 sugar’s).
 If a mutant grew on the vitamin medium but not the amino acid medium,
it must be unable to make one or more vitamins.

8.  Beadle and Tatum further pinpointed the "broken" pathway in each mutant
through a second round of tests. 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 20amino 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.
 In this way, Beadle and Tatum
linked many nutritional
mutants to specific
amino acid and vitamin
biosynthetic pathway.

9. "One gene-one enzyme"
 Their work produced a revolution in the study of genetics and showed
that individual genes were indeed connected to specific enzymes.
 "One gene-one enzyme’’
-The initially discovered link between genes and enzymes was called the
“one gene-one enzyme” hypothesis.
 For their work on Neurospora biochemical mutants they concluded that
each gene controls a single specific biochemical reaction.
 Genes control chemical reactions by controlling the production of
specific enzymes that catalyze these reactions.
 One gene one enzyme hypothesis simply states that each gene
specifies a single specific enzyme.

10. Reference :
 https://www.khanacademy.org/science/biology/gene-expression-
central-dogma/central-dogma-transcription/a/one-gene-one-
enzyme-hypothesis
 https://www.mun.ca/biology/scarr/Gr09-01.html
 http://nlhawksapbiology.weebly.com/chapter-17.html
 http://www.biologydiscussion.com/genetics/use-of-neurospora-in-
understanding-genetical-principle/12083