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BIOLOGY 207 - Dr.Locke
Lecture#3 – Genes, Biochemical Pathways, and
1 gene : 1 enzyme
Required readings and problems:
Reading: Open Genetics, Chapter 1
Problems: Chapter 1
Optional
Griffiths et al. (2008) 9th Edition: Reading: pp 230-235
Problems: 9th Ed. Ch. 6: 1-7, 25, 28, 50
Campbell (2008) 8th Ed. Readings: Concept 17.1
Concepts:
How are genes, proteins, and biochemical pathways related?
1. A mutation in a gene -> changes in a protein -> changes in a phenotype.
Leads to -> one gene - one enzyme Hypothesis
2. Work of Beadle and Tatum
3. Mutation in different genes can be identified by location and phenotype.
Biol207 Dr. Locke section
Lecture#3
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Central Dogma
Classic:
DNA
-> Protein
DNA sequence specify the structure of proteins
Expanded:
Normal (wild type) situation.
Gene-> protein-> function -> phenotype
Mutant (changed DNA sequence)
Gene-> protein-> function -> phenotype
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How did we arrive at this point?
Archibald Garrod
- early in last century – 1902
- human hereditary defects were due to defects in basic body biochemistry
-> now called "inborn errors in metabolism"
Focused attention on the metabolic control by genes
genes -> enzymes -> metabolism
- ultimately led to the idea called:
1-gene:1-enzyme hypothesis
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Biosynthetic pathways – basic principles
- Macro molecules are synthesized in a series of steps
- Each step catalyzed by an enzyme
Each enzyme is encoded by a gene.
Substrate
Product/
Product/
Substrate
Substrate
Product
1 ------------------> 2 --------------> 3 -----------------> 4
Enzyme
A
B
Gene
a+
b+
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Lecture#3
C
c+
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Suppose:
If we inactivate (mutate) one gene
- say b - in one strain of organism
- in that strain no B enzyme
- cannot carry out 2 -> 3 conversion
Therefore no product 3 or 4 is produced
-> if product 4 is needed for growth-> no growth
(providing this is the only pathway)
What if -> we add back 4 -> growth occurs
What if -> add back missing compound (3)
-> then it can convert 3 -> 4 (grow again)
But - if add either 1 or 2 -> no growth
Compound:
Growth:
Block:
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1
No
before
Lecture#3
2
No
before
3
Yes
after
4
Yes
after
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Experiments Beadle & Tatum (1941) - Nobel Prize
Neurospora - a fungus with a haploid genome.
Series of steps in their experiments Fig.
1 - irradiated wildtype Neurospora -> produce mutations
2 - crossed with wild type of opposite mating type -> go through fruity body stage ->
single haploid spores
3 - collect individual ascospores from fruiting body
Result: each spore is a potentially different mutant
4 - culture each single spore in tube with "complete medium"
(contains all the compounds needed to grow)
5 - grow up 100's -> 1000's of different spores
6 - test each culture to see if it could grow on "minimal medium"
(contains only those compounds needed for growth and reproduction of wildtype
strains - usually contains only salts and a carbon source
- no AA or vitamins)
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Beadle & Tatum found:
1- Many mutant strains that were unable to grow on the minimal medium (MM)
(although they grew fine on complete medium (CM))
-> called auxotrophs
- a strain that can't grow on MM but can on CM
prototrophs - grow on MM (and CM, of course)
2- They did a test to examine the inheritance pattern
- inherited as 1:1 ratio when crossed to wildtype (haploid) 1 mutant: 1 wildtype
-> they concluded each was a simple mutation in a single gene (see meiosis
section later)
3- They did a test with MM supplemented with amino acids or vitamins
- test each amino acid individually
- collect many Arg- auxotrophic mutations
- each an independently produced mutation.
Identified many arg- mutants
-> strains that needed arginine added to the Minimal Medium in order to grow.
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Arg mutants provided the focus for Beadle & Tatum's further
work
Beadle & Tatum found the many arg mutants mapped to only 3 different locations, on
separate chromosomes
-> therefore there were 3 different genes
Genes called
arg-1, arg-2, arg-3
All mutations, in each gene, were Auxotrophic for arginine
Also found that these auxotrophic mutations in the three genes differed in their
response to two chemicals related to arginine
Compound able to rescue mutants?
Mutant
ornithine
citrulline
arg-1
yes
yes
arg-2
no
yes
arg-3
no
no
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arginine
yes
yes
yes
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B & T - Knew that cellular enzymes often inter-converted related compounds - proposed
biochemical model for Neurospora
precursor----------> ornithine ---------> citrulline ------>arginine
Enzyme
A
B
C
Gene
+
+
arg-1
arg-2
+
arg-3
Assume:
1. that a mutation in a particular gene interferes with the production of a single enzyme
2. defective enzyme creates a block in the biosynthetic pathway
3. the block can be circumvented by adding the compound that comes after the block
Note:
- the entire model was inferred from the properties of the mutants (phenotype)
- later the presence of defective enzymes was demonstrated by independent
biochemical analysis
History: B&T 1941
- first insight into the function of genes and how they worked
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(remember it wasn't until 1944 that DNA was shown to be the genetic material)
Mutation in different genes can be identified by
location and phenotype.
Phenotype – different response to precursors
-> different genes (likely, but not definitive)
Location – different locations indicate different genes
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