Pathways
Gene A
Gene B
Enzyme A
Enzyme B
Substrate ------- intermediate ------ product
Most often the final product of the biochemical
pathway is something essential to life, like
amino acids, nucleotides, etc.
Pathways
• One-gene-one-enzyme hypothesis
– George Beadle and Edward Tatum
• Neurospora
Pathways
• Wild type Neurospora (all genes
intact), will grow on minimal media of
mineral salts, glucose and biotin
– Has all the genes to make all the
proteins to make everything the beasty
needs to live and survive
Pathways
• Auxotrophs (mutation in one or more
genes), will not grow on minimal
media of mineral salts, glucose and
biotin
– Will grow on minimal media only if
supplemented with one or more specific
nutrients that it cannot make
Arginine auxotroph
Pathways
Gene A
Gene B
Enzyme A
Enzyme B
Substrate ------- intermediate ------ product
Most often the final product of the biochemical
pathway is something essential to life, like
amino acids, nucleotides, etc.
Pathways
Gene A
Enzyme A
Substrate ------- intermediate
Mutant Gene B
No Enzyme B
No Product
Can use mutants to work out pathways, and
identify which gene catalyzes which step
+ = growth
- = no growth
+
+
+
Minimal ornithine citrulline arginine
Wildtype
Mut 1
Mut 2
Mut 3
+
+
+
+
4
-
+
-
+
+
-
+
+
+
3
2
1
No. 1
substrate
No.
+’s

ornithine
No. 2

citrulline
No. 3

arginine
Unknown Pathway
Mutant
No.
Methionine
Homoserine
Cysteine
Homocysteine
Cystathionine
1
+
-
-
-
-
2
+
-
-
+
+
3
+
-
-
+
-
4
+
-
+
+
+
5
+
+
-
+
+
No. 4
 cysteine
No. 2
No. 3
1
3
2
4
4
No. 1
 cystathionine  homocysteine  methionine
No. 5
 homoserine