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Molecular Basis for
Relationship between Genotype and Phenotype
genotype
DNA
DNA sequence
transcription
RNA
translation
protein
function
phenotype
organism
amino acid
sequence
One Gene/One Enzyme Hypothesis (Beadle and Tatum)
Wild-type Neurospora crassa
(red bread mold) is prototrophic.
Prototrophs are able to grow on
minimal medium.
Minimal medium contains only
inorganic salts, simple carbon
source, and water.
Auxotrophs require nutritional
supplement(s) for growth.
One Gene/One Enzyme
Hypothesis (Beadle and Tatum)
Conidia are mutagenized with
X-ray.
Possible mutants are crossed
to wild-type and individual
ascospores are cultured on
complete (permissive) medium.
Cultures are plated on minimal
medium and auxotrophs are
isolated.
Auxotrophs are tested for
nutritional requirement.
One Gene/One Enzyme Hypothesis (Beadle and Tatum)
Each requirement for a specific nutrient (mutant effect)
followed Mendelian pattern of inheritance.
Beadle and Tatum deduced that each mutant was deficient
for an essential enzyme in a biosynthetic pathway.
A_
B_
enzyme A
enzyme B
compound Y
compound X
A_
bb
enzyme A
compound X
compound Z
compound Y
Not All Gene Products are Enzymes!
One Gene/One Enzyme
One Gene/One Polypeptide
(structural proteins, subunits)
One Gene/One Product
(rRNA, tRNA)
Mutants of Arginine Biosynthetic Pathway
cit GSA arg orn
glu
Mutant 1
+
0
+
0
0
Mutant 2
+
+
+
+
0
Mutant 3
+
0
+
+
0
Mutant 4
0
0
+
0
0
cit
GSA
arg
orn
glu
citrulline
glutamic semi-aldehyde
arginine
ornithine
glutamic acid
Mutants of Arginine Biosynthetic Pathway
Order of Compounds in Pathway
Determination of Mutant Effects
cit
GSA
arg
orn
glu
citrulline
glutamic semi-aldehyde
arginine
ornithine
glutamic acid
Molecular Basis for
Relationship between Genotype and Phenotype
genotype
DNA
DNA sequence
transcription
RNA
translation
protein
function
phenotype
organism
amino acid
sequence
Gene Sites Sensitive to Mutation
null if splice junction is
affected
no effect if mutation is in the
middle of the intron or not
the branch point