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Genes and Proteins
• Another defective enzyme in the same pathway,
phenylalanine hydroxylase (PAH), leads to phenylalanine
accumulation which causes the condition known as
phenylketonuria (PKU)
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• Incidence of PKU, characterized by severe mental
retardation, is about one in 8000 among Caucasian births.
• A defective enzyme results from a mutant gene
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Figure 1.9: Metabolic pathway for the breakdown of phenylalanine and
tyrosine
Figure 1.10: Inborn errors of metabolism in the breakdown of
phenylalanine and tyrosine
Genes and Proteins
• In the 1940s George W. Beadle and Edward L. Tatum,
using a filamentous fungus Neurospora crassa,
demonstrated that each enzyme is encoded in a
different gene.
• Their experimental approach, now called genetic
analysis, led to the one gene–one enzyme hypothesis.
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Figure 1.11: Beadle and Tatum obtained mutants of the fi lamentous
fungus Neurospora crassa
Genes and Proteins
Figure 1.12A: Mutant spores can grow in complete medium but not in minimal mediu
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Figure 1.12B: Each new mutant is tested
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Figure 1.12C: Mutants that can grow on minimal medium supplemented
with amino acid are tested
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Figure 1.12D: Mutants unable to grow in the absence of arginine are tested
with likely precursors of arginine
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Figure 1.13: Metabolic pathway for arginine biosynthesis inferred from
genetic analysis of Neurospora mutants