"One Gene, One
Enzyme"
In Principle: Proteins
are the products of genes.
Proteins
catalyze biochemical reactions.
Such reactions produce phenotypes, either directly
or indirectly.
Different alleles produce
different phenotypes
Interaction between alleles in
diploid organisms is
the subject matter of Genetics
How do genotypes produce phenotypes?
Beadle & Tatum
experiment (1940s) on haploid Neurospora bread mold
haploid organisms have one allele at
each gene locus
prototroph
wild-type
grows on simple
medium
auxotroph mutants cannot grow
on simple medium,
require supplementation with specific amino acids
[AKA autotrophs ("self feeder") and heterotrophs ("other feeder"), respectively] HOMEWORK
Hypothesis:
"No-growth" phenotype results from a change in the genotype:
inability to synthesize amino acid is the result of loss of enzyme activity
each mutant corresponds to a defect in a particular enzyme:
"One gene, one
enzyme" ()
[ Remember: Beadle
& Tatum did not know about DNA in 1940 ]
Ex.:
arg-
mutants
cannot grow without arginine, always grow with added arginine
particular mutant classes sometimes
grow with other amino acids
(such as citrulline and/or ornithine)
Growth response to
added amino acids
|
mutants
|
none
|
ornithine
|
citrulline
|
arginine
|
arg-
4 |
-
|
+
|
+
|
+
|
arg-
2 |
-
|
-
|
+
|
+
|
arg-
1 |
-
|
-
|
-
|
+
|
=> These other
amino acids are involved in the arginine pathway:
Enzyme
defect
blocks
interconversion
of
precursors in the pathway.
Inference
of a haploid biosynthetic pathway
Each mutant class (arg4, arg2, & arg1)
affects a different enzyme in arginine biosynthesis
:
1. In the above discussion, I have used "mutant" but carefully
avoided "mutation": WHY?
2. A common source of confusion for
students is a seeming circularity, that since metabolites in
this pathway are amino acids, and enzymes are made out of amino
acids, therefore enzymes defects are equivalent to the absence
of correspondong amino acids.
Bearing this in mind,
critique the following statements:
"arg- mutants result in defective
arginine."
"arg- mutants are defects of
arginine."
"arg- mutants are due to absence of
the gene for arginine."
"arg- mutants are enzymes that block
synthesis of arginine."
[
new]
"arg4,
arg2, and arg1 are
different alleles of the arg
gene."
3.
Would you expect to observe an arg mutant class with the following
phenotype? Explain why or why not
Growth response to
added amino acids
|
mutant
|
none
|
ornithine
|
citrulline
|
arginine
|
arg-
X
|
-
|
+
|
-
|
+
|
4. HOMEWORK: Consider the nature
of mutant strains in the methionine metabolic
pathway in Neurospora.
Biochemical
Basis
of Human Genetic Diseases
In a haploid
organism, discovery of DNA shows the
origin of metabolic "mutants":
Mutations affect
the genes responsible
for different enzymes:
mutate
the DNA sequence of a gene modify / eliminate the
enzyme
What about diploid organisms?
Diploid organisms have two alleles at each gene locus: one from each
parent
Interactions between alleles at a
locus are the subject matter of classical
(Mendelian) genetics
"Online Mendelian Inheritance in
Man" (OMIM) database
Examples from human
biochemical genetics: "Inborn errors of
metabolism"
First three involve disruptions of phenylalanine metabolism
Phenylketonuria (PKU) (Folling 1934) (OMIM citation 261600)
phenylalanine &
byproducts accumulate in Central
Nervous System mental
retardation
A defect of phenylalanine hydroxylase
phenyalanine
metabolized to phenylpyruvic acid in alternative pathway
Detection & treatment
biochemical testing of newborns: Guthrie Test [Homework
#18]
phenylalanine-restricted
diet
corrects
inborn condition (Euphenics)
Maternal PKU
occurs in children high fetal [phe] born to treated,
asymptomatic mothers
[
Further information on PKU & related Inborn Errors of Metabolism
]
PKU arises from genetic
variation at the Phenylalanine Hydroxylase
(PAH)
gene locus
Important: This
gene is not a
gene "for" PKU: it is a gene "for" PAH
Diploid humans each have two alleles at this locus
Allelic variants produce
different levels of PAH activity
Consider diploid
combinations of three (hypothetical) alleles: A, B,
& C :
Phenotypic consequences of
interactions between alleles at the PAH locus
|
Genotype |
PAH Activity |
[phe]
uM
|
PKU Phenotype |
AA |
100%
|
60 |
Standard (non-PKU) |
AB |
30% |
120
|
Standard (non-PKU) |
CC |
5% |
200 ~
300
|
Hyperphenylalanemia:
no special diet required |
BB |
0.3% |
600 ~ 2400
|
Classic PKU:
special diet required |
[Alleles B
& C) arise from DNA mutations in the PAH gene]
PKU is called a classic "recessive" genetic disease: What does this mean?
AB genotype shows same PKU
phenotype as AA genotype: non-PKU
A allele shows haplosufficiency:
one "allele's worth"
of product is sufficient for standard phenotype
or, expression
of A allele "masks" expression of B allele
A is described as "dominant" to B in
influencing PKU phenotype
B is described as "recessive"
to A
and BB
phenotype is different from AA /
AB phenotype: PKU
but PAH activity phenotype of AB is intermediate between AA
& BB
AB phenotype
is closer to BB than AA (0% < 30% <<
100%)
Is B therefore an "incomplete dominant" to A ?
and
B produces a higher [phe] phenotype than A:
Is B
therefore "dominant"
to A ?
Distinguish molecular versus phenotypic expression
(Homework #19)
Homework #20
Predict the
PAH activity,
[phe], & PKU phenotypes of
the AC
and CB
genotypes.
Explain your
reasoning.
Would you expect
to find a dominant mutation in this
pathway?
Why or why not?
What might be the nature of such a mutation?
|
Other molecular disorders of Phenylalanine metabolism
Alkaptonuria
(OMIM
203500): an illustration of "Classical" versus "Reverse"
Genetics
(Updated 08 Oct 2016)
an introduction to Ascertainment Bias
Oculo-cutaneous Albinism
(OMIM 203100)
All text material © 2016 by Steven M. Carr