Page 89 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

248 Chapter 7 Anatomy and Function of a Gene: Dissection Through Mutation

Investigators working in the first half of the twentieth Garrod studied several other inborn errors of metabo-
century studied carefully the biochemical changes caused lism and suggested that all arose from mutations that pre-
by mutations in an effort to understand the genotype– vented a particular gene from producing an enzyme
phenotype connection. required for a specific biochemical reaction. In today’s ter-
In one of the first of these studies, conducted in 1902, minology, the wild-type allele of the gene would allow pro-
the British physician Dr. Archibald Garrod showed that a duction of functional enzyme (in the case of alkaptonuria,
human genetic disorder known as alkaptonuria is deter- the enzyme is homogentisic acid oxidase), whereas the mu-
mined by the recessive allele of an autosomal gene. Garrod tant allele would not. Because the single wild-type allele in
analyzed family pedigrees and performed biochemical anal- heterozygotes generates sufficient enzyme to prevent the
yses on family members with and without the trait. The accumulation of homogentisic acid and thus the condition
urine of people with alkaptonuria turns black on exposure to of alkaptonuria, the mutant allele is recessive.
air. Garrod found that a substance known as homogentisic
acid, which blackens upon contact with oxygen, accumu-
lates in the urine of alkaptonuria patients. Alkaptonuriacs A Gene Contains the Information for
excrete all of the homogentisic acid they ingest, while peo- Producing a Specific Enzyme: The One
ple without the condition excrete no homogentisic acid in
their urine even after ingesting the substance. Gene, One Enzyme Hypothesis
From these observations, Garrod concluded that peo- In the 1940s, George Beadle and Edward Tatum carried
ple with alkaptonuria are incapable of metabolizing homo- out a series of experiments on the bread mold Neurospora
gentisic acid to the breakdown products generated by crassa (whose life cycle was described in Chapter 5) that
normal individuals (Fig. 7.26). Because many biochemical demonstrated a direct relationship between genes and the
reactions within the cells of organisms are catalyzed by enzymes that catalyze specific biochemical reactions.
enzymes, Garrod hypothesized that lack of the enzyme that Their strategy was simple. They first isolated a number of
breaks down homogentisic acid is the cause of alkapton- mutations that disrupted the synthesis of the amino acid
uria. In the absence of this enzyme, homogentisic acid ac- arginine, a compound needed for Neurospora growth.
cumulates and causes the urine to turn black on contact They next hypothesized that different mutations blocked
with oxygen. He called this condition an inborn error different steps in a particular biochemical pathway: the
of metabolism. orderly series of reactions that allows Neurospora to ob-
tain simple molecules from the environment and convert
them step-by-step into successively more complicated
Figure 7.26 Alkaptonuria: An inborn error of molecules culminating in the end product arginine.
metabolism. The biochemical pathway in humans that degrades
phenylalanine and tyrosine via homogentisic acid (HA). In Experimental evidence for one gene,
alkaptonuria patients, the enzyme HA hydroxylase is not functional,
so it does not catalyze the conversion of HA to maleylacetoacetic one enzyme
acid. As a result, HA, which oxidizes to a black compound, Figure 7.27a illustrates the experiments Beadle and Tatum
accumulates in the urine. performed to test their hypothesis. They first obtained a set
Normal pathway Alkaptonuria of mutagen-induced mutations that prevented Neurospora
Phenylalanine Phenylalanine from synthesizing arginine. Cells with any one of these
Enzyme 1 1 mutations were unable to make arginine and could there-
Tyrosine Tyrosine fore grow on a minimal medium containing salt and sugar
only if it had been supplemented with arginine. A nutri-
2 2
tional mutant microorganism that requires supplementation
p-Hydroxyphenylpyruvate p-Hydroxyphenylpyruvate with substances not needed by wild-type strains is known
3 3 as an auxotroph. The cells just mentioned were arginine
auxotrophs. (In contrast, a cell that does not require the ad-
4 4
dition of a substance is a prototroph for that factor. In a
Homogentisic acid (HA) Homogentisic acid (HA)
more general meaning, prototroph refers to a wild-type cell
HA oxidase 5 that can grow on minimal medium alone.)
• HA oxidase nonfunctional
Maleylacetoacetic acid • HA accumulates Recombination analyses located the auxotrophic
• Turns urine black in air arginine-blocking mutations in four distinct regions of the
6 • Pathway stops
genome, and complementation tests showed that each of
7 the four regions correlated with a different complementa-
8 tion group. On the basis of these results, Beadle and Tatum
CO + H O concluded that at least four genes support the biochemical
2