is present, however, the second chemical reaction can take place with the addition
of B to the minimal medium, and arginine can be synthesized and developed.
Mutant II, in contrast, lacks Enzyme 2, which means that arginine cannot be
synthesized even if B is added. Because both types are genetically stable, a
mutation in Gene 1 governing the formation of Enzyme 1 results in failure to
synthesize Enzyme 1 in Mutant I, whereas a mutation in Gene 2 governing the
formation of Enzyme 2 results in failure to synthesize Enzyme 2 in Mutant II. In
other words, the enzymes that catalyze each chemical reaction are governed by
different genes. This is the famous “one gene-one enzyme hypothesis,” which holds
that a single gene is involved in formation of only one type of enzyme, governing
its particularities and influencing the phenotype. If a change occurs in some gene, it
becomes impossible to produce that enzyme; even if it is produced, it is incomplete.
As a result, the reaction stops and the target material is not synthesized. If that
material is a red pigment in the eye, it manifests in the visible trait of white eyes. In
the case of arginine, the result is the physiological trait of bread mold being unable
to grow in a minimal medium. Because the enzyme is a protein, a single gene is also
seen as governing the formation of specific proteins. The explanation can be
summarized as follows:
Gene ! Enzyme ! Chemical reaction !
Product
Protein
ð
Þ
Phenotypic expression
ð
Þ
The idea of a gene governing the formation of proteins means that it contains a
diagram bearing information for protein synthesis. This means that if any mutation
occurs in the gene, it results in irregularities in the protein diagram, so that protein
either is not formed or is formed but does not function normally. The resulting
organism bears an altered trait; in other words, it is a mutant. Because that mutation
is the result of a genetic change, it is also passed down to offspring.
2.4 What Is Nucleic Acid?
Explorations into the nature of the gene as a chemical began around more or less the
same time as Mendel’s research in a field—biochemistry—that bore no connection
whatsoever to genes. In 1896, the Swiss biochemist Miescher was researching pus,
a then poorly understood material extracted from white blood cells (Miescher and
Schmiedeberg 1896). Because it contained nitrogen and phosphorus, it was a new
material that different from a protein. The same material was also found in salmon
milt (the white mass through which a male fish secretes sperm within its stomach)
and yeast. Contained with the cell’s nucleus, it was named “nuclein.” Its main
components were subsequently found to be nitrogenous bases, pentose (a
monosaccharide consisting of five carbon atoms), and phosphoric acid. Because its
pH was acidic, it became known as nucleic acid (Miescher and Schmiedeberg
1896).
2.3 Genes and Phenotypic Expression
31
of B to the minimal medium, and arginine can be synthesized and developed.
Mutant II, in contrast, lacks Enzyme 2, which means that arginine cannot be
synthesized even if B is added. Because both types are genetically stable, a
mutation in Gene 1 governing the formation of Enzyme 1 results in failure to
synthesize Enzyme 1 in Mutant I, whereas a mutation in Gene 2 governing the
formation of Enzyme 2 results in failure to synthesize Enzyme 2 in Mutant II. In
other words, the enzymes that catalyze each chemical reaction are governed by
different genes. This is the famous “one gene-one enzyme hypothesis,” which holds
that a single gene is involved in formation of only one type of enzyme, governing
its particularities and influencing the phenotype. If a change occurs in some gene, it
becomes impossible to produce that enzyme; even if it is produced, it is incomplete.
As a result, the reaction stops and the target material is not synthesized. If that
material is a red pigment in the eye, it manifests in the visible trait of white eyes. In
the case of arginine, the result is the physiological trait of bread mold being unable
to grow in a minimal medium. Because the enzyme is a protein, a single gene is also
seen as governing the formation of specific proteins. The explanation can be
summarized as follows:
Gene ! Enzyme ! Chemical reaction !
Product
Protein
ð
Þ
Phenotypic expression
ð
Þ
The idea of a gene governing the formation of proteins means that it contains a
diagram bearing information for protein synthesis. This means that if any mutation
occurs in the gene, it results in irregularities in the protein diagram, so that protein
either is not formed or is formed but does not function normally. The resulting
organism bears an altered trait; in other words, it is a mutant. Because that mutation
is the result of a genetic change, it is also passed down to offspring.
2.4 What Is Nucleic Acid?
Explorations into the nature of the gene as a chemical began around more or less the
same time as Mendel’s research in a field—biochemistry—that bore no connection
whatsoever to genes. In 1896, the Swiss biochemist Miescher was researching pus,
a then poorly understood material extracted from white blood cells (Miescher and
Schmiedeberg 1896). Because it contained nitrogen and phosphorus, it was a new
material that different from a protein. The same material was also found in salmon
milt (the white mass through which a male fish secretes sperm within its stomach)
and yeast. Contained with the cell’s nucleus, it was named “nuclein.” Its main
components were subsequently found to be nitrogenous bases, pentose (a
monosaccharide consisting of five carbon atoms), and phosphoric acid. Because its
pH was acidic, it became known as nucleic acid (Miescher and Schmiedeberg
1896).
2.3 Genes and Phenotypic Expression
31