Download Chapter9.2a

Role of DNA
Proteins
• Order of bases in one gene codes for a
protein
• Proteins are often used in the cell as
enzymes
• Different enzymes cause different cells to
behave differently
What happens if the enzyme is
wrong?
• Phenylketonuria (PKU)
– Due to deficiency in one enzyme
(phenylalanine hydroxylase)
– Correct enzyme converts phenylalanine 
tyrosine
– Discovered in 1934 by Asbjørn Følling
– Gene located on chromosome 12
– If gene not present, enzyme lacking, and
phenylalanine  toxic phenypyruvate
What happens?
• Phenylpyruvate builds up in the body
• Causes mental retardation, heart disease,
death
• Occurrence is about 1 in 15 000 births
• About 2% of US population carries the
gene (heterozygous)
• Heterozygous individual can make normal
enzymes
Beadle and Tatum
• George Beadle and Edward Tatum, 1941
• Exposed spores of Neurospora crassa (a bread mold) to
X-rays or UV radiation and studied the resulting
mutations
• Each mutant has specific nutrition needs
• Discovered that each mutant strain differed by only one
gene
• Beadle and Tatum were able to create mutants that
could not form specific enzymes
• Formulated the “one gene one enzyme” hypothesis
– Each gene is responsible for building one enzyme
One gene, one protein
• Linus Pauling separated different forms of
hemoglobin (normal and sickled cells)
• Discovered that the two forms differ by only one
amino acid (out of 300)
• Further investigation determined that a gene is
responsible for coding for the order of amino
acids that make up one polypeptide
• Proteins are polymers of polypeptides
Genes code for
• Proteins!
• Read pages 237-242