15.1 Experiments Have Revealed the Nature of Genes
Nutritional mutation: a mutation affecting a synthetic pathway for a vital compound such as
amino acid or vitamin.
- Resulting from damage to genes encoding functions necessary to make important
biological molecules
- They concentrated on the ability to synthesize the amino acid arginine where the
biosynthetic pathway was known called “arg” mutants.
- Beadle and Tatum found that all the arg- could be assigned to one of four chromosomal
locations
oargE
oargF
oargG
oargH
One-gene/one-enzyme hypothesis
- To determine where each mutation was blocked in the biochemical pathway for arginine
biosynthesis, the researchers supplemented the medium with each intermediate in the
pathway to see which would support each mutant’s growth.
oIf mutation affects an enzyme in the pathway that acts prior to the supplement,
then growth should be supported
oIf not, the mutation affects a step after the intermediate used.
- Through this approach, they were ablet. Isolate a mutant strain defective for each enzyme
in the biosynthetic pathway.
oEach of the mutants they examined had a defect in a single enzyme caused by a
mutation at a single site on a chromosome.
- In conclusion they believed that genes specify the structure of enzymes, and that each
gene encodes the structure of one enzyme.
oOne-gene/one-enzyme hypothesis
- Today, because many enzymes contain multiple polypeptide subunits, each encoded by a
separate gene, the relationship is more commonly referred to one-gene/one-polypeptide
hypothesis
Crick States the Central Dogma
- How is the information stored in DNA converted to protein enzymes?
oInformation passes in one direction from the gene (DNA) to an RNA copy of the
gene, which directs the sequential assembly of a chain of amino acids into a
protein
DNA mRNA protein
Transcription: the enzyme-catalyzed assembly of an RNA molecule complementary to a strand
of DNA
- Produces an exact copy of the DNA
