The One Gene One Enzyme Hypothesis

Beadle and Tatum used the bread mold Neurospora to study the biochemical result of mutations. Neurospora is a good model organism because it is easy to cultivate in the laboratory and because the main vegetative part of the fungus is haploid; the haploid state allows the effects of recessive mutations to be easily observed (< Figure 15.1).

Wild-type Neurospora grows on minimal medium, which contains only inorganic salts, nitrogen, a carbon source such as sucrose, and the vitamin biotin. The fungus can synthesize all the biological molecules that it needs from these basic compounds. However, mutations may arise that disrupt fungal growth by destroying the fungus's ability to synthesize one or more essential biological molecules.

^ The vegetative haploid mycelium produces haploid spores through mitosis.

yA These spores may germinate and produce a genetically identical mycelium (asexual reproduction)..

Haploid (n) mycelium mating type A

■wasaaaa

/ Sexual

Asexual reproduction reproduction I

Haploid mycelium mating type a

Haploid ^spores

Haploid mycelium mating type a

Haploid (n) mycelium mating type A

■wasaaaa

/ Sexual

Asexual reproduction reproduction I

Haploid ^spores

The spores are released from the ascus and germinate to form new mycelia.
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