Biology 12

Neurospora

irradiated to

increase mutations

wild, untreated

Neurospora

minimal

medium

mated

fruiting body meiosis

growth

occurs

spore

case

complete

medium

individual

spore placed

on medium

fungi

transferred

mutant fungi

transferred

growth

occurs on

complete

medium

fungi that

do not

grow are

mutants

growth

occurs on

minimal

medium

supplemented

with various

amino acids

minimal

medium

fungi transferred

minimal medium

supplemented

with only one

amino acid

choline nucleic acid arginine niacin thiamine control

242 MHR • Unit 3 Molecular Genetics

they were able (in many cases) to identify the

specific stage in a metabolic pathway that was

blocked by a particular mutation. This work,

illustrated above in Figure 7.30, led them to

conclude that each mutant variety of the mould

had one defective gene that caused the mould to be

deficient in one enzyme that catalyzed a particular

step in a given metabolic pathway. Their

hypothesis became known as the one gene-one

enzyme theory of gene function.

In the years that followed, this functional

definition was broadened to one gene-one protein,

since some genes were found to code for proteins

other than enzymes. (Examples include structural

proteins such as collagen and the silk of spider

webs.) This definition was later modified to one

gene-one polypeptide when scientists found that

the different polypeptides in a single protein

complex may be coded for by entirely separate

genes. An example of this is hemoglobin —

different genes code for each of the two types of

polypeptide sub-units that make up one

hemoglobin molecule.

In short, the precise functional definition of a

gene has become more complex as scientists have

learned more about how genes work. The one

gene-one polypeptide theory still does not account

for all aspects of gene function. In eukaryotes, a

single gene can code for several different polypeptide

products. Other genes code for the synthesis of

Figure 7.30Beadle and Tatum isolated mutant strains of

the mould that could grow on a complete medium but not

on a minimal medium. By gradually adding one nutrient at a

time to the minimal medium, they were able to isolate

different mutant strains. Here, the mutant strain isolated

lacks the ability to produce arginine, and so will grow only

on a medium in which this nutrient has been added.