The
Beadle - Tatum
experiment:
Identification (a-c) & characterization (d-e) of nutritional
mutants of Neurospora
(a) Wild-type Neurospora are
able to grow on minimal medium (CHNOPS salts only). Individual conidia (asexual, haploid spores)
are exposed to X-rays, which
are known to be mutagenic. The mutagenized spores are grown on complete medium (CHNOPS salts plus a full range of amino acids),
and are crossed with non-mutagenized Neurospora.
The diploid offspring then
produce haploid ascospores, which are dissected and
transferred individually to new tubes with complete medium. (b) The haploid spores grow asexually
as haploid fungi on complete medium. (c)
Spores from each of the haploid fungi are tested for ability to grow on
minimal medium: most grow, a small number do not. Inability to grow indicates that a mutation affecting ability to
produce some critical growth substance was induced in the parent spore
by the X-rays.
Returning to the cultures in part (b)
that showed a "no growth" phenotype in part (c), individual haploid spores are
grown on (d) minimal and complete media as controls, and on a series of minimal
media each supplemented with one
amino acid. Typically, spores from each culture will grow on one and only one of the
supplemented media. In the example, growth on minimal medium plus arginine
indicates that the mutation altered the ability of the
particular fungal strain to synthesize arginine.
The strain will therefore be designated arg- [read, 'arg minus'] to indicte a defect of
arginine synthesis.
Beadle and Tatum repeated this experiment for hundreds of mutagenized Neurospora, and obtained scores of
nutritional mutants that were unable to grow on minimal media, without
the addition of particular amino acids.
Homework:
The test in (d) includes
minimal and complete media growth experiments as "controls": what do these experiments
"control" for? Hint: supposing
growth occurred on both minimal and complete medium, what would you
conclude?
Figure © 2000 by
Griffiths
et
al. ; text © 2010 by Steven M.
Carr