which scientists carried out an experiment to determine how mutations arise?
Luria and Delbruck
Luria and Delbruck experimental system (+ test
analysing a bacteriophage (TI) that infects and kills e coli
made the fluctuation test
what was the result of luria and delbruck
most e coli died but a few were resistant and survived to give colonies
- progeny also resistant to TI
RESISTANCE IS AN INHERITABLE GENETIC TRAIT
what were the two hypotheses for how mutations arose
EITHER
Induced mutations (adaptive response) - resistance due to phage introduction
OR
Darwanian evolution - resistant mutations were pre-existing due to spontaneous mutations
outcome of fluctuation test
consistent with mutations arising spontaneously and mutant bacteria being selected by T1 phage
outcomes in bulk culture?
to test if the fluctuations they saw in individual cultures were due to sampling errors
- much less variation in different samples from the same bulk culture
jackpot cultures = most number mutation
limitations of fluctuation test
bacteria are rapidly killed - not time to adapt
can take some time (several generations) for resistance mutations to give rise to resistant phenotypes
2 ways to test for mutagens
direct = laboratory animals
using microorganisms
why would we use microorganisms rather than animals
simple, effective, large numbers. genes in microorganisms work in teh same way as in eukaryotes
ames test overview
use strains of salmonella typhimurium that are AUXOTROPHS (his- aka NEED his to grow)
spread onto growth medium WITHOUT his
any bacteria growth have had a second mutation and are revertance - reverted back to wild type
example of ames test
compound = aflatoxin
conclusion = does increase frequency of mutation (revertance) therefore aflatoxin is a mutagen
