BIOL205:week 6, lecture 17
Lethal genes are essential genes
what is an example?
→ gene that controls heart formation
How do geneticists work wth lethal alleles?
In diploids, recessive lethal alleles can be maintained as heterozygotes.
In haploids, temperature sensitive lethal allele are useful. Their phenotype is wild type at the permissive
temperature (often room temperature) but mutant at some higher restrictive temperature.
Q: What type of mutation in a gene would lead to a temperature sensitive genotype?
Missense: still function but maybe not optimally at higher temps
• signal transduction: transmit instructions from an extracellular signal such as a hormone from
another part of the body
◦ this signal activates a cellular protein that, in turn, activates another, and so on, in a cascade
of activation that generally ends in the activation or inactivation of transcription genes
• developmental pathways: a diverse set of processes, many of which are contrlled by signal
transduction pathways that promote the growth and differentiation of the body of an organism
Isolation of an arginine auxotropic mutation
(Beadle and Tatume 1941)
→ x-rays to mutagenize the Neurospora (fungi)
Neurospora usually prototrophs:
can grow on minimal media
those that can't: oxitrophic • they found that three arginine auxotrophic mutants mapped to three different chromosomes
• therefore they isolated 3 different genes needed for arginine synthesis
◦ arg-1, arg-2, and arg-3 genes
• a key breakthrough was their discovery that the auxotrophs for each of the three genes differed
in their responses to the structurally related compounds ornithine and citrulline
◦ precursors to arginine
Argiine and its analogs
→ this brillian model, which was initially known as the one-gene-one enzyme hypothesis, then later
was changed to one-gene-one-polypeptide
→ Birth of Molecular Biology
Will double mutant arg-1 and arg-2 grow on citrulline?
→ don't have arg-3: behaves like arg-3 mutant A synthetic pathway and associated disease
Some pathways in PKU What can gene interactions tell us?
→ general strategey for geneticists using model organisms
• step 1: treat cell or organisms w/ mutagen (ex: x-rays). This produces random mutations in