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In 1952, Joshua and Esther Lederberg performed experiments to determine if adaptation in E. coli arose from a process of random mutation followed by selection in a new environment, or whether environmental stressors directly induce mutations that confer an adaptive advantage. The observation that motivated their study was this. If one exposes a population of antibiotic sensitive bacteria (strepS, for example) to the antibiotic (streptomycin), the vast majority of thebacterial cells will be killed. But, if the original population was large enough, it will soon be repopulated with bacteria, all of which are resistant to streptomycin (strepR). The question is then: does streptomycin select for rare cells pre-existing in the population that have the mutation to strepR, or does the streptomycin itself induce the cells to produce new mutations that confer resistance? In order to answer this question, the Lederbergs developed a technique known as replica plating.Bacteria from a non-selective media (containing no antibiotics) are grown on a 'master plate'. Apiece of sterile velvet is then used to transfer some bacteria to fresh media in several different replica plates which contain a selective agent (streptomycin). If a few bacteria from the masterplate contain pre-existing mutations that, by chance, allow them to grow in the presence o fstreptomycin, then what pattern of colonies will be present on the replica plates? If the antibiotic induces the bacteria to produce new mutations that confer resistance, what pattern of colonies is expected on the replica plates? -------------------------------------------------------------------------------- . 4MUTATION IS THE ULTIMATE SOURCE OF ALL GENETIC VARIATIONHow much mutation is there? -------------------------------------------------------------------------------- Compare sequences for species pairs w/ known divergence times:Human/chimp: 1.3x10-9per site per year; 15-20 yr generation time2x10-8per generation; 6x109nucleotide pairs at least 120 new mutations per genome per generation!However, not all these mutations have phenotypic effects. What is the per gene mutation rate?We can measure the rate of mutation for certain kinds of mutations with phenotypic effects, t odetermine whether mutation is a powerful force for changing allele frequencies between onegeneration and the next.
Approximate Word count = 1273
Approximate Pages = 5.1
(250 words per page double spaced)
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