Introduction
Mutations are occurring constantly in every population of every species in our environment that are either benign, or cause a change in the specific gene product. Some common causes of mutations are radiation, chemical mutagens, mistakes during genetic replication, or viral interference. In this experiment, colonies of E.coli are introduced to a virulent bacteriophage, T2.
Bacteriophage T2 attacks E.coli cells by adsorbing to a specific outer membrane protein called ompF. Because T2 cannot reproduce extracellularly, it uses the E.coli as host cells by injecting its DNA into the bacterial genome for further replication. The T2 DNA uses the E.coli cell's ribosomes to replicate until the cellular membrane lyses and the new phages are released to attack other cells.
However, because ompF is not necessary for E.coli cells to survive, there can be a mutation that will allow the cell to live in the presence of T2 bacteriophage by removing ompF. The purpose of this experiment was to determine if the mutations that occurred in the surviving E.coli were induced by the presence of T2, or were spontaneous.
Spontaneous mutations, for our purposes, are standing mutations caused by factors other than T2. Spontaneous mutations usually cause a base alteration, such as replacement of a base with another base, or a frameshift that results in a change in translation of all following codons. A frameshift mutation generally knocks out the gene in which it occurs. On the other hand, induced mutations are caused by natural or man-made mutagens which alter the structure or sequence of DNA.
Newcombe's experiment was a way to determine whether mutations are induced by a screening agent or occur spontaneously. Before Newcombe's experiment, there were only complex statistical analyses to determine if the resistance to a screening agent is induced by the screening agent. Newcombe proposed an experiment where two plates of bacteria, one re-spread after a short incubation period and one not, are introduced to a screening agent, such as bacteriophage T2. If mutations were induced, then both plates should include the same number of mutant colonies, as they both have the same viable cell count. However, if the number of colonies on the re-spread plate is much greater than the un-spread plate, then mutations were not induced by the screening agent. This result is caused by the re-distribution of the resistant micro-colonies.