One point mutation

Michael Benedik benedik at
Thu Jun 11 13:53:59 EST 1998

In article <6lnb1f$jq1$1 at>
"Emir" <em at> writes:

> Dear colleagues,
> I heard that to get UV mutants of bacteria with only one point mutation one
> should take only those UV-exposed plates, which have 50% of survived clones
> (as compared to non-irradiated control plates). Could someone give an
> explanation to this? Is it possible to have a published reference?
> Thank you in advance.
> Emir

The idea is as follows: if you take cells that have been heavily
mutagenized, as determined by a high percentage of dead cells,
virtually all of the cells will have multiple mutations. 

However if you use conditions where most of the cells survive, then the
rate of mutation will be much lower and most of the cells will have no
mutations. Those few cells having mutations will likely only have a
single mutation.

Ideally, what you want is enough mutagenesis so that most cells carry a
mutation, but not too much so that most cells only carry very few

50% survival with UV tends to approximate this mid point where you get
adequate mutagensis, but not too high a rate of mulitple mutations. 

You should be aware that this is no guarantee of only a single
mutation, the number of mutations per cell is distributed in a Poisson
fashion. In fact it is likely that there will be more than one mutation
per genome, however it is unlikely that there will be more than one
mutation leading to a phenotypic change.

Michael Benedik
Department of Biochemical Sciences
University of Houston
benedik at

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