Hi. Please answer the following two questions as concisely as
possible. If you can answer each question in one sentence, that's fine
with me. I don't want fancy research (thus the paltry $2). If you know
the answer off the top of your head, just give it to me. I do want
someone w/ a little background in genetics to answer this question
though. This is very important: I need the answer before 8AM EST
Friday November 2. If I am pleased with the answer, I will give a $1
tip. Here it
goes:

Mutations can often be reverted to wild type by treatment with
mutagens. The type of mutagen that will reverse a mutation gives us
information regarding the nature of the original mutation. Proflavin
is an intercalating agent that causes insertion or deletion of a base.
UV light causes single base pair substitutions. EMS causes transitions
to occur.

You have been given mutant cultures of E. coli (trp-). You treated
these cultures with the three kinds of mutagens separately and spread
onto a plates lacking tryptophan with the goal to isolate revertants.
You isolated four mutants with the following results:


Mutant number			Mutagen treatment 
			Proflavin	UV light	EMS 
1			-		+		- 
2			-		+		+ 
3			+		-		- 
4			-		-		- 
Key: (+) indicates presence of colonies; (-) indicates no colonies grew. 


a. What is the nature of the original mutation in each of these strains?
b. How do you know that the colonies that grew on the plates are
mutagen-induced mutants (revertants) or if they represent spontaneous
revertants?

Good luck with your exam.