Gene Order Transfer

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Gene Order Transfer

Post by greenpak07 on Sun Dec 07, 2008 5:39 pm

Hey guys!

I really need help with this problem and I have no one else to ask as everyone is gone for the day....I was hoping someone could help me out and explain it to me because I've been told the answer but I don't really understand how to approach the problem.
Thanks in advance and Have a terrific weekend! Smile

In a mating of Hfr pro+ trp+ ala+ tet-s x F- pro- trp- ala- gly+ tet-r, cells are
interrupted after several minutes and plated on the media containing various amino
acids shown below. The tet marker is known to be transferred very late. Based on the
number of colonies, what is the order of gene transfer? 2 PTS
Amino acid free media with the following amino acids added # of colonies
proline + alanine 290 colonies
tryptophan + proline 120 colonies
tryptophan + alanine 45 colonies
Ans: The gene order is trp-ala-pro. (BUt I don't get why or how to tell???)
NOTE: amino acid free media supplemented with tryptophan and proline is
selective for alanine.

greenpak07
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Re: Gene Order Transfer

Post by Alla on Sat Dec 20, 2008 8:56 am

It is really easy. You just need to understand what is being done.

1. Like you note implies, the missing amino acid is the selective factor. Only cells that are ala+ will be able to grow on plates without alanine, etc.
2. In this type of experiment the colonies are transferred to different media plates, meaning that out of total of 290 colonies (most colonies present on the plate) on the second plate only 120 grew and so on...

Thus, colonies on the 1st plate grow despite missing trp, are trp+; those on the 2nd plate missing alanine are ala+; and those on the last plate missing proline are pro+. Now, just looking at the numbers, you can see that all are trp+, approximately 1/2 of which are ala+, and only some are pro+. That indicates the gene transfer order of trp-ala-pro.

Good luck!

Alla
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