Tm and Hybridization temperature

CVRL microbio at
Wed Jan 21 03:53:33 EST 1998

My question concerns calculation of  Tm value for determining
hybridization temperature. 

I am trying to screen a genomic library with DIG labelled (CA)15 probe.
We order our oligos from MWG Biotech and the synthesis report states Tm
as 68 degrees (I have no idea how they caluclated this). Can I use this
as a starting value for choosing a suitable hybridization temperature
i.e Tm-25 degrees?

When I tried to calculate Tm uising the simple method: multiplying G and
C by 4 degrees and the number of A and T by 2 degrees the sum works out
to 90 degrees. With the more elaborate equation involving [Na+] and
length of the oligonucleotide, Tm works out to around 54 degrees. What
would you recommend as a starting value?

Any help on this matter would be much appreciated! Please respond to me
by e-mail.

E-mail: microbio at

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