Ask A Scientist Molecular Biology Archive

name         Vanessa S.
status       other
age          19

Question -   What is the molecular weight of DNA?  Has it even been calculated? 
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It would simply depend on the number of nitrogen bases (since the phosphates and sugars are 
a constant) and therefore the length of the DNA in question.

Peter Faletra
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DNA is comprised of base pairs and DNA from different life forms is made up of different numbers 
of bases. a mole of one base weighs 325 g.  We have about 3 x 10e9 bp for each copy of our DNA 
[two total, one from each parent]  There are about 6 picograms of DNA/cell

 Jeannine M. Durdik
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Vanessa,
This has been determined, and you can do the calculation yourself with some basic 
information.

DNA is made up of four building blocks, called nucleotides. The molecular weights of 
these four nucleotides (A,C,G,T) are not identical, so in routine work we usually 
average the molecular weights of all four and go with 330 daltons per nucleotide in 
DNA (=330 grams/mole of individual nucleotides). Since DNA is double stranded, i.e., 
one nucleotide pairs with a nucleotide on the
opposing strand, then we can say the average molecular weight of a nucleotide PAIR in 
DNA is 660 daltons. Now you just have to multiply 660 daltons by the length of DNA you 
are working with. For example, let us say a bacterial cell has a million (10E6) 
nucleotide pairs per cell. So, 660 daltons/nucleotide pair times 1,000,000 nucleotide 
pairs equals the DNA's molecular weight:  660 million daltons, or 6.6 x 10E8 daltons). 
Mammals have about 1000 times more than this per cell.

Paul Mahoney, PhD

(PS. My wife, who is also a scientist, says the average is 342, not 330. I do not have a 
reference handy to look it up here, but I think you get the picture
:)
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