[Bio101] DNA Molecular Weight Calculation

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This method is to roughly estimate DNA molecular weight. One of its applications is to calculate the ratio of vector to insert in a ligation reaction (please see Standard DNA Cloning protocol).
Anhydrous molecular weight of each nucleotide is (see reference 1):
A= 313.21
T= 304.2
C= 289.18
For rough estimation, typically average the molecular weights of all four, average weight of a DNA nucleotide (in salt solution) = 325 Daltons (see reference 2)
MW of a single-strand DNA molecule= (#of bp) x (325 Daltons/per base)
Moles of a ssDNA molecule = (grams of DNA)/ (MW in Daltons)
For accurate estimation of MW of ssDNA, an online calculator1 is available.


  2. New England Biolabs catalog book.
How to cite: He, F. (2011). DNA Molecular Weight Calculation. Bio-protocol Bio101: e46. DOI: 10.21769/BioProtoc.46.

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viji ravi
alagappa university,karaikudi.
I have 6 micrograms of plasmid dna(puc 18) from 2 ml of axcenic my question is how many plasmids ( puc 18) expect from 6 micrograms of dna
7/7/2016 7:48:50 PM Reply
yoandry Hinojosa
i would like to ask you something that I can undersatnd. I had read in some guide book of molecular biology and had found that average weiht of nuckleotide is 330 Dantons and I have read now in your page that is 325 Daltons, although I had knwe that before now the doub returned, I am not sure which can I use if the first or the second or both
12/2/2014 9:31:17 AM Reply
Fanglian He
Department of Biology, University of Pennsylvania, USA

I have also seen 330 Daltons in other company's catalogs. "325" was cited from New England Biolabs catalog book. If you want to have a rough estimation of the MW of a DNA nucleotide, 330 or 325 might not matter that much.

But, If you know the number of As Cs Gs and Ts (or the number of GC and AT pairs), you can do a more accurate determination of MW of DNA (or dsDNA) based on the molecular weight of each monophosporylated nucleotides. I should also mention that the numbers provided in the current protocol are the molecular weight of each nucleotides given that they loose a water molecule of 18.0 when polymerized.

Hope this helps.


12/7/2014 6:51:39 PM

Kh Hussein
What is the different between molecular weight and molecular mass of DNA please?
3/2/2014 6:15:16 AM Reply
Fanglian He
Department of Biology, University of Pennsylvania, USA

I think, at least in practice, they are considered to be the same. For more explanation, you may find this link helpful,


3/10/2014 5:30:05 PM

Kh Hussein

Many thanks for reply, regards

3/11/2014 1:08:24 AM

Thanks alot - your asnewr solved all my problems after several days struggling
3/17/2012 12:33:58 AM Reply