What is the relationship between Daltons and kilobase pairs?

Answer 1

Having done some searching myself, I have read a range of answers that, for one kilobase of double stranded DNA, the molecular mass is between 650kDa to 684kDa, with a commonly used average of about 660kDa and a proviso that the exact amount will vary (very) slightly depending on the DNA sequence and the AT to CG ratio.

However, having sat here this afternoon calculating the molecular mass of DNA nucleotide pairs from the chemical formula of double stranded DNA using either the relative atomic weight or the Atomic Mass (assuming the DNA comprises of the most abundant isotope only), I seem to get 1kb of double stranded DNA being about 616kDa (for both atomic weight and mass, assuming CG and AT ratio of 1:1).

Doing the same calculations with 1kb of double stranded RNA gives about 641kDa (for both atomic weight and mass, assuming CG and AT ratio of 1:1).

I considered that there may be some water molecules (H2O) around which I'm not counting, as water is a byproduct of DNA synthesis. To get to the assumed of 660kDa, I would need 2.5 water molecules. As I'm assuming double stranded DNA, i.e. AT or GC pairs, 2.5 may be considered near to the theoretical water release by DNA synthesis of double stranded DNA (1 water molecule per nucleotide, therefore 2 water molecules for double stranded).

However, if you're calculating the molecular mass of a double stranded DNA molecule of more than 1 base, then it should be calculated without the water molecules (ignoring the 2 water molecules "left" at the (potentially) unjoined ends of your chosen DNA which won't significantly change the molecular mass of 1kb). Possibly, people who have worked out 660kDa as a good assumed average have calculated from single nucleotides and neglected to consider the changes the nucleotides undergo when they join to form DNA.

Overall though, I'm a bit stumped as to the exact answer. As I've calculated from the chemical formula directly, I would personally go with the molecular mass of 616kDa, however, as I've never seen it used that low before I wouldn't blame you if you used 660kDa.

Answer 2

A Dalton is the name given the unified atomic mass unit. A single base pair has, on average, 650 Dalton of mass.