# Thread: What is a mega dalton?

1. OK the title is a little misleading as I suspect that a Mega Dalton is simply a way of counting the overall size of something, when you don't know what all the constituent parts are? Is this correct? I mean looking it up on Wikipedia it says it says one Dalton is equivalent to the mass of one proton. Which is fair enough. So if a protein was one million Daltons in size (or one mega Dalton), that would just mean that it contains overall one million protons? (These are questions BTW).

However the question is if this is correct, why is the Dalton used over any other measure in biology, how is it calculated and how is it denoted?

Thanks!

2.

3. One Dalton is just really a short hand way of saying "atomic mass unit(s)" and is common in molecular biology when quantifying the masses of macromolecules like proteins or nucleic acids. It does have a precise value, but can be approximated to be the same as the mass of one proton.

To calculate it all you really do is add up the molecular weights of the atoms in the molecule. To save you doing that there are scripts and programs into which you can past protein or nuclei acid sequences and get an answer. To do it roughly in your head you'd carry around in your head some rough conversion factors. For example, the average molecular weight of an amino acid is 110 Da, so a protein of 200 amino acids in length would have a size of roughly 22 KDa. This is quite a common formula to learn (no. of aa's X 110 = average MW of a protein).

There are a whole bunch of such useful conversion factors to get a rough idea of the mass of large molecules.

4. Thanks man.

5. Oh, forgot to mention that 1 MDa doesn't mean that there are 1 million protons in the molecule. Instead, it means the mass of the molecule is nearly the same as the mass of one million protons. Recall that the atomic nucleus also as neutrons in it.

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