Question

The largest human DNA molecule is a double helix consisting of two complementary polynu- cleotide chains, each chain in turn being made up of about 250 million linked nucleotides (i.e., 250 Mbp). Given that the average molecular weight of a nucleotide is about 325 g/mol and that there are 2.2 pounds in one kilogram, show the calculation of the mass in pounds of one mole of this DNA molecule.

Answer 1

Ans. Total number of nucleotides in the dsDNA molecules =

                                    2 strands x (Number of nucleotides / strand)

                                    = 2 strands x (250 x 10⁶ nucleotides/ strand)

                                    = 500 x 10⁶ nucleotides.

Given, molar mass of 1 mol nucleotides = 325 g/ mol

So, Mass of 1 molecule of nucleotide = Molar mass / Avogadro number

                                                = (325 g/ mol) / N_A              ; [N_A = Avogadro number]

Mass of 1 DNA molecule = Number of nucleotides x Mass of 1 nucleotide

                                                = (500 x 10⁶ nucleotides) x [(325 g/ mol) / N_A]   

Molar mass of DNA = Mass of 1 DNA molecule x Avogadro number

                                    = (500 x 10⁶ nucleotides) x [(325 g/ mol) / N_A]    x N_A

                                    = 1.625 x 10¹¹ g/ mol

Thus, the mass of 1 mol DNA = 1.625 x 10¹¹ g

                                                = 1.625 x 10¹¹ x 10^-3 kg                               ; [1 g = 10^-3 kg]

                                                = 1.625 x 10⁸ kg

                                                = 1.625 x 10⁸ x 2.2 lb                                    ; [1 kg = 2.2 lb]

                                                = 3.575 x 10⁸ lb

Therefore, mass of 1 mol DNA = 3.575 x 10⁸ lb