Biochemistry 4th Edition by Christopher Mathews,Kensal van Holde, Dean Appling, Spencer Anthony Cahill

Exercise 3

The buoyant density of DNA can be increased by incorporation of either heavy stable isotopes, such as 15 N, or base analogs, such as 5-bromouracil. This problem asks you to calculate the density increment generated by each technique. In all calculations the density of "light" E. coli DNA is 1.710 g/mL. The G + C content is 51 mol % (that is, 51 moles of G + C per 100 moles of DNA nucleotides). Residue molecular weights of cesium salts of nucleotides are as follows: dAMR 445; dTMP, 436; dGMP,461; dCMP, 421; 5-bromo-dUMP, 501.
(a) Calculate the buoyant density of E. coli DNA when all 14 N atoms are replaced with 15 N atoms.
(b) Calculate the buoyant density of E. coli DNA when all thymine residues are replaced with 5-bromouracil.