Question

10. Assume that a strain of E. coli is grown for many generations in SN, a heavy isotope of nitrogen (14N). Under this condition, assume that the DNA has a density of 1.723 gm/cm (with water being 1.00 gm/cm'). Assume that DNA containing only I'N has a density of 1.700. Bacteria from the SN culture were washed in buffer and transferred to 4N medium for one generation. DNA was extracted. a. What would the expected density of the extracted DNA? Explain. (6 points)

Answer 1

a.

Since DNA was extracted from E. coli grown for many generations on N-15 containing medium, followed by one generation on N-14 containing medium, this DNA would contain one strand made of DNA with N-15 and the other strand with N-14. This is because DNA replication is semi-conservative and each daughter DNA molecule has one newly synthesized strand, and one parental strand.

This DNA would have a density which is the average of the two densities since half of this DNA molecule has N-15 (Parental strand) while the other half has N-14 (Daughter strand). Therefore, the density of this DNA strand is 1.7115 g/cm³ [= (1.723 + 1.7)/2].

b.

If this DNA is completely denatured, the two strands would separate and one would contain only N-15 while the other would only contain N-14. Therefore, two bands would be observed, a heavy band of density 1.723 g/cm³ (containing N-15, the parental strand) and a light band of density 1.7 g/cm³ (containing N-14, the daughter strand)