You Discover a Virus with a Number of Unusual Properties

Multiple Choice

You discover a virus with a number of unusual properties, and decide to assign it the name virus U238. The genome of U238 consists of double-stranded DNA. To study the mode of DNA replication in U238, you carry out a Meselson-Stahl type of experiment, and first observe that viral DNA containing only normal "light" nitrogen (¹⁴N) has a density of 1.715 gm/cm³. You then allow the virus to replicate in the presence of nucleotides containing "heavy" nitrogen (¹⁵N) until both strands of its genome are "heavy." At this point the density of the viral DNA is 1.722 gm/cm³. You allow the virus with "heavy" DNA to undergo one round of replication in the presence of only "light" nucleotides, and you observe that half of the viral progeny have DNA with a density of 1.722 gm/cm³ and the other half of the viral progeny have DNA with a density of 1.715 gm/cm³. How would you interpret these results?

A) The viral DNA is replicated like any other double-stranded DNA molecule.
B) The viral DNA is replicated in very short pieces that reassociate at random.
C) After replication, pairing of the parental strands is restored.
D) This result is impossible and is therefore most likely due to an error.