During the synthesis of mRNA, one DNA strand is transcribed into message. Meselson and Stahl grew E. coli bacteria in the presence of the heavier 15Nitrogen isotope. The E. coli were then switched to a growth medium which only contained the lighter 14Nitrogen isotope. After one round of DNA replication, the two daughter strands are "tagged" with the lighter 14N isotope, while the original parental strands of DNA remain "tagged" with the heavier 15N isotope. Assume there are on average about 1,000 protein coding genes on a typical chromosome. Are both of the 15N isotope tagged anti-parallel parental strands required to transcribe all 1,000 proteins of a typical chromosome, OR is only one of those 15N isotope tagged parental strands ever used as a template for transcription of all of those approximately 1,000 protein coding genes on a typical chromosome?.