Nucleic Acids and the Genetic Material Problem Set 1

Problem 15: DNA template for mRNA synthesis

You have obtained a sample of DNA, and you transcribe mRNA from this DNA and purify it. You then separate the two strands of the DNA and analyze the base composition of each strand and of the mRNA. You obtain the data shown in the table to the right. Which strand of the DNA is the coding strand, serving as a template for mRNA synthesis?
DNA strand #1 19.1 26.0 31.0 23.9 0
DNA strand #2 24.2 30.8 25.7 19.3 0
mRNA 19.0 25.9 30.8 0 24.3

Summary of DNA transcription

The diagram depicts DNA transcription. The coding strand of DNA in blue is used as a template for the mRNA shown in green. Complementary base pairing is used to determine the base sequence. A key feature is that the sequence and polarity of the mRNA is the same as the noncoding strand of DNA shown in red. Thus, since both the non-coding strand and the mRNA are transcribed from the same coding strand, their sequences are the same, except that U replaces T. In question 15, the correct answer has to be DNA strand #2. Strand #1 is noncoding since it has the same A, G, and C composition and has its T percentage equal to U of mRNA.

The Biology Project
University of Arizona
Sunday, September 29, 1996
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