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Structure of Nucleic Acids



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Problem : What is the name of the sugar group in RNA? How is this different from the sugar group in DNA?

The RNA sugar group is a ribose group. DNA contains a 2' deoxyribose group. They are different in that the DNA sugar lacks a 2' –OH group that is present in the RNA sugar.

Problem : RNA does not contain the base group thymine. What base is found in its place? What is the structural difference between this base and thymine?

RNA contains the base uracil. Structurally, uracil is identical to thymine except that it lacks a methyl group attached to its 5' carbon.

Problem : What is the main three-dimensional structural difference between DNA and RNA?

While DNA is found in a double-stranded helix, RNA is found in single strands. Although it is not found in double-strands, it still adopts complex three-dimensional helical structures.

Problem : What prevents RNA from adopting the B helical form?

RNA contains an additional –OH attached to its 3' carbon. This additional group prevents the B helical form because it is too large. As a result, RNA adopts the A helical form.

Problem : Why does Chargaff's rule not apply to RNA?

RNA is found as a single stranded molecule. Chargaff's rule states that DNA helices contain equal molar ratios of A to T and G to C. This is because DNA is found as a double stranded helix in which A and T and G and C bases pair complementarily. RNA only forms local helices meaning that it doesn't necessarily contain equal ratios.