Two samples of nucleic acid are analysed…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Two samples of nucleic acid are analysed. Sample X holds adenine, guanine, cytosine and uracil, and its adenine and uracil are not equal. Sample Y holds adenine, guanine, cytosine and thymine, with adenine equal to thymine and guanine equal to cytosine. (a) Which sample is DNA and which is RNA? (b) Which sugar does each contain? (c) Which sample is probably double-stranded? Give the evidence.
Need a reminder? The key ideaShow
Nucleic acids are polynucleotides. A nucleotide is a nitrogenous base, a pentose sugar and a phosphate group; a base plus a sugar is a nucleoside. Purines: adenine and guanine. Pyrimidines: cytosine, thymine and uracil. DNA has deoxyribose and thymine; RNA has ribose and uracil. Double helix of Watson and Crick: antiparallel strands, A with T by two hydrogen bonds and G with C by three, a turn of 3.4 nm with 10 base pairs, so 0.34 nm between base pairs. Chargaff: A equals T and G equals C.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Which is which. Sample X has uracil, so it is RNA. Sample Y has thymine, so it is DNA.
Step 2. The sugars. RNA has ribose. DNA has deoxyribose.
Step 3. The double strand. Sample Y. Its adenine equals its thymine, and its guanine equals its cytosine, as Chargaff's rules say for a double strand with paired bases. Sample X has no such equal amounts, so it is probably a single strand.
Answer: (a) X is RNA and Y is DNA; (b) X has ribose and Y has deoxyribose; (c) Y, because its paired bases are in equal amounts.
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