A student draws four structures. (i) Adenine alone…
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A student draws four structures. (i) Adenine alone. (ii) Adenine joined to ribose. (iii) Adenine, ribose and one phosphate group. (iv) Adenine, deoxyribose and one phosphate group. (a) Say whether each is a base, a nucleoside or a nucleotide. (b) Give the usual names of structures (ii) and (iii). (c) Which of the four could be a building block of DNA?
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. Naming each one. (i) is a nitrogenous base, a purine. (ii) is a nucleoside. (iii) and (iv) are nucleotides, because each also has a phosphate group.
Step 2. The usual names. Adenine joined to ribose is adenosine. Adenine, ribose and a phosphate group make adenylic acid.
Step 3. DNA. DNA contains deoxyribose, so the building block is (iv).
Answer: (a) base, nucleoside, nucleotide, nucleotide; (b) adenosine and adenylic acid; (c) structure (iv).
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