Nucleic Acids

A nucleotide has three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. In DNA the sugar is deoxyribose and the bases are adenine, thymine, guanine, and cytosine. Four letters sounds limiting, but it is not about variety of parts, it is about sequence: just like 26 letters build every word in a language, strings of A, T, G, and C spell out instructions for every protein a cell makes. Nucleotides link by forming bonds between the phosphate of one and the sugar of the next, building a sugar-phosphate backbone with the bases sticking off to the side like rungs.

That backbone is not symmetric. Each sugar has a 5 prime carbon, where the phosphate attaches, and a 3 prime carbon, which has a free hydroxyl group available to bond to the next nucleotide's phosphate. Because new nucleotides can only be added onto that free 3 prime hydroxyl, a strand always grows in one direction, 5 prime to 3 prime. Add a nucleotide at the wrong end and there is no hydroxyl waiting to react, so the bond cannot form; this is why DNA polymerase can only extend a strand at its 3 prime end, and why one new strand during replication is built continuously while the other must be made in short backward-reading fragments.

RNA uses the same backbone logic but swaps deoxyribose for ribose, which carries an extra hydroxyl group, and swaps thymine for uracil. That extra hydroxyl makes RNA more chemically reactive and less stable, and RNA usually stays single-stranded instead of pairing into a double helix. Those differences are not cosmetic: they are exactly what let RNA fold into shapes that catalyze reactions and carry messages quickly, while DNA's stability suits it to long-term storage.