If it is a long run, 16 gauge is preferred. If the run is very short, you probably can use 18 gauge with no apparent difference in sound. But the cost difference is fairly small, so 16 gauge is preferred. There is no shielding on speaker wire, so the covering is of little import.
The backbone of the DNA strand is made from alternating phosphate and sugar residues. The sugar in DNA is 2-deoxyribose, which is a pentose (five-carbon) sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings.
Since DNA is thought to be like a twisted ladder the rungs, not including the rails, of DNA are made up of nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The nitrogenous bases are part of nucleotides, which include what is commonly called the sugar-phosphate backbone (deoxyribose sugar and phosphate groups) which could be equated as a rail in the ladder metaphor.