Organic chemistry question?

There are a number of hard and fast rules for numbering the positions of ring systems and sugars, etc in organic chemistry. They are complex and depend largely on the type of system in chemistry the compound is found.

For example, your sugars. If you have the stick structure, then the aldehyde carbon is # 1 and the number increasing linearly to the end of the chain. If the sugar is a ketone (single O on # 2) then that carbon is 2 and the one above it, or in an initial carbon adjacent to it is #1. If that structure is projected to a haworth structure, then the carbon # is a projection of the stick.

For the others you mention, for example, nucleic acids; they are heterocyclic compounds; containing rings with elements other than carbon. These can have a guaning base, pyrimidine base, pyrazine base, etc. The general rule with heteros is that if only N and C make up the structure, then the N is #1 then go around the ring so that any other substituents will carry the lowest number on the ring carbon attached. If there is more than 1 N, then the numbering goes in way so that #1 is the N in which the other Ns have the lowest possible number. If the rings have more than C and N, then they are numbered to give the other elements an alphabetical sequence.

Simply carbon rings (other than benzene deriv) use the apex of one of the rings as #1, such as in 2-naphthol. The #2 carbon carries the OH and the apical adjacent carbon is #1.

There are many other rules, but I think this will give you a start.