What is an analogy reference for rRNA, and the four nitrogen bases that would be common in a cookbook?
rRNA is an acronym for "ribosomal RNA," or the RNA that the ribosomes are partially composed of (in addition to some proteins). Ribosomes are the cell structures directly responsible for synthesizing proteins by aligning amino acids with the mRNA instructions "fed" to them, typically by the nucleus. In terms of something that could be analogous in a cookbook (or, by extension, a kitchen) we could say that the oven, or the person doing the cooking, is the best analogy, because they're the things that assemble the ingredients according to the recipe and produce the final product.
The four nucleotide bases (actually five, if we're counting both RNA uracil and thymine in DNA) are a little more difficult to describe in terms of a cookbook, because they aren't actually part of the final product; they're a part of the recipe itself. Just as the cookbook is made of words and pages, the protein recipe is made of RNA, which is made of those nucleotide bases. So, properly speaking, the bases are most similar to the words and letters found in the cookbook, rather than any of its physical products.