starting from the base

adenine

Canto: You know, just as an aside on this earliest forms of life stuff, it's interesting how the pioneers of modern biology were interested in getting down to the smallest, simplest forms of life, and studying them, to see just how far down they could go, in size, and still find life. I mean they were asking, just how small can a living organism be?
Jacinta: Right, and they were no doubt amazed at the complexity of those little critters, once they developed microscopes powerful enough to detect them and look inside them.
Canto: Well you know Darwin took a microscope with him on the Beagle, a state-of-the-art instrument of the time, and his Beagle notes were more about zoophytes than anything else. Zoophytes were sea creatures - the term is obsolete now - that seemed to have the qualities of plants as well as animals. But he was also intrigued by what he called infusoria, another obsolete term, then given to what we now know are diverse forms of largely water-dwelling eukaryotic micro-organisms. And I'm sure his interest in such organisms ran along these 'how small can life be' lines.
Jacinta: And yet the pioneering microbiologist Christian Ehrenberg considered that there were no 'lower' creatures, and that 'the infusorian has the same sum of organisation-systems as a man'. These were the beginnings of heretical ideas, questioning the role of humanity as the pinnacle of the earth's, or God's, creation. Darwin was definitely being influenced by these new lines of thinking.
Canto: So anyway, we were talking about ribosomal RNA as I recall.
Jacinta: Well you know that the ribosome is the protein-making factory of the cell, right?
Canto: Of course. And so ribosomal RNA is - the RNA in ribosomes, right?
Jacinta: No, it's nowhere near as simple as that, mate. First we know that 'DNA makes RNA makes protein', but it's not a simple process. Certainly rRNA is central to that protein-making process, but there are two other forms of RNA, messenger RNA [mRNA] and transfer RNA [tRNA], that need to be understood and differentiated.
Canto: Yeah, I can see this is going to be horribly complex, but let's just do it. How does DNA make RNA and why does it make these three different types?
Jacinta: Well this first step is called transcription or RNA synthesis. We know that both DNA and RNA are nucleic acids.
Canto: Made up of nucleotides. But what's a nucleotide?
Jacinta: Okay, if you're serious, I'll tell you. A nucleotide has three basic components - a nucleobase, a five-carbon sugar, and a phosphate group [or two, or three]. A nucleobase is involved in base pairing in DNA and RNA. They're often called bases for short. The four bases in DNA are cytosine, guanine, adenine and thymine.In RNA the first three are the same, and uracil replaces thymine.
Canto: Fine, you've named some nucleobases, but what are they?
Jacinta: Well, if you want their chemical structure, you'll find all that here. But aren't we getting a little bogged down?
Canto: No, no, there's no use talking of RNA until we understand the basics, and that includes bases. Let's take it slowly, inch by painstaking inch.
Jacinta: Okay, you're the boss.

Source: Rebecca Stott: Darwin and the Barnacle. Faber & Faber, 2003.