Nonproteincoding RNA

In addition to the protein-coding transcripts, an organism's DNA produces other RNA transcripts that assist with the production of proteins. These other RNAs fall into several categories. For purposes of this book, I won't bore you by listing them. All you need to remember is that the most important category for evolution is ribosomal RNA.

Ribosomal RNA (abbreviated rRNA) is the class of RNA molecules that makes up the ribosomes, the cellular factories that produce proteins.

Ribosomal RNA is of special interest to evolutionary biologists because all organisms need ribosomes for protein production; therefore, ribosomal RNA can be used for a couple key evolutionary tasks:

1 Understanding relationships between different organisms: Ribosomes provide a character that can be compared across all branches of life. Most characteristics aren't shared across all branches of life. Take eyes, for example. Few things have eyes. As a result, comparing a human with, say, a stalk of broccoli and a mushroom based on similarities and differences in the structure of the eye is impossible. But humans, mushrooms, and stalks of broccoli do all have ribosomal RNA. (In case you're curious, you and the mushroom are a fair bit more similar to each other at the ribosomal level than either of you is to the broccoli!)

1 Determining historical relationships between species: Beyond explaining the level at which people and fungi share similarities, the really fascinating thing about ribosomal RNA is that evolutionists can use nucleotide sequences to determine the historical relationships among species. In other words, it can help clarify which branch of the tree of life an organism belongs to. You can read more about the tree of life in Chapter 9.

Ribosomal RNA isn't important just as a tool for evolutionary biology. It's also important to the proper functioning of the cell — so much so that 80 percent of the RNA in a given cell can be ribosomal RNA. The other categories of non-coding RNA are transfer RNA (abbreviated tRNA), which is involved in assembling the amino acids that make a protein, and a growing collection of small RNA molecules (we keep discovering new ones) that seem to be involved with the regulation of gene expression — a fancy way of referring to the process of deciding which genes get turned on and off in any given cell. (Think about it: All your cells have all your genes, but the genes needed to make your eyes aren't expressed in your fingers, or vice versa.)

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