FIGURE 1. MicroRNAs in action.

In the early days of molecular biology, RNA was known only as a way of transferring information from DNA to protein. But microRNAs do not make protein and they prevent protein from being made when they bind to messenger RNAs. a, When unbound, messenger RNA of the TCP gene-transcription factors makes protein. Each set of three nucleotides (green) is a codon for a specific amino acid (blue). b, The JAW microRNA (red) binds by complementary base pairing to the TCP messenger RNA and leads to cleavage of the messenger RNA, thus preventing protein from being made. c, Palatnik et al.⁶ changed the third nucleotide in the codons of the TCP messenger RNA. This drastically reduces the ability of the JAW microRNA to bind to its target, but does not change the protein that is made. Introducing the gene for this modified TCP messenger RNA into wild-type plants resulted in defective leaf development, indicating that microRNAs are controlling the process.