Fig. 3.

The evolution of helix 25/ES 7 shows serial accretion of rRNA onto a frozen core. This image illustrates at the atomic level how helix 25 of the LSU rRNA grew from a small stem loop in the common core into a large rRNA domain in metazoans. Each accretion step adds to the previous rRNA core but leaves the core unaltered. Common ancestors, as defined in Fig. 1, are indicated. Pairs of structures are superimposed to illustrate the differences and to demonstrate how new rRNA accretes with preservation of the ancestral core rRNA. Each structure is experimentally determined by X-ray diffraction or Cryo-EM.