Biochemistry, 4th Edition P39. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | What Are the Secondary and Tertiary Structures of RNA 343 Three-nucleotide bulge Hairpin loop Single-nucleotide bulge Symmetric internal loop Asymmetric internal loop FIGURE Bulges and loops formed in RNA when aligned sequences are not fully complementary. Adapted from Appendix Figure 1 in Gesteland R. F. Cech T. R. and Atkins J. F. RNA World 2nd ed. New York Cold Spring Harbor Press. Mismatch pair or symmetric internal loop of two nucleotides strongly tilted from the plane perpendicular to the helix axis see Figure . A-form double helices are the most prominent secondary structural elements in RNA. Both tRNA and rRNA have large amounts of A-form double helix. In addition a number of defined structural motifs recur within the loops of stem-loop structures such as U-turns a loop motif of consensus sequence UNRN where N is any nucleotide and R is a purine and tetraloops another class of four-nucleotide loops found at the termini of stem-loop structures . Stems of stem-loop structures may also have bulges or internal loops where the RNA strand is forced into a short single-stranded loop because one or more bases along one strand in an RNA double helix finds no base-pairing partners Figure . Regions where several stemloop structures meet are termed junctions Figure . Stems loops bulges and junctions are the four basic secondary structural elements in RNA. The single-stranded loops in RNA stem-loops create base-pairing opportunities between distant complementary single-stranded loop regions. These interactions FIGURE Junctions and coaxial stacking in RNA. Stem junctions or multibranched loops are another type of RNA secondary structure. Coaxial stacking of stems or stem-loops as in stacking of stem 1 on stemloop 4 is a tertiary structural feature found in many RNAs. Adapted from Figure 1 In Tyagi R. and Matthews D. H. 2007. Predicting coaxial stacking in multibranch loops. RNA 13 1-13. 344 Chapter 11 Structure of Nucleic Acids .
