8.2 Secondary Structures of RNA
8.2.1 A-form double helix
Figure 8.4: A Form Double Helix RNA
Two strands of RNA that are complementary to one another can form an A form7 double helix (just like the A form of DNA). However, do note that most RNA molecules in vivo are produced in the single-stranded state!
Nonetheless, single-stranded RNA molecules (like the one shown in figure 8.4) fold up into structures that maximize the number of self-complementary double-stranded regions. Hence, there are also regions of A form double helix RNA that are accompanied by a great variety of loops, bulges, distortions, and various other pairings.
8.2.2 Bulges
Figure 8.5: RNA Bulges
RNA bulges can involve a single nucleotide, three nucleotides, or a hairpin loop (see figure 8.5).
8.2.3 Internal loops
Figure 8.6: Internal RNA Loops
Like figure 8.6 suggests, there are many kinds of internal loops present in RNA: mismatch pairs (i.e., a symmetric internal loop of two nucleotides), symmetric internal loops, and asymmetric internal loops.
The 2’ OH group in RNA makes it impossible for RNA to form a B-form double helix!↩︎