Folding of RNA molecules

The stability of RNA molecules is much increased if, similarly to what happens in DNA, the hydrophobic bases may get away from the aqueous medium. This can happen by formation of hydrogen bonds between complementary bases although, contrary to DNA, both bases belong to the same RNA chain.

That is, the chain folds onto itself in zones where the sequences are partially complementary, forming short stretches of double helix.

Here we can see, as an example, the structure of the 5S ribosomal RNA from Xenopus laevis (formed by 128 nucleotides). Other RNAs are much bigger and adopt more complex structures.

In the case of rRNAs and tRNAs, it is common that a major part of the molecule has complementary sequences that allow this kind of folding.

sphere or spacefilling model.

backbone (hydrophilic, polar) and bases (hydrophobic, nonpolar).

Note how some bases, not having a complementary base opposite, turn outwards.

Let's get a closer view of one of the loops or places where the chain turns onto itself, the end of a double helix stretch: 
Note the base pairs (face to face in the same plane) and bases in the loop, not paired.