 Let us see the structures of ribonucleic acids. So mRNA is always single stranded when it is formed from DNA during the process of transcription. But soon after its formation, this single strand assumes a double helical conformation just after formation. This conformation is achieved mainly due to base stacking interactions. You might be remembering base stacking interactions which were also present in the DNA helix. But here it is slightly different, base stacking interaction is slightly different here. Here the interaction between purine and purine is stronger as compared to the base stacking interaction between purine and pyrimidine or pyrimidine and pyrimidine. So this is the helical structure which is assumed by a mRNA molecule. In this structure the white line is representing the nitrogenous bases while this green line representing the backbone consisting of ribose sugars and phosphoric acids. So this is a single stranded helical structure in contrast to the double helical structure of DNA where two strands are helically coiled around an imaginary axis. But here only single strand is coiled around an imaginary axis and it forms a helix. In the structure some self-complementary sequences may occur in RNA molecules and these self-complementary sequences may produce more complex structures. So RNA can base pair with complementary regions of either RNA of the same or of the other or DNA of molecules. RNA has no any regular secondary structure that serves as a reference point. As you know in case of DNA it is double helical structure which is a reference structure but here there is no any reference structure present in the ribonucleic acids. Rather the three-dimensional structures of many RNAs are complex and unique. Breaks in the helix caused by mismatched or unmatched bases in one or both strands are common and these breaks result in bulges or internal loops. It means in a RNA molecule there are complementary regions but there are also breaks where one strand is not complementary to the other. So at these points either a bulge is formed or a loop is formed. Similarly another type of loop which is called hairpin loop it may form between nearby self-complementary sequences. So this can be clear by this illustration. In this illustration you can see a number of secondary structures assumed by the ribonucleic acids. For example here you can see this is hairpin loop and this is internal loop. This is not hairpin loop and this is a small bulge where only one or two bases are not complementary to each other. So this small bulge is formed and if more than two three bases are not complementary to each other then a big bulge loop is formed. So at some points it is double helically coiled. At some point it is single-stranded. So in this way a number of secondary conformations may be present in a ribonucleic acid. So here you can see the structure of transfer RNA. This is the secondary structure assumed via molecule of tRNA. And similarly in this structure you can see there are number of loops and these are hairpin loops. And at number of points this is double-stranded. At some points it is single-stranded and it also contains a bulge. So this is the secondary structure of a tRNA molecule.