Is RNA present in the cytosol as double stranded structures? I know that tRNA can assume double stranded structure but isn't mRNA single stranded? I just want to make sure because I was reading about northern blots and it was states that formaldehyde agarose gel is used to separate the RNA bands specifically to denature RNA and stop it from assuming a self complementary structural shape. Or is formaldehyde used because some RNA assumes this shape and so to equilibrate the solution formaldehyde is used to ensure all RNA is single stranded. Would you deem this principle similar to how sds is used in sds page for protein?

RNA is mostly present as single-strand, but it can form secondary structures with itself or other nucleic acids. Thus a certain level of denaturation is required to prevent that. The role for SDS in proteins serves several purposes, actually, of which denaturation is only one.