1), each with their own properties and functions. These small RNAs include: MicroRNAs (miRNAs). These are 22-24 nucleotides in length, and downregulate gene expression by attaching themselves to messenger RNAs (mRNAs) and preventing them from being translated into proteins.Short interfering RNAs (siRNAs).he major world of microRNAs. Steven Buckingham. Until recently, the way in which genes control cells was summed up by a simple formula: DNA makes RNA makes protein, and proteins are the cellular workhorses that carry out all the crucial tasks.