Types of RNA

There are three principal types of RNA, each with a distinct structure and role in gene expression. Messenger RNA (mRNA) is single-stranded and carries a working copy of the instructions needed to assemble amino acids into a specific protein; it is chemically very unstable and makes up only about 5% of a cell's total RNA. In prokaryotes, mRNA is often polycistronic, meaning a single mRNA molecule carries the coding sequences for several different polypeptides; in eukaryotes, mRNA is typically monocistronic, carrying the information for just one polypeptide per molecule. Transfer RNA (tRNA) translates the genetic code carried by mRNA and physically transfers the corresponding amino acids to the ribosome to be built into a protein; it folds into a highly elaborate three-dimensional structure and makes up about 15% of total cellular RNA - it is also sometimes called soluble RNA. Ribosomal RNA (rRNA) is single-stranded but, unlike mRNA, is metabolically stable, and it makes up the two structural subunits of the ribosome itself; rRNA constitutes roughly 80% of total cellular RNA, making it by far the most abundant RNA type. It exists as a polymer of widely varying length, from about 120 to 3,000 nucleotides depending on the specific rRNA molecule, and gives the ribosome its overall shape. Because gen …