While RNA is built on the same backbone and structural concept as DNA (read this post for a refresher!), its slight biological differences lead to significant functional diversity. We’ll examine the structure of RNA molecules first, then dig more deeply into the myriad of functions performed by these molecules within (and even outside of!) the cell.
Here is the basic structure of a nucleic acid, like we saw before. Unlike DNA, the RNA molecule has a hydroxyl group (an oxygen bound to a hydrogen) bound to both red-marked carbon atoms on the central sugar molecule (which is why it’s called a ribose sugar instead of a deoxyribose sugar). This additional hydroxyl group adds a certain amount of instability to the RNA molecule, as it can attack the phosphate bond in the backbone and cleave the RNA strand if not part of a double helix – and RNA is quite often not found in a double helix.
Another difference between DNA and RNA is the nitrogenous bases used to transmit information. While RNA still uses four primary bases, the thymine of DNA is here replaced with uracil, and those four bases are modified in over one hundred different ways to fine-tune a given molecule’s function within the cell.
And considering how many roles RNA plays inside the cell, it makes sense that it would have this type of flexibility!
We’ll look at all (or most, anyway) of these functions individually, but here is a brief introduction to some of the most important, common, and well-studied functional forms of RNA:
ribosomal RNA (rRNA): these RNA molecules provide both structure and catalytic activity for the cell’s ribosomes – large complexes of protein and RNA that translate messenger RNA strands into proteins. rRNA makes up about 80-90% of all the RNA within a given cell, and is highly conserved across species (meaning that the rRNA sequence is relatively unchanged between different organisms, although it does differ enough to use it for rough species identification).
messenger RNA (mRNA): these molecules are RNA copies of specific regions of the cell’s DNA, which can exit the nucleus, find a ribosome, and be copied into protein. They are key for the cell’s self-regulation since they enable genes – DNA sequences coding for specific enzymes – to be expressed and the necessary enzymes to be produced.
transfer RNA (tRNA): these small molecules have a unique shape allowing them to bind to specific amino acids and recognize specific short RNA sequences on messenger RNA molecules, and essentially bring the ribosome the correct amino acids it needs to translate messenger RNA accurately.
micro RNA (miRNA): miRNA strands are only 20-22 base pairs long and contribute to the regulation of gene expression by degrading or preventing transcription of specific messenger RNAs within the cell.