The central dogma of molecular biology: The idea that, within a cell, information is transcribed from DNA into RNA and is translated from RNA to proteins. The DNA molecule is essentially a book containing the blueprints to make everything; RNA is a short copy of the blueprint that is necessary at a given moment; and the protein is the actual IKEA furniture.
Over the years, this one-way flow of information, so crucial to our understanding of molecular biology, has been refined through important discoveries. RNA can go back to DNA by a process known as “reverse transcription”, which is used by retroviruses like HIV and by human cells to elongate the ends of our chromosomes. Moreover, not all RNA molecules serve as templates for proteins. A subset of RNA molecules plays an important role in assisting the translation of RNA into protein, while other RNA molecules, like microRNAs, interfere with this process, essentially ensuring a particular RNA molecule is never turned into a protein.
The central dogma does however remain as a general rule. Like most rules, it has exceptions.