the central dogma of molecular biology
The central dogma is the single sentence that organizes the whole subject: in a cell, sequence information generally flows from DNA to RNA to protein. DNA is the master copy of the instructions; the cell makes RNA copies of the bits it needs (transcription); and those RNA copies are read out to build proteins (translation). Once that information has reached protein, it does not get written back into the sequence of a gene. Think of it as reading a master cookbook, photocopying a recipe, and cooking the dish: you can copy and cook, but tasting the finished dish never edits the original recipe.
Francis Crick stated the idea in 1958 and clarified it in 1970, and it is worth being precise about what he actually claimed, because the name is often misunderstood. Crick's rule was about the direction in which detailed sequence information can be transferred. He said information can flow from nucleic acid to nucleic acid and from nucleic acid to protein, but it can never flow out of protein back into nucleic acid or into another protein. He did not say RNA can never make DNA, and he did not say a protein can never affect which genes are used.
This matters because the famous misstatement 'DNA always makes RNA and never the reverse' is simply wrong. Reverse transcriptase, an enzyme found in retroviruses such as HIV, copies RNA back into DNA, and that backward step is still completely consistent with the central dogma, because the information stays within nucleic acids. What the dogma rules out is the one thing nature appears never to do: take the amino-acid sequence of a finished protein and use it as a template to write a new gene. That asymmetry is one of the deepest facts in biology.
HIV is a retrovirus: it carries its genes as RNA and, on infecting a cell, uses reverse transcriptase to make a DNA copy that inserts into the host genome. This famous backward step (RNA -> DNA) does not break the central dogma, because information never leaves a protein to become a gene.
Reverse transcription runs information 'backward' yet still obeys the dogma.
The single most common error is to state the dogma as 'DNA to RNA to protein and never the reverse'. The real claim is narrower: information that has reached protein can never flow back into nucleic acid. RNA-to-DNA (reverse transcription) is allowed and happens.