Genetic information flows from DNA to RNA but never from RNA back into DNA.

A simple diagram of molecular biology is easy to remember: DNA is copied into RNA, and RNA is used to make protein.

That common summary encouraged an extra assumption—that the DNA-to-RNA arrow could never run in reverse.

Some RNA tumor viruses did not fit comfortably with that rule. Howard Temin proposed that after entering a cell, such a virus might make a DNA copy of its RNA. In 1970, Temin and David Baltimore independently found the enzyme that makes this possible.

The enzyme is called reverse transcriptase. It reads an RNA molecule and builds a corresponding DNA molecule.

Retroviruses use this process as part of their normal life cycle. After infection, viral RNA is copied into DNA, and that DNA can be inserted into the host cell’s genome.

Reverse transcription also occurs outside retroviruses. Related mechanisms are used by retrotransposons and by telomerase, an enzyme involved in maintaining chromosome ends.

There is an important historical nuance. Francis Crick’s carefully stated central dogma did not actually forbid RNA-to-DNA transfer. The discovery overturned a widespread simplified version of the rule, not the most precise formulation Crick gave.