The step in protein synthesis where RNA directs the assembly of a polypeptide is called translation.

Here's a breakdown:

* Transcription: DNA is copied into mRNA (messenger RNA).

* Translation: mRNA is used as a template to assemble a chain of amino acids, forming a polypeptide.

During translation, the mRNA sequence is read by ribosomes, which then recruit tRNA (transfer RNA) molecules. Each tRNA carries a specific amino acid, and they bind to the mRNA according to the genetic code. The ribosome links the amino acids together, forming the polypeptide chain.