The order of amino acids in a polypeptide chain is determined by the sequence of nucleotides in messenger RNA (mRNA).

Here's how it works:

1. DNA to mRNA: DNA contains the genetic code for proteins. During transcription, the DNA sequence is copied into mRNA, which carries the code out of the nucleus to the ribosomes.

2. Codons: The mRNA sequence is read in groups of three nucleotides called codons. Each codon corresponds to a specific amino acid.

3. Translation: At the ribosome, transfer RNA (tRNA) molecules, each carrying a specific amino acid, recognize and bind to the mRNA codons. The order of codons on the mRNA dictates the order in which tRNA molecules, and therefore amino acids, are added to the growing polypeptide chain.

So, the order of nucleotides in mRNA directly determines the order of amino acids in the polypeptide chain.