In protein synthesis, the part of the DNA that is copied is a specific gene which contains the instructions for building a particular protein.

Here's how it works:

1. Transcription: The DNA sequence of the gene is copied into a messenger RNA (mRNA) molecule. This occurs in the nucleus of the cell.

2. Translation: The mRNA molecule travels out of the nucleus and into the cytoplasm, where it attaches to a ribosome. The ribosome reads the mRNA sequence in codons (three-nucleotide units), each of which codes for a specific amino acid.

3. Protein Assembly: The ribosome brings together amino acids in the order specified by the mRNA codons, creating a polypeptide chain. This chain eventually folds into a functional protein.

So, while the entire DNA molecule isn't copied, the specific gene containing the protein's code is transcribed and translated to create the final protein.