Here's why:

* DNA is the master copy: DNA holds the original genetic code, but it stays safely tucked away in the nucleus of a cell.

* mRNA is the copy: mRNA is a copy of a specific gene from the DNA. This copy is made through a process called transcription.

* mRNA carries the code: mRNA then travels out of the nucleus to the ribosomes, where it acts as a blueprint for protein synthesis. The sequence of nucleotides in mRNA dictates the sequence of amino acids in the protein being built.

So, while DNA holds the original code, it's mRNA that acts as the blueprint for protein synthesis.