Here's why:

* DNA contains the genetic code: DNA holds the blueprint for all the proteins your body needs. This blueprint is organized into genes, which are segments of DNA that code for specific proteins.

* Transcription: The process of creating mRNA from DNA is called transcription. It's like making a copy of a recipe from a cookbook. The DNA sequence (the gene) is used as a template to create a complementary mRNA molecule.

* Translation: The mRNA molecule then travels out of the nucleus to the ribosomes, where it is used as a template to build a protein. This process is called translation.

So, DNA is the source of the instructions, and mRNA acts as the messenger that carries those instructions to the ribosomes for protein synthesis.