Final answer:
The instructions for building a protein come from DNA, which is transcribed into mRNA. The mRNA then guides the assembly of amino acids into proteins at the ribosome with the help of rRNA and tRNA during translation.
Step-by-step explanation:
The instructions for building a protein originally come from DNA. The sequence of DNA is transcribed into messenger RNA (mRNA). During this process, the DNA's instructions are copied onto the mRNA, which then travels from the nucleus to the ribosomes. The ribosomes, which contain ribosomal RNA (rRNA), use the information on the mRNA as a guide to assemble amino acids into a protein. This process is known as translation, where transfer RNA (tRNA) plays a crucial role by bringing the correct amino acids to the ribosome based on the codons specified by the mRNA.