Final answer:
The blueprint for a protein is originally carried by DNA, with mRNA serving as a temporary copy used during protein synthesis.
Step-by-step explanation:
The molecular sequence, or blueprint for a protein, is originally carried by DNA. The cellular process known as transcription generates messenger RNA (mRNA), which is a mobile copy of the DNA sequence of one or more genes. mRNA carries the genetic message from the DNA in the cell's nucleus to the ribosomes in the cytoplasm, where protein synthesis (translation) occurs. During this synthesis, tRNA brings specific amino acids to the ribosome to be incorporated into the growing polypeptide chain based on the codon sequence of the mRNA.
Therefore, the answer to the question is B. DNA. DNA contains hereditary information that is handed down from one generation to the next. mRNA, on the other hand, is like a temporary copy of the genetic information that cells use during protein synthesis.