The molecule that serves as the template for protein synthesis is mRNA. During translation, mRNA carries the genetic code transcribed from DNA in the form of codons, which specify the sequence of amino acids in the protein. Ribosomes read the mRNA sequence to ensure accurate assembly of amino acids delivered by tRNA. Without mRNA, the ribosome would lack the instructions needed to build proteins, making it essential for gene expression and subsequent protein synthesis.

Reference: Molecular Biology of the Cell, Alberts et al., 6th Edition.