Final answer:
François Jacob and Jacques Monod proposed the existence of messenger RNA in 1961, which was confirmed by experiments by Sydney Brenner.
Step-by-step explanation:
The existence of messenger RNA (mRNA) was proposed by French scientists François Jacob and Jacques Monod in 1961. Jacob and Monod hypothesized that mRNA was an intermediary between DNA and protein synthesis. This proposal was a significant step in understanding the central dogma of molecular biology, which describes the flow of genetic information within a biological system. The validity of their hypothesis was later confirmed by experiments conducted by scientists like Sydney Brenner, who performed classic experiments showing that mRNA carries information from genes to ribosomes for protein synthesis. These discoveries paved the way for detailed studies of the genetic code and set the foundation for modern molecular biology.