To send instructions given by DNA to the rest of the body.
Because it is the one on which the DNA info is copied, and then it goes to the ribosome and brings him the "map" to synthesize the proteins encoded in your DNA.
RNA is necessary as a messenger because it transfers genetic information from DNA to ribosomes where proteins are synthesized. This process involves transcription to create mRNA and translation where mRNA is used to assemble proteins. Without RNA, the information in DNA could not be effectively utilized to produce proteins essential for life.
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