MRNA is called messenger RNA because

Our body is made up of different types of cells. ... All this means that specific cells produce specific proteins. Thus each cell requires to copy few specific recipes from the DNA stored within it. This is done by Transcription of genes on DNA to generate corresponding RNAs which are called messenger RNA.Jul 31, 2016

Explanation:

Cells of different tissues require to produce different proteins. This means that all the specific cells produce specific proteins. Thus each cell requires to copy few specific recipes from the dna stored within it. This is done by transcription of genes on dna to generate corresponding rnas which are called messenger rna

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a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.