In eukaryotes, immediately after transcription, a newly formed RNA molecule has two structures added to it. At the 5' end, a methylguanylate cap is attached. At the 3' end, a long sequence of adenine nucleotides, called a poly(A) tail, is added. The cap and tail on a mature mRNA molecule protect it from enzymes called ribonucleases, which are enzymes in the cytoplasm that destroy mRNA molecules.
Why do these ribonucleases exist? What function would be served by an enzyme that destroys mRNA transcripts? Answer this question by using the example of an mRNA for a hormone that increases human heart rate.