When eukaryotes make mRNA, they add a 5' cap and a poly-A tail to protect the mRNA from being destroyed by ribonucleases (enzymes that break down RNA). But why do cell have ribonucleases to begin with? Why would cells to break down the mRNA it went through all the trouble to make? Use an example to explain your answer.

In an experiment examining the effects tai chi on arthritis pain, callahan (2010) selected a large sample of individuals with doctor-diagnosed arthritis. half of the participants immediately began a tai chi course and the other half (the control group) waited 8 weeks before beginning. at the end of 8 weeks, the individuals who had experienced tai chi had less arthritis pain that those who had not participated in the course.