Not how but why does acid rain form?

Burning of fossil fuels emits sulphur dioxide.  
S (in coal) + O2SO2  

Sulphur dioxide reaches the atmosphere, it oxidizes to first form a sulphate ion.  

2 SO2 + O2 2 SO3 

The sulphate ion joins with hydrigen atoms in the air and becomes sulphuric acid. This then falls back down to earth as "acid rain"         

SO3 + H2O H2SO4

Hope this at least helped a little c:

Ribonuclease activity is a type of nuclease that catalyzes the degradation of rna into small components.