Problem:

I am learning RNA-seq analysis. I always encounter this phase "poly(A)+ RNA". After searching, I got this: "Most messenger RNAs contain a poly(A) tail, while structural RNAs do not. Poly(A) selection therefore enriches for messenger RNA. The technique has proved essential for construction of cDNA libraries."

Question: Does it mean that when constructing the cDNA, we always use the ones with a A's tail? Then other RNAs like tRNA are filtered and not used to get their cDNA? I do find some RNAs records on UCSC where microRNAs are included.

Question: When we say poly(A)+ RNA library, what is the difference between normal one? Or normal RNA library is a poly(A)+ RNA library?