Statistical significance of find a certain number of reads in a certain number of bases

Question

[isaacmtz_12]

I´m working with small RNAs which are produced by transposons, in my data set I have different transposons with different sizes, by chance I expect that transposons with more bases could produce more small RNAs, but in my data set it is no necessary true for all transposons. I want to know if you could recommend a statistical test that help me to decide if my data are significantly different than expected in a null model.

For example, I have 1743044 small RNAs in 860753789 bases, in 4415840 bases I expect 8942 small RNAs in a null model, but in the RNA-seq experiment I found 211372, that are produced in those 4415840 bases. Mi question is, how can I know how many significant is the difference between 8942 and 211372?.