I am new to research of miRNA and I have been asked to do a knockdown on a miRNA by miRNA sponge technology. However, I am still confused about the maturation process of miRNA.

According to figure 2 of this review

http://www.ncbi.nlm.nih.gov/pubmed/?term=MicroRNA+biogenesis%3A+coordinated+cropping+and+dicing.

One of the strand will be degraded.

However, according to another reference, both 5p and 3p strands will become mature miRNA.

http://abcommunity.lifetechnologies.com/community/real-time_pcr/blog/2013/03/29/why-so-many-different-mirna-names-3p-vs-5p-what-does-it-mean

I am so confused, which strand should I targeted for when I design the plasmid of miRNA sponge?

I want to knockdown mir221

http://www.ncbi.nlm.nih.gov/nuccore/NR_029806.1

but it doesn't annotate which strand would be the mature strand.