Final answer:
siRNA is preferred over miRNA because siRNA can completely block translation of a specific mRNA by cleaving it, while miRNA may only inhibit translation. siRNA also requires chemical modifications for in vivo application.
Step-by-step explanation:
siRNA is preferred over miRNA as a means for blocking translation of a specific mRNA because siRNAs are completely complementary to the mRNA transcript of the target gene, while miRNAs are mostly complementary. siRNAs can bind to RNA-induced silencing complex (RISC) and cleave the mRNA, completely blocking translation. On the other hand, miRNAs may only inhibit translation of the mRNA. In addition, unmodified siRNAs are highly efficient in cell culture but require chemical modifications for in vivo application.