A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2016. / Primary microRNA (pri-miRNA) mimics have been shown to mediate effective gene silencing and are well-suited for therapeutic applications. Pri-miRNA mimics, like natural pri-miRNA, are processed in the endogenous microRNA (miRNA) biogenesis pathway. Elements of the secondary RNA structure are crucial for processing by the Drosha-DGCR8 microprocessor, including a basal stem of - 11 bp. However, structural variation is common and the exact determinants of pri-miRNA processing have been elusive. The aim of this project were to explore the use of natural pri-miRNAs with exceptional basal stem in the design of correspondingmimics and to identify optimal stem features.[Abbreviated Abstract. Open document to view full version] / LG2017