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Micro RNAs (miRNAs) are molecules synthesized in the nucleus (more precisely called small noncoding RNAs) as long primary transcripts (pri-miRNA). This transcript is cleaved in the so-called microprocessor complex, made up of the enzymes Drosha and Pasha, into a stem-loop structure of about 70 base pairs (pre-miRNA).
The pre-miRNA then moves into the cytoplasm, where the RNA-induced silencing complex (RISC) is activated and cuts the pre-miRNA into 20-25mer oligonucleotides, which target corresponding mRNAs, which are then cleaved and inactivated.
Microprocessor Complex: Refers to the enzyme complex, where the major components are the enzymes called Drosha (a class II RNAse III enzyme) and Pasha (a double-stranded RNA binding protein, also known as DGCR8). This complex converts the primary RNA transcript (pri-miRNA) into a 70-base pair mi-RNA (pre-miRNA).
Dicer is an RNase III endonuclease enzyme that cleaves double-stranded RNA (dsRNA) and pre-microRNA (miRNA) into short double-stranded RNA fragments called small interfering RNA (siRNA) of about 22 nucleotides long that have a two-base overhang on the ends. Dicer initiates the first step in the RNA interference (RNAi) pathway. It is part of the RNA-induced silencing complex (RISC), whose catalytic component Argonaute is an endonuclease able to degrade messenger RNAs (mRNAs) whose sequence is complementary to the siRNA guide strand. Therefore, Dicer plays a role in RISC by providing the initial RNA material to activate the complex and the first RNA substrate molecule. Dicer cleaves dsRNA and pre-miRNAs to form a double-stranded 20-25 base pair RNA fragment.
RNA-Induced Silencing Complex (RISC): This refers to the multi-protein aggregate. In this complex, the RNAse enzyme Dicer is the principal endonuclease that cleaves dsRNA and binds short antisense RNA strands (guide strands), binding complementary strands. Upon binding the complementary strand, it activates RNase activity and cleaves the RNA. This mechanism is vital in both microRNAs' gene expression control and fending off viral infections, where double-stranded RNA is frequently the infectious vector.
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Diagram of the Micro RNA Process
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