Question

Where are miRNAs encoded? Describe the steps in miRNA biogenesis. Identify two features of RNA cleavage events that occur in this pathway.

Answer 1

miRNAs are small RNA molecules encoded by eukaryotic genomes, involved in gene regulation. They are produced through transcription, cropping by Drosha, cytoplasmic processing by Dicer, assembly into RISC, and target recognition. RNA cleavage features specificity and enzymatic activity.

Step-by-step explanation:

MicroRNA (miRNA) are small non-coding RNA molecules encoded by eukaryotic genomes. They play a crucial role in gene regulation by targeting specific mRNAs for degradation or by inhibiting their translation. miRNAs are approximately 21-24 nucleotides long and function within the RNA-induced silencing complex (RISC).

Steps in miRNA Biogenesis

1. Transcription: MiRNAs are transcribed in the nucleus as primary miRNAs (pri-miRNAs) by RNA polymerase II.
2. Cropping: The pri-miRNA folds into a hairpin structure that is recognised and processed by the microprocessor complex, which includes Drosha, an RNase III enzyme, to form a precursor miRNA (pre-miRNA).
3. Cytoplasmic Processing: The pre-miRNA is exported to the cytoplasm and further cleaved by Dicer, another RNase III enzyme, yielding a miRNA duplex.
4. Assembly: One strand of the miRNA duplex is incorporated into RISC while the other is degraded.
5. Target Recognition: The RISC-miRNA complex binds to complementary sequences on target mRNAs leading to their downregulation either by degradation or translational repression.

Two features of RNA cleavage events in miRNA pathway include:

• Specificity: Both Drosha and Dicer specifically recognise and bind to distinct features of the miRNA precursors.
• Enzymatic activity: The RNA cleavage is enzymatic, relying on the RNase III activity of Drosha and Dicer.