Question 1

What is microRNA and how does it regulate gene expression?

Accepted Answer

MicroRNA (miRNA) is a small regulatory RNA approximately 22 nucleotides long that does not code for protein. Instead, it regulates gene expression by inhibiting messenger RNA (mRNA) translation into protein. MiRNA forms a complex with proteins called RISC (RNA-induced silencing complex), which binds to specific mRNA sequences through complementary base pairing, typically in the 3-prime untranslated region, silencing gene expression through mRNA cleavage or translation interference.

Question 2

How is mature microRNA produced from its precursor?

Accepted Answer

Mature miRNA is produced through sequential enzymatic processing. Primary miRNA (pri-miRNA) forms a stem-loop structure and is shortened by the Drosha enzyme into hairpin-shaped pre-miRNA within the nucleus. After methylation to prevent degradation, pre-miRNA is exported to the cytoplasm where the Dicer enzyme cleaves it into a 21-24 nucleotide miRNA duplex, then releases a single strand of mature miRNA ready for loading into RISC.

Question 3

What determines whether miRNA causes mRNA degradation or translation inhibition?

Accepted Answer

The extent of complementary base-pairing between miRNA and the 3-prime untranslated region of target mRNA determines the silencing mechanism. Extensive or near-perfect complementarity causes irreversible mRNA degradation. Limited base-pairing inhibits translation, which is reversible since stable mRNA can resume translation after repressors are eliminated.

Question 4

Why is dysregulation of microRNA associated with cancer?

Accepted Answer

Loss of let-7 miRNA is observed in lung, liver, breast, prostate, and ovarian cancers. Let-7 normally inhibits oncogenes—genes that promote cell growth, survival, and proliferation. When let-7 is lost, these oncogenes remain active, promoting tumor formation. Dysregulation of miRNA is correlated with potentially deadly diseases including cancer and heart disease.

Question 5

Where are microRNAs transcribed from in the genome?

Accepted Answer

MicroRNAs are transcribed from introns, which are non-coding regions within genes, or from intergenic regions, which are stretches of DNA located between genes. These genomic sources are processed through multiple enzymatic steps to produce biologically active mature miRNA that can regulate target mRNA molecules.

Question 6

How does the RISC complex function in gene silencing?

Accepted Answer

The RNA-induced silencing complex (RISC) is a protein complex that loads mature miRNA and guides it to complementary regions of target mRNA. Once bound through base-pairing interactions, RISC either facilitates mRNA degradation or blocks translation, depending on the degree of sequence complementarity. This mechanism represents an important type of post-transcriptional regulation in gene expression transcription splicing and translation.

Question 7

What happens to pre-miRNA after it is exported from the nucleus?

Accepted Answer

After pre-miRNA is methylated at its ends to prevent degradation and exported from the nucleus into the cytoplasm, the Dicer enzyme cleaves it into a 21-24 nucleotide miRNA duplex. Dicer then separates one strand from the duplex, releasing the single-stranded mature miRNA that is subsequently loaded into the RISC complex for target mRNA recognition.

MicroRNAs

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