Micro ribonucleic acid (miRNA) is a class of small, non-coding molecules that are fundamental to regulating gene expression within cells. These molecules are typically about 21 to 25 nucleotides in length and do not contain the instructions for building proteins like their counterpart, messenger RNA (mRNA). Instead, miRNA acts as a regulator by controlling the amount of protein produced from specific mRNA transcripts. This regulatory function is performed after the mRNA has been created, a process known as post-transcriptional regulation. Ultimately, the presence of a microRNA dampens or entirely stops the production of a particular protein.
The Genesis of microRNA
The process of creating a functional microRNA begins within the cell nucleus, where miRNA genes are transcribed by RNA polymerase II. This initial, long transcript is known as primary microRNA (pri-miRNA), which can be several thousand nucleotides long and contains one or more hairpin-shaped sections. The pri-miRNA is then recognized by a protein complex called the Microprocessor, which includes the enzyme Drosha. Drosha acts as a molecular scissor, cleaving the pri-miRNA to release the hairpin structure.
This cleavage results in a shorter, folded molecule known as precursor microRNA (pre-miRNA), which is approximately 70 nucleotides in length. The pre-miRNA retains a characteristic hairpin shape and possesses a two-nucleotide overhang. This distinct structure signals the molecule for export out of the nucleus and into the cytoplasm. The transport is facilitated by a protein called Exportin-5, moving the pre-miRNA into the cell where the next steps of processing occur.
Assembly into the RISC Complex
Once the pre-miRNA reaches the cytoplasm, it encounters the enzyme Dicer. Dicer recognizes the hairpin structure of the pre-miRNA and cuts away the loop, yielding a small, double-stranded segment of RNA approximately 22 nucleotides long, known as the microRNA duplex. The creation of this duplex is the final cutting step in the maturation pathway.
The microRNA duplex is then loaded onto the RNA-Induced Silencing Complex (RISC). The core component of RISC is a protein from the Argonaute family. During this loading process, the two strands of the duplex are separated. One strand, designated the mature or guide strand, is retained by the Argonaute protein, becoming the active component that guides the complex. The other strand, the passenger strand, is discarded and degraded.
Target Recognition and Gene Silencing
The activated RISC complex, with the single-stranded microRNA guide, scans the cytoplasm for messenger RNA (mRNA) molecules that carry complementary sequences. The microRNA uses a small sequence at its 5’ end, known as the seed region, to initiate binding to a matching sequence on the target mRNA. In human and animal cells, this binding most frequently occurs in the 3’ untranslated region (3’ UTR) of the mRNA.
The level of sequence matching determines the outcome of the silencing action. In mammalian cells, the binding is usually an imperfect or partial match. This partial complementarity leads primarily to translational repression, where the RISC complex physically blocks the cellular machinery from translating the mRNA into a protein. The partially bound mRNA is often shunted to specialized regions within the cell for storage or subsequent degradation, reducing protein output.
If the microRNA achieves a near-perfect sequence match with the target mRNA, a different outcome occurs. In this less common scenario, the Argonaute protein within the RISC complex acts as an endonuclease, directly cleaving and destroying the target mRNA. This immediate destruction permanently stops protein production from that specific transcript. MicroRNA-mediated silencing is a flexible regulatory system that can either repress protein synthesis or trigger mRNA destruction based on the precision of the sequence match.
Broader Biological Significance
The ability of microRNA to precisely regulate the expression of hundreds of genes makes it a fundamental component of cellular control. This fine-tuning is necessary for maintaining cellular homeostasis, the stable internal environment required for a cell to function correctly. MicroRNAs help cells adapt dynamically to changes in their environment, ensuring genes are expressed at the right time and in the correct amounts.
MicroRNAs are deeply involved in controlling cell differentiation and development. They help guide stem cells to mature into specialized cell types, such as muscle cells or neurons, by turning off the expression of inappropriate genes. Furthermore, microRNAs play a prominent role in the immune system, regulating the development and function of immune cells like T-cells and B-cells. They modulate the intensity of the immune response, helping the body manage both infection and the prevention of autoimmune reactions.