Ribonucleic acid (RNA) acts as a versatile intermediary in the expression of genetic information. While deoxyribonucleic acid (DNA) functions as the stable, long-term archive of hereditary material, RNA performs a wide array of active roles in the cell, from carrying instructions to regulating gene activity. RNA structure is similar to DNA but differs in three significant ways. RNA is typically a single-stranded molecule, making it more flexible and reactive, whereas DNA forms a stable double helix. The sugar component in RNA is ribose, which contains an extra hydroxyl group compared to the deoxyribose sugar found in DNA. Finally, RNA utilizes the nitrogenous base uracil (U) in place of the thymine (T) found in DNA, with uracil pairing with adenine.
Messenger RNA
Messenger RNA (mRNA) functions as the direct carrier of genetic instructions, acting as the intermediary between the DNA blueprint in the nucleus and the protein-building machinery in the cytoplasm. The process begins with transcription, where the sequence of a gene on the DNA strand is copied to create a complementary mRNA molecule. Once synthesized, this mRNA transports the message out of the nucleus.
The information carried by mRNA is encoded in a sequence of nucleotides read in groups of three, known as codons. Each codon specifies a particular amino acid, the building blocks of proteins. For example, the codon AUG signals for methionine and serves as the start signal for protein synthesis. By serving as a template, the mRNA dictates the precise order in which amino acids are linked to form a specific protein.
Transfer RNA
Transfer RNA (tRNA) acts as the molecular adapter that decodes the mRNA sequence into a growing chain of amino acids during protein synthesis. Each tRNA molecule is small, generally 70 to 90 nucleotides long, and folds into a unique three-dimensional L-shape. This structure bridges the nucleic acid language of mRNA with the amino acid language of proteins.
The tRNA structure features two specialized regions that perform its decoding function. At one end is the amino acid attachment site, where a specific amino acid is covalently linked. At the opposite end is the anticodon loop, which contains a three-nucleotide sequence called the anticodon. The anticodon is complementary to an mRNA codon, allowing the tRNA to bind accurately to the mRNA template and ensure the correct amino acid is delivered to the growing protein chain.
Ribosomal RNA
Ribosomal RNA (rRNA) is the main structural and catalytic component of the ribosome, the cellular machine responsible for assembling proteins. Ribosomes are complex structures composed of a large ribosomal subunit and a small ribosomal subunit. These subunits are built from multiple protein molecules combined with distinct strands of rRNA.
The rRNA performs the enzymatic work of protein synthesis, making the ribosome a ribozyme. The large subunit contains the peptidyl transferase center, a region made almost entirely of rRNA. This center catalyzes the formation of peptide bonds, the chemical links that join amino acids to extend the protein chain.
Regulatory and Processing RNAs
Beyond the machinery of protein synthesis, a group of non-coding RNAs exists whose primary function is to regulate gene expression and process other RNA molecules.
MicroRNA (miRNA)
MicroRNA (miRNA) is composed of short strands, around 22 nucleotides in length. MicroRNAs regulate gene expression at the post-transcriptional level. They bind to target messenger RNA molecules, either blocking the translation of the message into protein or promoting the degradation of the mRNA.
Small Interfering RNA (siRNA)
Small interfering RNA (siRNA) performs a similar regulatory function but is often associated with cellular defense mechanisms. These double-stranded RNA fragments, 20 to 24 nucleotides long, are effective at silencing specific genes. They trigger the cleavage and degradation of target mRNA that shares a complementary sequence. This mechanism is important for protecting the cell against viral RNA or for precise gene knockdown in research settings.
Small Nuclear RNA (snRNA)
Small nuclear RNA (snRNA) is involved in RNA splicing. Before mRNA can be translated, non-coding sections called introns must be removed, and the remaining coding sections (exons) must be accurately re-joined. Small nuclear RNAs associate with proteins to form complexes that recognize the boundaries of these non-coding sections and catalyze their removal, ensuring the mature mRNA contains the correct instructions for protein assembly.