Ribonucleic acid (RNA) is a single-stranded nucleic acid molecule that plays a crucial role in the cellular machinery of life. While deoxyribonucleic acid (DNA) functions as the stable blueprint for genetic information, RNA acts as the dynamic intermediary, carrying out the instructions encoded in the DNA. This flow of information, from DNA to RNA to protein, is a foundational concept in biology known as the Central Dogma. Structurally, RNA differs from DNA in two key ways: it uses the sugar ribose instead of deoxyribose, and it substitutes the base uracil (U) for DNA’s thymine (T). This difference in sugar and its single-stranded nature make RNA less stable and more flexible for its multiple cellular roles.
Messenger RNA (mRNA)
Messenger RNA (mRNA) serves as the direct copy of a gene, carrying genetic instructions from the DNA in the nucleus to the ribosomes in the cytoplasm. The function of mRNA is to act as the template that dictates the precise sequence of amino acids needed to build a specific protein. The information on the mRNA strand is read in a series of three-base units called codons, each specifying a particular amino acid.
Before a eukaryotic mRNA molecule is ready, it must undergo several modifications known as RNA processing. A modified guanine nucleotide, called a 5′ cap, is added to one end; this protects the message from degradation and helps initiate translation. A long chain of adenine nucleotides, the poly-A tail, is added to the opposite end. The poly-A tail enhances stability and facilitates the mRNA’s export from the nucleus.
Non-coding segments called introns are precisely cut out from the initial RNA transcript, and the remaining coding segments, or exons, are spliced together. This processing ensures that only the correct protein-coding sequence is delivered to the ribosome. The resulting mature mRNA molecule is then ready to be translated into the final protein product.
Transfer RNA (tRNA)
Transfer RNA (tRNA) acts as the molecular adaptor, linking the information in the mRNA code to the specific amino acids that form a protein. Each tRNA molecule carries one specific type of amino acid to the ribosome. Despite their small size, tRNAs possess a distinctive three-dimensional L-shape.
This structure features two important regions: the anticodon loop and the acceptor stem. The anticodon loop contains a three-nucleotide sequence complementary to a specific codon on the mRNA template. The base-pairing between the codon and anticodon ensures the correct amino acid is delivered according to the genetic code.
The opposite end of the L-shape is the 3′ acceptor stem, which universally ends in the sequence CCA. This is the site where a specific amino acid is covalently attached, or “charged,” by enzymes called aminoacyl-tRNA synthetases. The accurate work of these synthetases ensures the right amino acid is matched to the right tRNA, which is necessary for the integrity of protein synthesis.
Ribosomal RNA (rRNA)
Ribosomal RNA (rRNA) is the most abundant type of RNA in the cell and forms the core structural and functional component of ribosomes, the complexes where protein synthesis occurs. Ribosomes are large molecular machines composed of rRNA molecules and numerous ribosomal proteins. These components assemble into two distinct subunits: one large and one small. The rRNA is synthesized and assembled with proteins in the nucleolus of eukaryotic cells.
The small ribosomal subunit is responsible for binding the mRNA and mediating the interaction between mRNA codons and tRNA anticodons. The large subunit contains the peptidyl transferase center, the site where the chemical reaction of protein bond formation takes place. Evidence indicates that the catalytic activity of the ribosome—the formation of the peptide bond—is performed entirely by the rRNA itself, not by a protein.
This makes the rRNA a ribozyme, an RNA molecule with enzymatic capabilities. The peptidyl transferase activity catalyzes the transfer of the growing polypeptide chain from the tRNA in one site to the amino acid carried by the tRNA in the adjacent site. This ability of RNA to perform catalysis underscores the ancient and central role of RNA in biological systems.
Small Regulatory RNAs
Beyond the molecules directly involved in protein synthesis, a diverse class of small regulatory RNAs works to fine-tune gene expression. These non-coding RNA molecules do not carry instructions for making proteins. Instead, they control the abundance and translation of other messenger RNA transcripts. Two prominent types are microRNAs (miRNAs) and small interfering RNAs (siRNAs).
MicroRNAs are short, endogenous RNA strands, typically 20 to 25 nucleotides in length, transcribed from the cell’s own genome. They function by binding to target mRNA molecules, usually with only partial sequence complementarity. This binding inhibits the process of translation or accelerates the degradation of the target mRNA, reducing the amount of protein produced from that gene.
Small interfering RNAs (siRNAs) are often derived from exogenous sources, such as viruses, or produced from long double-stranded RNA precursors. siRNAs bind to their target mRNA with perfect sequence complementarity. This highly specific binding triggers the cleavage and rapid degradation of the target mRNA, resulting in precise gene silencing. Both miRNAs and siRNAs operate by being loaded into a protein complex called the RNA-induced silencing complex (RISC), which locates and represses or destroys their target messages.