RNA
RNA (ribonucleic acid) is a nucleic acid molecule present in all living cells that serves essential functions in gene expression and protein synthesis. Like DNA, RNA is composed of nucleotides with a sugar-phosphate backbone and nitrogenous bases. However, RNA typically exists as a single strand rather than the double helix structure of DNA, and contains the sugar ribose instead of deoxyribose. The structural differences between RNA and DNA reflect their distinct biological roles and chemical properties.
Structure and Composition
RNA molecules are built from four types of nucleotides, each containing one of four bases: adenine, guanine, cytosine, and uracil (uracil replaces the thymine found in DNA). The single-stranded nature of most RNA allows it to fold into complex three-dimensional structures that are crucial for its function. These secondary and tertiary structures, formed through base pairing within the molecule itself, enable RNA to perform catalytic and regulatory functions beyond simple information storage.
Biological Functions
RNA acts as an intermediary between the genetic instructions encoded in DNA and the proteins synthesized by cells. Messenger RNA (mRNA) carries genetic information from DNA to ribosomes, where the genetic code is translated into amino acid sequences. Transfer RNA (tRNA) brings amino acids to the ribosome during protein synthesis, while ribosomal RNA (rRNA) is a structural and catalytic component of the ribosome itself. Additionally, many non-coding RNAs regulate gene expression, control cellular processes, and participate in defense mechanisms against viral infections and foreign genetic elements.