Ribonucleic acid, commonly known as RNA, is a foundational nucleic acid present in all known living cells. It plays a central role in gene expression, acting as an intermediary that translates the genetic information stored in DNA into functional proteins. RNA molecules are involved in numerous cellular processes, from carrying genetic instructions to catalyzing biochemical reactions.
The Common Structure of RNA
RNA is a single-stranded molecule, distinguishing it from the double-helical structure often associated with genetic material. It is built from a chain of nucleotides, each with a five-carbon sugar (ribose), a phosphate group, and one of four nitrogenous bases. The four bases are adenine (A), guanine (G), cytosine (C), and uracil (U). Uracil replaces thymine (T) found in DNA, pairing with adenine during synthesis.
While generally single-stranded, RNA’s flexible nature allows it to fold back on itself. This often results in localized regions where complementary bases within the same strand pair up, forming short double-helical segments or stem-loop structures. These intramolecular base pairings, along with other interactions, enable RNA to adopt complex three-dimensional shapes. These folds are important for RNA’s specific functions.
Key Differences Between RNA and DNA
RNA and DNA are both nucleic acids, but they exhibit distinct structural features that influence their biological roles. A primary difference lies in their sugar components: DNA has deoxyribose, while RNA has ribose. Deoxyribose lacks an oxygen atom at the 2′ carbon position, making DNA more chemically stable and suitable for long-term genetic information storage. Ribose, with an extra hydroxyl (-OH) group at this position, contributes to RNA’s relative instability, fitting its role in transient cellular functions.
Another distinguishing feature is their nitrogenous bases. DNA uses adenine, guanine, cytosine, and thymine (A-T, G-C). In contrast, RNA contains adenine, guanine, cytosine, and uracil (A-U, G-C). This substitution of uracil for thymine further differentiates the two molecules. Finally, DNA exists as a stable double helix with two intertwined strands, whereas RNA is predominantly single-stranded.
Instances of Double-Stranded RNA and Complex Shapes
Despite its common single-stranded form, RNA can occur as a double helix in specific biological contexts. Double-stranded RNA (dsRNA) serves as the genetic material for certain viruses, such as rotaviruses and reoviruses, which cause diseases like gastroenteritis. In these viruses, the dsRNA genome is replicated and transcribed by viral enzymes within the viral capsid. The presence of dsRNA is uncommon in eukaryotic cells, often signaling a viral infection to the host’s immune system.
These self-complementary sequences form structures like stem-loops, prevalent in functional RNA molecules such as transfer RNA (tRNA) and ribosomal RNA (rRNA). For instance, tRNA molecules fold into a cloverleaf-like secondary structure and a compact L-shaped tertiary structure due to extensive internal base pairing. These complex three-dimensional arrangements are important for RNA’s diverse functions, including its ability to act as enzymes or participate in protein synthesis.