RNA — Structure and Types

Ribonucleic acid, or RNA, is chemically very similar to DNA but differs from it in three important structural respects. First, the pentose sugar in RNA's nucleotides is ribose rather than deoxyribose — ribose carries an extra hydroxyl (-OH) group at its 2' carbon that deoxyribose lacks, and this single chemical difference is what makes RNA generally less chemically stable than DNA, and therefore better suited to being a short-lived, disposable "working copy" of genetic information rather than DNA's long-term, stable storage role. Second, RNA uses the pyrimidine base uracil (U) in place of DNA's thymine (T); uracil pairs with adenine just as thymine does, so this substitution does not change the logic of complementary base pairing at all. Third, and most fundamentally, RNA is normally single-stranded, whereas DNA is normally double-stranded — although a single RNA strand can still fold back on itself and base-pair internally, producing complex secondary structures (as seen very clearly in tRNA, described below).

Because most RNA molecules exist only fleetingly and serve immediate, functional roles inside the cell rather than acting as permanent hereditary storage, RNA's genetic role in most organisms is described as being an intermediate messenger and functional working molecule, rather than the genetic material itself (RNA viruses are the interesting exception, using RNA as their own genetic material). In cells, however, RNA is typically further specialised into three main functional types, each playing a distinct role in gene expression. Messenger RNA (mRNA) carries the coded genetic information copied from a gene on DNA (during transcription) to the ribosome, where that code is read out during translation to build a protein — mRNA is therefore often described as carrying a "readable copy" of a gene's instructions. Ribosomal RNA (rRNA) is a major structural and catalytic component of the ribosome itself, the cellular machine on which translation takes place; rRNA molecules combine with ribosomal proteins to build up both the small and large subunits of the ribosome. Transfer RNA (tRNA) is the smallest of the three RNA types and acts as an adapter molecule during translation, physically carrying a specific amino acid to the ribosome and matching it to the correct triplet codon on the mRNA via its own complementary anticodon sequence — tRNA folds into a characteristic cloverleaf secondary structure through internal base pairing, which then further folds into a compact, L-shaped three-dimensional tertiary structure. …