Ribonucleic acid, commonly known as RNA, is a fundamental molecule present in all living cells. It plays a central role in converting genetic information from DNA into proteins, the workhorses of the cell. RNA molecules are typically single-stranded, consisting of a single chain of nucleotides. This single-stranded nature is crucial for its diverse functions in carrying genetic codes and facilitating protein production.
The Standard: Single-Stranded RNA
A typical RNA molecule exists as a single, linear chain of nucleotides, setting it apart from the double helix structure of DNA. This single-stranded form arises from chemical distinctions in its molecular makeup. RNA contains the sugar ribose in its backbone, unlike DNA which has deoxyribose; ribose has an extra hydroxyl group, making RNA less stable than DNA.
Another chemical difference is the presence of uracil (U) instead of thymine (T) as one of its four nitrogenous bases. While DNA pairs adenine (A) with thymine (T), RNA pairs adenine (A) with uracil (U). These chemical features contribute to RNA’s flexibility and its tendency to remain single-stranded, unlike DNA’s preference for a double-helix configuration.
Why RNA’s Single Strand is Key to Its Roles
The single-stranded nature of RNA is important for its biological functions. This flexibility allows RNA molecules to fold into specific three-dimensional shapes. These shapes are important for their biological activity, enabling RNA to perform specialized tasks.
Messenger RNA (mRNA) carries genetic instructions from DNA to ribosomes, where proteins are assembled. Transfer RNA (tRNA) molecules, with their distinctive structures, act as adaptors, bringing amino acids to the ribosome during protein synthesis. Ribosomal RNA (rRNA) forms the structural and catalytic components of ribosomes, facilitating the formation of peptide bonds between amino acids. The single-stranded flexibility of these RNA types allows them to interact precisely with other molecules and carry out their distinct cellular functions.
When RNA Forms More Complex Structures
While RNA is single-stranded, its molecules often fold back on themselves, creating internal structures. These structures arise from base pairing within the same strand, forming regions like hairpin loops, bulges, and internal loops. Such folding results in secondary structures, which then arrange into more complex tertiary structures, such as the L-shape of tRNA or the architecture of rRNA within ribosomes.
These internal pairings are not equivalent to a double helix like DNA, but rather a single strand bending and interacting with itself. In rare instances, some viruses possess true double-stranded RNA (dsRNA) genomes, where two entirely separate RNA strands pair together. This double-stranded viral RNA is distinct from a single RNA strand’s folding patterns and plays a role in the viral life cycle and host immune responses.