The iconic image of DNA is the double helix, a twisted ladder holding the genetic blueprint for most life. This leads many to wonder if its molecular relative, ribonucleic acid (RNA), shares this form. The answer is complex, as RNA’s structure is more varied and dynamic due to its chemical makeup and diverse functions within the cell.
RNA’s Predominantly Single-Stranded Structure
Unlike DNA, RNA is most often found as a single strand of nucleotides. This difference stems from two variations in its chemical composition. The first is the sugar in the molecule’s backbone: RNA has ribose, while DNA has deoxyribose. Ribose has a hydroxyl (-OH) group on the second carbon of its ring that deoxyribose lacks.
This hydroxyl group makes the RNA molecule more reactive and prone to breaking down. It also introduces steric hindrance—meaning the atoms are physically in the way—making it difficult for two RNA strands to form a stable, long double helix like DNA. The second difference is in the nitrogenous bases. While both molecules use adenine (A), guanine (G), and cytosine (C), RNA uses uracil (U) instead of DNA’s thymine (T).
This single-stranded nature also contributes to RNA molecules being much shorter than DNA molecules. A human DNA chromosome can be 250 million nucleotide pairs long, while most RNA molecules are only a few thousand nucleotides. This compact, transient nature is related to its role as a temporary message, not a permanent genetic archive.
When RNA Folds Into Helical Shapes
Although RNA is a single strand, it rarely remains a simple linear chain. The flexible strand can fold back on itself, allowing complementary bases along its length to pair: adenine with uracil (A-U) and guanine with cytosine (G-C). This self-pairing creates short, double-stranded helical regions.
These localized double-stranded segments determine RNA’s three-dimensional structure. A common motif is the hairpin loop, where the strand forms a stem of paired bases and a loop of unpaired bases. These specific folded structures are required for the RNA’s function.
For example, transfer RNA (tRNA), which helps build proteins, folds into a defined cloverleaf shape with several helical stems. This precise 3D structure allows tRNA to be recognized by other molecules and perform its job. These folded helices are distinct from DNA’s continuous double helix, as they are localized structures within one self-folding strand.
Double-Stranded RNA in Viruses
The main exception to RNA’s single-stranded state is found in some viruses. Viruses like rotavirus, a cause of severe diarrhea in infants, use RNA as their genetic material instead of DNA. In these cases, the viral genome is double-stranded RNA (dsRNA), forming a complete double helix similar to DNA with two separate strands.
For these viruses, the dsRNA genome is a stable way to store genetic information. Inside a host cell, viral dsRNA can trigger an immune response. Human cells detect dsRNA as a sign of viral infection, which leads to the production of antiviral proteins.
This viral dsRNA differs from the short, folded helices in cellular RNAs like tRNA. In dsRNA viruses, the two strands are complementary along their entire length, forming a continuous helix. This shows that RNA can adopt a DNA-like structure when it serves a purpose, such as forming a compact viral genome.
How Structure Determines Biological Role
The structural differences between DNA and RNA are tied to their distinct biological roles. DNA’s stable and rigid double helix is suited for its primary function: long-term storage of the genetic blueprint. Its stability protects the genetic code from damage and ensures information is passed on accurately, while the double-stranded format also facilitates replication and repair.
In contrast, RNA’s single-stranded, flexible, and less stable nature is suited for its diverse and temporary tasks. Messenger RNA (mRNA) is a short-lived transcript of a gene, carrying instructions from DNA to the cell’s protein-making machinery. Its transient nature allows cells to produce proteins only when needed and to adjust to changing conditions by degrading the mRNA to halt production.
Other types of RNA, such as tRNA and ribosomal RNA (rRNA), depend on folding into specific three-dimensional shapes to function. As mentioned earlier, tRNA’s shape allows it to participate in protein synthesis. Ribosomal RNA is a structural and catalytic part of ribosomes, where its intricate folding creates the active sites for building proteins, linking RNA structure directly to enzymatic function.