6.1.6—RNA (mRNA) structure

RNA is a nucleic acid made from nucleotides and is typically a single polynucleotide strand. Messenger RNA (mRNA) is an RNA transcript copy of a gene that carries information from DNA to a ribosome.

• Sugar: DNA nucleotides contain deoxyribose; RNA nucleotides contain ribose.
• Bases: DNA uses adenine, thymine, cytosine and guanine (A, T, C, G). RNA uses adenine, uracil, cytosine and guanine (A, U, C, G); U replaces T in RNA.
• Strands: DNA is typically a double-stranded antiparallel helix. RNA molecules are typically single-stranded, so an RNA molecule is not automatically a second DNA-like helix.
• Stability and role: DNA is relatively stable and suited to retaining genetic information. mRNA is a more temporary, mobile transcript that can be read by a ribosome; this lets information be used without moving the DNA molecule.
• Shared structure: Both are polynucleotides with sugar–phosphate backbones joined by phosphodiester bonds and nitrogenous bases projecting from the backbone.

The sugar and base differences distinguish the two nucleic acids, while strand arrangement supports their typical roles: DNA provides a stable information store, whereas a single-stranded mRNA copy can carry a selected sequence to a ribosome. “Typically” matters—RNA structure is not being defined as an absolute rule that every RNA molecule must be single-stranded or equally short-lived.

mRNA is not DNA with thymine: it contains ribose and uracil. A DNA coding strand and its mRNA transcript can have a related sequence, but the RNA uses U where DNA uses T. This card establishes RNA structure and mRNA’s transcript role; detailed transcription/translation steps belong to the next protein-synthesis topic.