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sci-modeling-bench
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design-bench

RNA is a directional polymer whose sequence and intramolecular interactions jointly determine an ensemble of structures. Watson–Crick A–U and G–C pairs, G–U wobble pairs, stacking, ions, temperature, and molecular context all contribute to folding. A single minimum-free-energy structure is a model of one favorable state, not a complete description of the conformations present in solution or in a cell.

Scope

Covered

RNA nucleotide identity, 5′-to-3′ polarity, and DNA-alphabet notation.
Canonical and wobble base pairing.
Secondary-structure elements, folding free energy, and structural ensembles.
Dependence of RNA structure on sequence and environment.

Not covered

A particular transcript, reporter construct, or sequence library.
A particular folding program or feature-engineering method.
Translation-specific regulatory mechanisms, which are covered separately.

Key concepts and notation

Term or symbol	Definition	Notes
5′ and 3′	Ends defined by ribose-carbon numbering	RNA sequences are conventionally written 5′ to 3′
A, C, G, U	RNA bases adenine, cytosine, guanine, and uracil	DNA uses T in place of U
Base pair	Hydrogen-bonded interaction between bases	A–U and G–C are canonical RNA pairs
G–U wobble	Common non-Watson–Crick RNA pair	Its geometry and stability differ from canonical pairs
Secondary structure	Pattern of base pairs and unpaired regions	Includes stems, hairpins, bulges, and internal loops
(\Delta G)	Gibbs free-energy change	Depends on temperature, ions, and the thermodynamic model

Core knowledge

Chemical identity and direction

RNA nucleotides contain ribose, phosphate, and one of A, C, G, or U. Adjacent nucleotides are connected by phosphodiester bonds, giving the polymer distinct 5′ and 3′ ends. Sequence direction is therefore chemically meaningful [1].

DNA-alphabet representations commonly replace RNA U with T. This substitution changes notation rather than the base order: a 5′-to-3′ DNA string can denote the corresponding RNA transcript by replacing every T with U.

Pairing and structural elements

Complementary segments within one RNA molecule can pair. A–U and G–C are the principal Watson–Crick pairs, while G–U is a frequent wobble interaction. Paired runs form helices or stems; unpaired regions form hairpin loops, internal loops, bulges, and multibranch junctions. Higher-order contacts can produce tertiary structure [1,2].

Base pairing alone does not determine stability. Base stacking, loop geometry, strand entropy, electrostatics, counterions, temperature, and solvent conditions also contribute [2,3].

Folding is an ensemble

At equilibrium, an RNA can populate multiple structures (s). Their relative probabilities are related to free energy:

[ P(s)=\frac{\exp[-\Delta G(s)/(RT)]} {\sum_{s'}\exp[-\Delta G(s')/(RT)]}. ]

Here (R) is the gas constant and (T) is absolute temperature. A minimum-free-energy structure is the structure with the lowest predicted (\Delta G) under a specified model. Alternative structures may nevertheless have appreciable probability, especially when their energies are close [3].

Cellular RNA is actively remodeled

In cells, RNA-binding proteins, helicases, ribosomes, chemical modifications, transcription kinetics, and molecular crowding can shift the structural ensemble. Structure measured or predicted for an isolated RNA in vitro may therefore differ from structure in a translating messenger ribonucleoprotein complex [2,4].

Conditions, limitations, and uncertainty

Thermodynamic parameters are empirical and are most reliable within the sequence, salt, and temperature regimes used to estimate them.
A more negative predicted folding free energy does not by itself identify where a structure forms or prove that it persists in a cell.
Long-range and pseudoknotted interactions are not represented equally by all secondary-structure models.
Sequence composition affects both possible pairing and other biochemical properties; an observed composition effect need not be caused only by folding.
DNA notation should not be confused with the chemical composition of the transcribed RNA.

Related knowledge resources

eukaryotic_mrna_and_translation_initiation: how ribosomes engage mRNA.
five_prime_utr_regulatory_elements: regulatory consequences of 5′-leader sequence and structure.

References

Alberts B, et al. Molecular Biology of the Cell, 4th ed. “The RNA World and the Origins of Life.” NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK26876/ [Textbook]
Mustoe AM, Busan S, Rice GM, Hajdin CE, Peterson BK, Ruda VM, Kubica N, Nutiu R, Baryza JL, Weeks KM. Pervasive Regulatory Functions of mRNA Structure Revealed by High-Resolution SHAPE Probing. Cell. 2018;173:181–195.e18. https://doi.org/10.1016/j.cell.2018.02.034
Turner DH, Mathews DH. NNDB: the nearest neighbor parameter database for predicting stability of nucleic acid secondary structure. Nucleic Acids Research. 2010;38:D280–D282. https://doi.org/10.1093/nar/gkp892
Bevilacqua PC, Ritchey LE, Su Z, Assmann SM. Genome-Wide Analysis of RNA Secondary Structure. Annual Review of Genetics. 2016;50:235–266. https://doi.org/10.1146/annurev-genet-120215-035034