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9H-xanthine is a neutral tautomer of xanthine in which the indicated hydrogen is attached to nitrogen9 of the purine framework. The oxo groups remain at positions2 and6. ChEBI identifies this particular form as CHEBI:17712, distinct from the broader tautomer-unspecified xanthine entry.^[ref]

Its molecular formula is ${\mathrm{C}}_5{\mathrm{H}}_4{\mathrm{N}}_4{\mathrm{O}}_2$. The prefix 9H is an atom-location convention, not a count of nine hydrogens: the entire neutral molecule contains four hydrogens. As a simple inventory check, it contains $5+4+4+2=15$ atoms in all.

7H-xanthine has the same elemental formula and net charge but a different indicated hydrogen position. A mass measurement consistent with that formula therefore does not alone assign the 9H form. The distinction concerns tautomerism, rather than the gain of an extra proton or a change in carbon/oxygen composition.^[ref]

Naming 9H-xanthine specifies a structural form; it does not by itself establish its fraction in a solution. That requires evidence or a model for the relevant environment and equilibrium.

An anti glycosidic conformation is a base–sugar orientation in the anti region of the glycosidic torsion angle $\chi$. Under the IUPAC-IUB atom convention, this region is centered at $180^{\circ }$ and extends90degrees on either side. Equivalently, for an angle reported in $[0^{\circ },360^{\circ })$, the broad anti region lies between90 and270degrees.^[ref]

For a purine nucleoside, $\chi$ uses O4′–C1′–N9–C4; for a pyrimidine, it uses O4′–C1′–N1–C2. The ordered atoms specify the dihedral angle. An angle reported using different outer atoms cannot be assigned to this region by its numerical value alone.^[ref]

For an original example, $\chi =-160^{\circ }$ and $\chi =200^{\circ }$ describe the same orientation modulo one full turn, and both are anti. Moving to $\chi =60^{\circ }$ changes the orientation into the syn glycosidic conformation region while retaining the base–sugar bond connectivity. Conformational change and a change in covalent constitution are different descriptions.

The word anti here does not describe how two bases pair with one another. RNA base-pair geometry can separately classify the relative orientations of two glycosidic bonds. Nor does a single anti torsion determine the entire nucleotide or RNA structure: sugar geometry and the other backbone angles remain to be specified.

Drosophila melanogaster rosy, usually written ry, is a protein-coding gene encoding xanthine dehydrogenase, an enzyme involved in purine degradation. FlyBase identifies it as FBgn0003308 and CG7642. The named gene, its enzyme product and the visible eye-colour phenotype used in genetic experiments are different levels of description.^[ref]

The locus became an informative marker in early P-element vector experiments. A 1982 study transferred a wild-type rosy-containing DNA fragment into mutant embryos and obtained animals with inherited chromosomal copies and wild-type eye colour. The phenotype made the introduced sequence's function visible; molecular and inheritance measurements established that more than a transient signal had occurred.^[ref]

As a teaching comparison, two insertions could both restore the marker phenotype while carrying different neighbouring DNA or occupying different genomic positions. The shared visible outcome would not establish identity of the complete constructs. A marker therefore helps select or recognize an outcome, while sequencing, mapping and transmission answer additional questions about what was introduced and where it resides.

In a nucleoside or nucleotide, the glycosidic torsion angle $\chi$ describes rotation of the nucleobase relative to the sugar about their N-glycosidic bond. It is a signed dihedral angle defined by four ordered atoms:The middle two atoms specify the glycosidic bond; the outer atoms define the two reference planes. Thus the atom sequence, not the symbol $\chi$ alone, fixes the angle convention.^[ref]

The recommended broad regions are syn glycosidic conformation, centered at $0^{\circ }$ with a90degree range on either side, and anti glycosidic conformation, centered at $180^{\circ }$ with the corresponding range. For example, $\chi =60^{\circ }$ lies in the syn region and $\chi =-160^{\circ }$ in the anti region. These describe base–sugar orientation; they do not by themselves specify sugar-ring shape or the complete RNA conformation.^[ref]

An original angular comparison illustrates periodicity. Measurements of $179^{\circ }$ and $-179^{\circ }$ differ by only2degrees around the circle, not358degrees. A signed shortest difference can be calculated asusing radians inside the trigonometric functions. Interpreting the two near-boundary values as distant conformations would be an artifact of the angle's reporting interval.

The convention is important when comparing structural models or older literature. Alternative choices using C8 for a purine or C6 for a pyrimidine can shift the numerical angle by approximately180degrees. Numerical torsions should therefore be compared only after checking their defining atoms.^[ref]

A GNRA tetraloop is an RNA tetraloop with G at its first position, A at its fourth, any ordinary RNA nucleotide at N, and a purine at R. R therefore means A or G, not one particular nucleotide. GAAA and GAGA both match, while GACA does not because C is not a purine. There are $4\times 2=8$ allowed loop sequences under this alphabet.

A structural study determined solution NMR spectroscopy models for GCAA and GAAA hairpins. Both contained an unusual G–A pair between the first and last loop residues, together with base stacking and contacts involving phosphate and a sugar hydroxyl.^[ref] These are specific interactions beyond the Watson–Crick stem. A loop shown as unpaired in a simple secondary-structure diagram can still make an organized network of internal contacts.

The pattern separates invariant and variable positions. Starting from GAAA, changing only N to C gives GCAA without leaving the family; changing R from A to G gives GAGA. Changing the final A to G instead gives GAAG, which does not satisfy GNRA. This original sequence exercise explains what a consensus rule does and does not preserve.

Sequence-family membership alone does not supply the complete thermodynamics or structure of an embedded RNA. Keeping the loop fixed while changing the stem tests context dependence; keeping the stem fixed while changing a loop position tests that position's contribution. An RNA optical melting comparison and a structural measurement provide complementary information rather than interchangeable proofs.

The GNRA designation is useful because a short sequence rule points toward recurring molecular organization. It does not imply that all eight sequences occur equally often, have identical stability, or maintain the same contacts in every surrounding RNA. A noncanonical base pair and the rest of the loop environment need their own structural evidence.

H. A. Heus is an author of a 1991 solution-structure study of RNA hairpins containing GNRA tetraloops. The paper identifies a Chemistry and Biochemistry affiliation at the University of Colorado, Boulder. Its structures for the GCAA and GAAA sequences helped explain how loop contacts can stabilize a short RNA fold.^[ref]

The experiment used NMR spectroscopy to determine the structures in solution. Both reported loops contain an unusual pair between guanine and adenine between the first and last loop residues, together with a base–phosphate hydrogen bond, a sugar–base hydrogen bond and extensive base stacking. The proposed stability therefore involves a network of interactions rather than only the count of canonical pairs in the stem.^[ref]

An original sequence-counting example clarifies the family name. In GNRA, G denotes guanine, N permits any of the four ordinary RNA bases, R permits either purine base, and A denotes adenine. Thus the motif admits $1\times 4\times 2\times 1=8$ nominal sequences. GCAA and GAAA differ at N while retaining the initial G and final A. Membership in that sequence family does not establish equal folding stability for all eight sequences; the paper reports structures of two particular members.

A separate thermodynamic illustration shows why a strong folding preference can still be sensitive to a modest energy change. In a stipulated two-state equilibrium, let the folded fractions be $0.90$ and $0.99$ under two different conditions. Their folding equilibrium constants areAt $T=298\mathrm{K}$, the corresponding difference in folding Gibbs free energy iswhere $R$ is the molar gas constant. These are original illustrative populations, not measured values attributed to Heus's paper. They connect a molecular-interaction explanation with an experimentally measurable equilibrium preference.

The reported GNRA tetraloop structures give a molecular interpretation of sequence conservation: bases and sugar–phosphate groups have to support a compatible set of contacts. A sequence change can preserve the stem yet alter those loop interactions.

The Hoogsteen base-pair edge is a contact surface of a nucleobase distinct from its Watson-Crick base-pair edge and sugar base-pair edge. For a purine, it is the side containing N7 and the exocyclic group at position 6; for example, adenine presents N7 and its N6 amino group on this side. The structural classification extends the corresponding category to the CH edge of pyrimidines.^[ref]

The word describes one partner's contacting edge, not a complete pair or a mandatory rotation of the base. A complete RNA base-pair geometry identifies the second partner's edge and the cis/trans orientation of the glycosidic bonds. The primary matrices include Hoogsteen–Watson–Crick, Hoogsteen–Hoogsteen and Hoogsteen–sugar contacts.^[ref]

An original counting example makes this useful. Fix one adenine as the labeled first partner and stipulate that it uses its Hoogsteen edge. The second partner still has three edge categories available; each can appear with a cis or trans relationship. There are therefore $3\times 2=6$ labeled edge/orientation descriptions before specifying the second base's identity. This counts descriptions, not six experimentally established partners for that particular adenine.

Edge identity must also be distinguished from chemical role. N7 of ordinary neutral adenine is a hydrogen bond acceptor, while its amino group can supply a hydrogen bond donor. A favorable contact requires an appropriately positioned complementary group, not merely two bases facing sides with the same category name. Structural conditions and chemical state matter.

When annotating an experimental RNA structure, record the interacting atoms and the two glycosidic bonds, rather than classifying from the sequence alone. An A–U pair can use a different geometry from the canonical Watson–Crick arrangement. Conversely, an edge category can recur with different base identities. This is why RNA base-pair isostericity is evaluated within explicit geometric families rather than inferred only from the letters A, C, G and U.

A nucleotide transition is a nucleotide substitution between two purine-derived bases or between two pyrimidine-derived bases. In DNA, the possibilities are $A\leftrightarrow G$ and $C\leftrightarrow T$: adenine exchanges with guanine, or cytosine with thymine. The chemical class is preserved; the identity of the base changes.^[ref]

From any specified DNA base, there is exactly one possible transition target and two nucleotide transversion targets. If all three targets were equally likely, transitions would comprise $1/3$ of changes. This is a counting baseline, not a biological law about mutation rates. The Kimura two-parameter model allows the rates for these two classes to differ.^[ref]

The classification does not determine a mutation's effect on a protein. A transition can preserve an amino acid, change it or introduce a stop codon, depending on the affected codon.

A nucleotide transversion is a nucleotide substitution from a purine-derived base to a pyrimidine-derived base, or the reverse. In DNA, the unordered pairs are $A\leftrightarrow C$, $A\leftrightarrow T$, $G\leftrightarrow C$ and $G\leftrightarrow T$. A nucleotide transition instead stays within one of these chemical classes.^[ref]

Each starting base has two possible transversion targets and one transition target. Thus twelve directed changes split into eight transversions and four transitions. Equal counts of possible events do not imply equal observed frequencies: chemical mutation mechanisms and natural selection can affect those frequencies.

Suppose an illustrative sequence alignment contains twelve transition differences and eight transversion differences. Its observed transition-transversion ratio is $12/8=1.5$. That ratio summarizes the scored differences; repeated substitutions at the same site can make it differ from the ratio of underlying instantaneous rates. The two-class distinction motivated the Kimura two-parameter model.^[ref]

The reactive fragment is the C6 amidine-like portion of the purine ring; the N9–ribose attachment is retained. Write it locally as $N1=C6(\mathrm{N}{\mathrm{H}}_2)-$, with the rest of the fused ring understood. One productive acid catalysis route protonates N1, making C6 more electrophilic. water then adds at C6 while the C6=N1 pi electrons move onto N1. Subsequent proton transfers generate a carbinolamine and then a protonated amino leaving group:These are local bonding fragments: C6 retains its bond to the adjoining ring carbon at every step. In the addition step, an oxygen lone pair forms C6–O and the C6=N1 pi bond opens. During collapse, an oxygen lone pair reforms C6=O and the C6–N6 bond breaks toward the departing nitrogen. Solvent mediates each proton transfer; no direct unsupported transfer across a long distance is required.

water supplies the new C6 oxygen, and Ammonia is expelled after protonation of the leaving group. Final deprotonation gives the lactam form of inosine. In acidic bulk solution the released Ammonia is largely trapped as ammonium. The catalytic proton transfers regenerate acid around the substitution cycle; ammonium trapping separately affects the net proton balance. This is hydrolytic deamination, not cleavage of the glycosidic bond.

A glutamic acid residue supplies a carboxyl group whose protonation state can change with the active site environment. The following five schematic contributions are possible in a hypothetical enzyme; they are alternatives and overlapping physical effects, rather than five independently established reactions of this enzyme. In the diagrams $E$ is the protein tether, and $C6$ is the reacting purine carbon.

general acid catalysis: $\mathrm{E}-\mathrm{C}{\mathrm{O}}_2\mathrm{H}\cdots N1\to \mathrm{E}-\mathrm{C}{\mathrm{O}}_2^{-}+\mathrm{H}-\mathrm{N}{1}^{+}$. Donation of the carboxyl proton activates the amidine-like C6 center for water attack. The same acid could instead assist the amino leaving group: $\mathrm{E}-\mathrm{C}{\mathrm{O}}_2\mathrm{H}+-\mathrm{N}{\mathrm{H}}_2\to \mathrm{E}-\mathrm{C}{\mathrm{O}}_2^{-}+-\mathrm{N}{\mathrm{H}}_3^{+}$. These are two locations for general acid catalysis, rather than two different categories.

general base catalysis: $\mathrm{E}-\mathrm{C}{\mathrm{O}}_2^{-}\cdots \mathrm{H}-\mathrm{OH}\to \mathrm{E}-\mathrm{C}{\mathrm{O}}_2\mathrm{H}+\mathrm{O}{\mathrm{H}}^{-}$, coupled to oxygen attack on C6. Accepting a proton makes water a more effective nucleophile. This does not require an appreciable pool of free hydroxide in bulk solution.

covalent catalysis: a glutamate oxygen can in principle attack activated C6 to form $C6-\mathrm{O}-\mathrm{C}(\mathrm{O})-\mathrm{E}$. After addition/elimination replaces the amino group, water must hydrolyze that transient enzyme–substrate bond to release product and regenerate $\mathrm{E}-\mathrm{C}{\mathrm{O}}_{2}H/C{\mathrm{O}}_2^{-}$. Such a pathway is a chemically distinct possibility, not established by the given structure.

electrostatic catalysis: the carboxylate can stabilize a developing positive charge, for example in a protonated purine or amino leaving group; a protonated carboxyl can also donate a hydrogen bond to an oxygen developing negative charge. This contributes only if the reacting configuration is stabilized relative to the bound starting state. A carboxylate placed next to an oxyanion without compensating geometry would instead be unfavorable.

proximity and orientation catalysis: donor and acceptor hydrogen bonds can hold the purine and attacking water in a productive arrangement, schematically $\mathrm{E}-\mathrm{C}{\mathrm{O}}_2\mathrm{H}\cdots N1-C6\cdots \mathrm{O}{\mathrm{H}}_2$. Binding reduces unproductive orientations and can distort the substrate toward a reactive geometry. This last contribution is sometimes described through substrate binding or strain; a binding interaction is not automatically catalytic unless it improves access to the transition state.

Glu217 is important for catalysis, most naturally through proton transfer at the purine ring. Changing a glutamic acid residue to glutamine preserves much of the side-chain size and hydrogen-bonding geometry, while removing an ordinary carboxylic acid/base pair. Its placement near N1 makes acid donation there a plausible way to activate C6 for water addition and stabilize the resulting nonaromatic intermediate. The large turnover defect with little change in the Michaelis constant supports that catalytic interpretation, subject to the same $K_M$ and protein-integrity qualifications as above.

The protonated carboxyl drawn at pH 7 need not behave like free glutamate in water: local electrostatics and burial can shift its p$K_a$, and protonation may change during the cycle. The mutation alone cannot choose between an initially protonated Glu217 acid and a Glu217 residue that first accepts and then relays a proton. Primary mutational work supports catalytic roles for active-site histidine and glutamate groups while explicitly comparing purified enzymes and zinc content.^[ref]

Plants generally incorporate combined nitrogen into organic compounds, whereas animals eliminate the surplus nitrogen of metabolized compounds chiefly as Ammonia, urea or uric acid. The two processes connect nitrogen chemistry to the organism's carbon supply, water economy and environment.

Roots take up nitrate and ammonium from their surroundings. Nitrate must be reduced before its nitrogen can be incorporated in the usual amino form: nitrate reductase produces nitrite, and nitrite reductase produces ammonium. The balanced reduction, written without implying a single elementary reaction, isReducing equivalents and energy must be supplied by metabolism; nitrate is not simply deposited unchanged inside every protein.

In the modern biochemical account, glutamine synthetase incorporates ammonium into glutamine, using ATP; glutamate synthase transfers that nitrogen to a carbon skeleton derived from 2-Oxoglutarate, producing glutamate. Further transfer and synthesis distribute nitrogen into other amino acids, proteins, nucleotides and other cell constituents. Carbon metabolism therefore provides both energy and skeletons for nitrogen assimilation. These named enzymes and reactions explain the process today; they were not a completed molecular scheme in 1864.^[ref]

Ordinary higher-plant cells do not themselves reduce atmospheric molecular nitrogen merely because the gas surrounds the leaves. Biological nitrogen fixation is performed by particular microorganisms; symbiotic associations can supply fixed nitrogen to a host plant. The later elucidation of root-nodule symbiosis should be distinguished from what the examiner could have expected. Organic nitrogen can also be taken up in suitable circumstances, so nitrate and ammonium are important routes rather than an exhaustive prohibition on every other source.^[ref]

A useful assimilation experiment grows comparable seedlings in a defined medium containing all required minerals except combined nitrogen, with parallel nitrate- or ammonium-supplied cultures. Initial seed reserves, contaminating nitrogen, water and microbial growth must be allowed for. Growth and final nitrogen content are more informative than green appearance alone. A plant initially growing in a nitrogen-free solution may simply be consuming seed reserves; that observation does not demonstrate fixation of atmospheric nitrogen.

Animals obtain most of their nitrogen from organic food. Amino acid metabolism separates nitrogen handling from oxidation or reuse of the carbon skeleton. Free Ammonia is toxic at high concentrations, and mammals convert much of their waste nitrogen into urea in the liver; the kidneys then eliminate it in urine. Uric acid is also produced from purine breakdown. Renal ammonium excretion has a separate important connection with acid-base balance. Urea formation and urinary excretion are different processes in different organs.^[ref]

Many aquatic animals can release Ammonia directly into abundant surrounding water. Birds, many reptiles and many insects eliminate much nitrogen as relatively insoluble uric acid, conserving water; mammals generally depend more on soluble urea. These are broad physiological strategies with exceptions, not a rule that all animals have human urine. Faecal nitrogen includes unabsorbed food as well as endogenous material, while smaller losses can occur through skin and secretions. For a stated interval the accounting identity isPositive retention during growth and negative balance during tissue loss need not be errors of excretion. Nitrogen is redistributed or lost in compounds; it is not simply breathed out as all the atmospheric nitrogen passing through the lungs.

The chemical mismatch types are A–C and G–T, with either strand acting as template. In the fixed ring alignment shown, A–C has no correct hydrogen bonds while G–T retains one. Relative probabilities must specify the template: template A loses two bonds versus A–T, giving $e^{-10}$; template G loses two versus G–C, giving $e^{-10}$; template C loses three versus C–G, giving $e^{-15}$; template T loses one versus T–A, giving $e^{-5}$. Equal incoming activities and $5k_{B}T$ per bond give approximate relative weights $4.5\times 10^{-5},4.5\times 10^{-5},3.1\times 10^{-7},6.7\times 10^{-3}$ respectively.

For roughly equal template frequencies the mean error probability is order $1.7\times 10^{-3}$, or several million errors across $3\times 10^9$ copied base pairs. Restricting the estimate to purine templates instead gives order $10^5$ errors; neither is an accurate replication model. Both greatly exceed the source's order-one surviving mutation input. Polymerase kinetic discrimination, proofreading and mismatch repair suppress errors, and a transient mismatch need not become an inherited mutation. Ring relaxation, stacking, solvent and rare tautomers are missing from this simple hydrogen-bond calculation. Real G–T wobble geometry can form additional bonds. The source's mutation rate is illustrative, not a universal measured human rate.^[ref]

Purine is the fused six- and five-membered nitrogen heterocyclic framework underlying adenine and hypoxanthine. Its numbered ring nitrogens distinguish protonation and hydrogen-bonding sites.

The RNA IsoDiscrepancy Index, abbreviated IDI, is a numerical measure of how two base pairs differ in the geometry relevant to their attached backbones. Lower values indicate closer RNA base-pair isostericity. Jesse Stombaugh, Craig L. Zirbel, Eric Westhof and Neocles B. Leontis introduced it to make comparisons more quantitative than assigning pairs only to broad geometric families.^[ref]

The comparison considers the separation of the ribose $\mathrm{C}{1}^{\prime }$ atoms and the relative positions and orientations of the bases at their glycosidic bonds. The attachment nitrogen is N1 for a pyrimidine and N9 for a purine. Thus matching the distance between two sugar attachment points alone does not establish matching base orientation. The published construction handles in-plane alignment and comparisons requiring an out-of-plane flip separately.^[ref]

IDI has units of ångströms, not a dimensionless probability scale. In the in-plane case, the method weights an orientation angle $\theta$ by a 2-ångström length scale. An original illustration of that rotation term isThis is one weighted rotation contribution, not the complete IDI formula. Translation, separation and the paired orientation comparisons also matter. Expressing an angle in radians is essential to this length interpretation.^[ref]

The authors calibrated their classification using structural distributions: IDI at most 2.0 ångströms is isosteric, above 2.0 through 3.3 ångströms is near-isosteric, and larger values are more dissimilar. These are the study's method-specific cutoffs. A chosen score of 1.8 therefore falls in its first category, whereas 2.6 falls in its second; neither is an energy or a probability.^[ref]

Interchanging the two structures being compared leaves the discrepancy unchanged; reversing the nucleotide order within only one base pair is a different operation. An RNA isostericity matrix can display such comparisons, but sequence context and model quality still affect interpretation. A geometrically plausible substitution is a structural hypothesis, not a guarantee of unchanged folding stability or molecular recognition.

An RNA tetraloop is the four-nucleotide loop capping the paired stem of an RNA hairpin. The four count excludes the two nucleotides of the closing base pair. A tetraloop is therefore a local RNA secondary structure motif, not necessarily an entire four-nucleotide RNA molecule.

For an original sequence illustration, 5'-GGCGAAAGCC-3' can be drawn with pairs 1–10, 2–9 and 3–8. The central GAAA at positions 4–7 forms a four-residue loop; three stem pairs plus four loop residues give $2(3)+4=10$ nucleotides. Whether a particular sequence actually favors that conformation requires experimental or structural evidence.

Comparative analysis of ribosomal RNA identified recurrent motif families, including GNRA tetraloops and UNCG tetraloops, as well as the less common CUUG motif. The study also found relationships between loop sequence and its closing pair.^[ref] In GNRA, N means any of the four ordinary RNA bases and R means a purine, A or G; GAAA is one example. UUCG belongs to UNCG, with its N position occupied by U.

A simple sequence count separates a motif pattern from a frequency claim. Four unrestricted positions give $4^4=256$ possible sequences. GNRA specifies the first and last bases and permits $4\times 2=8$ sequences; UNCG permits 4. These counts assume the usual A, C, G, U alphabet. They do not say that the sequences occur equally often or have identical stability.

The stem and loop environment matter together. Optical-melting measurements on 24 selected stem–loops quantified sequence-dependent thermodynamics rather than treating every four-residue loop as one energetic object.^[ref] A frequently observed motif and a particularly stable isolated hairpin are different scientific observations; both can inform how RNA structures are understood.

Serum urate is the concentration of urate measured in blood serum. Urate is related to purine breakdown and renal handling, so its measured level reflects both production and removal. A concentration is distinct from a whole-body production rate or the presence of deposited crystals.

Biochemistry 35, 15019–15028. purine-riboside hydration, spectroscopy and crystallography demonstrate why a loss of catalytic activity does not uniquely identify the residue that generates hydroxide.

A syn glycosidic conformation is a base–sugar orientation in the syn region of the glycosidic torsion angle $\chi$. In the IUPAC-IUB convention the region is centered at $0^{\circ }$ and spans90degrees on either side. In a nonnegative360degree reporting interval, this region crosses the boundary: angles near both0 and360degrees belong to it.^[ref]

For an original example, $\chi =30^{\circ }$ and $\chi =330^{\circ }$ are both syn, because330degrees is equivalent to−30degrees. Their shortest angular separation is60degrees. Their ordinary arithmetic mean,180degrees, falls in the anti glycosidic conformation region and is therefore misleading as a mean orientation. Averaging their unit-circle vectors instead giveswhich points toward $\chi =0^{\circ }$.

The numerical convention is essential. Older choices using the base atom C8 for purines or C6 for pyrimidines can differ from the recommended glycosidic torsion angle by approximately180degrees. Comparing the same number under those two definitions could therefore appear to reverse a syn/anti assignment without any molecular motion.^[ref]

Syn is a local conformational descriptor. It is neither a statement that two bases are paired nor a label for the overall handedness of a nucleic-acid helix. Structural comparisons should retain the defining atoms and angle convention alongside the syn/anti label.