Codex @codex  0

A base pair is an association of two nucleobases through hydrogen bonds in a compatible arrangement. It commonly joins opposite strands of a nucleic acid double helix, but can also join two regions of one folded RNA strand. Canonical Watson–Crick DNA base pairs match adenine with thymine and guanine with cytosine; canonical RNA base pairs use adenine with uracil instead. The nucleobases belong to complete nucleotides, including their attached sugars and backbone phosphate. See NHGRI: base pair.

The abbreviation bp also measures the length of a paired region. Our ACGTAC/TGCATG duplex has six base pairs, containing twelve nucleotides altogether. Its upper strand contains two adenine, two cytosine, one guanine, and one thymine nucleobase; the lower contains one, one, two, and two respectively. Combined, the counts are three of each nucleobase. Equality of total adenine and thymine, or of total guanine and cytosine, need not hold within either individual strand.

Base pairing is the recognition and association of compatible nucleobases. In canonical Watson–Crick geometry, an adenine–thymine base pair has two hydrogen bonds, while a guanine–cytosine base pair has three. These counts refer to those specific geometries and usual chemical forms; they are not a definition of every possible base pair. RNA can also contain other pairings, and canonical adenine–uracil pairing replaces adenine–thymine pairing. See Clark, Douglas and Choi, Biology 2e: DNA structure.

For this illustrative duplex, the six base pairs are A–T, C–G, G–C, T–A, A–T, and C–G. Thus three contribute two hydrogen bonds each and three contribute three each:Each dotted connector in the diagram counts one hydrogen bond; coloured boxes denote whole nucleobases, not individual chemical atoms. The solid rails denote the covalent bond scaffold. Separating the two strands disrupts base pairing and base stacking without requiring the backbone to be cut.

The count of fifteen hydrogen bonds is not fifteen identical contributions to a duplex's Gibbs free energy. Base stacking, interactions with water and ions, temperature, and the order of neighbouring base pairs also matter. Knowing the fraction of guanine–cytosine base pairs alone does not fix a nucleic acid double helix's shape or stability. See Brown, Genomes: pairing and stacking interactions. What makes base pairing especially useful is its selectivity: an exposed DNA strand constrains the partner's nucleic acid sequence, giving DNA replication a chemical recognition rule.

A codon is a three-nucleotide unit read in a specified reading frame during biological translation. For example, GCU specifies alanine in the standard genetic code. Its letters are read $5^{\prime }\to 3^{\prime }$.

For an original example, choose the messenger RNA segment $5^{\prime }\text{-}\mathrm{AUG}\mathrm{GCU}\mathrm{UUC}\mathrm{UGA}\text{-}{3}^{\prime }$ and stipulate initiation at the displayed AUG in a suitable eukaryotic context. Under the standard genetic code, its first three codons specify methionine, alanine and phenylalanine; the fourth is a stop codon. The resulting peptide before processing is N–methionine–alanine–phenylalanine–C. Twelve nucleotides therefore specify three amino acid residues and termination. The displayed segment omits the surrounding initiation signals; its nucleotide sequence alone is not a complete expression system.

A codon belongs to an oriented nucleotide sequence: GCU and UCG are different ordered triplets. Four possible nucleobases, adenine, cytosine, guanine and uracil, allow $4\times 4\times 4=64$ triplets because each position can be chosen independently when enumerating possible sequences. This is a counting argument, not a claim that nucleobases occur independently in real RNA. Sixty-one triplets are sense codons, meaning amino acid assignments, and UAA, UAG and UGA are stop codons in the standard genetic code.^[ref]

The useful unit is the ordered triplet together with its reading frame and genetic code, rather than three letters viewed in isolation. That combination lets DNA sequencing data support predictions about protein sequences after identifying the relevant messenger RNA. The Nirenberg and Matthaei experiment illustrates how the correspondence became experimentally accessible.

A deoxynucleoside triphosphate is a nucleotide containing deoxyribose and three phosphate groups. DNA polymerases use these substrates to extend DNA; the four standard substrate types carry adenine, cytosine, guanine or thymine.

Deoxyribonucleic acid, or DNA, is a nucleotide polymer that stores biological information in its nucleotide sequence. The crucial variable is order: two DNA molecules can contain the same numbers of each nucleobase yet carry different nucleotide sequences. This makes a physical molecule an inheritable record, rather than merely a mixture of ingredients. Genes are functional regions of that record; the whole DNA nucleotide sequence is not one long gene. See NHGRI: DNA fact sheet.

Each DNA strand has a backbone of alternating deoxyribose sugars and phosphate groups joined through covalent bonds. Attached nucleobases supply the familiar four-letter alphabet: adenine A, thymine T, guanine G, and cytosine C. In ordinary double-stranded DNA, base pairing matches A with T and G with C. The backbone connects successive nucleotides along a strand; base pairing connects partners across strands. Keeping these two connections distinct explains how strands can separate while each retains its own nucleotide sequence.

The two backbones have directions, conventionally labelled $5^{\prime }$ and $3^{\prime }$ after numbered carbon chemical atoms in deoxyribose. They form antiparallel nucleic acid strands: following one from $5^{\prime }$ to $3^{\prime }$ takes you opposite to the other's $5^{\prime }$-to-$3^{\prime }$ direction. For the chosen DNA nucleotide sequence $5^{\prime }\text{-}\mathrm{ATGCC}\text{-}{3}^{\prime }$, write its partner directly underneath as $3^{\prime }\text{-}\mathrm{TACGG}\text{-}{5}^{\prime }$. Now read that partner from its own $5^{\prime }$ end: its reverse complement is $5^{\prime }\text{-}\mathrm{GGCAT}\text{-}{3}^{\prime }$. Complementing letters and reversing their order are separate operations. See Alberts et al., Molecular Biology of the Cell: the structure and function of DNA.

The diagram's rails represent sugar–phosphate backbones and its coloured blocks represent nucleobases, not individual chemical atoms. Reading left to right gives five matched positions: A–T, T–A, G–C, C–G, C–G. There are two A–T-type base pairs and three G–C-type base pairs; the complete two-strand example contains ten nucleotides. Reversing the bottom row changes the reading convention, not the physical partners. Real double-stranded DNA winds into a nucleic acid double helix; the flattened ladder emphasizes base pairing and direction without pretending to show its three-dimensional geometry.

The motivating consequence is copying by a template. After strands separate, each existing DNA strand specifies which incoming nucleotides should accompany it through base pairing. DNA replication uses enzymes to build complementary strands, allowing the record to persist through successive biological cells. Base pairing supplies a selection rule, while enzymes perform the chemistry: a matching diagram alone is not a complete copying mechanism. The connection between structure and copying was already central to the model published by Watson and Crick on 25 April 1953. See Watson and Crick, Molecular Structure of Nucleic Acids, Nature 171, 737–738 (1953).

Stored information needs interpretation. Some DNA regions specify proteins through RNA; others contribute to functional RNA or regulate gene expression, and some have no known function. Non-coding DNA means sequence that does not specify a protein's sequence of amino acids, not automatically useless sequence. Nor does a DNA nucleotide sequence alone determine an organism's fate: phenotypes also depend on biological context and environment. See NHGRI: non-coding DNA. Its double-stranded form is often modeled mechanically as a worm-like chain, with elasticity and effective excluded volume depending on solvent conditions.

DNA sequencing determines the order of nucleotide bases in DNA. Sequence data can test molecular rearrangements, as in the functional plastid-to-nucleus gene transfer experiment, or support phylogenetics after a suitable sequence alignment.

An ordered nucleotide sequence answers a different question from fragment size. The eight-nucleotide strands ACGTTAGC and AACCGGTT have equal lengths and identical counts of adenine, cytosine, guanine and thymine, yet different orders. A length-compatible gel band therefore cannot identify either strand by itself. Sanger sequencing makes length informative by deliberately coupling each stopping position to the identity of its last added nucleotide.

That coupling is the beautiful experimental idea: convert an invisible order into a collection of ordered endpoints. Sanger ladder reading recovers the newly synthesized strand; base pairing and the reverse complement then connect that readout to the original template. The distinction also matters when checking a mutation: a reliable local nucleotide sequence supports a particular molecular change, while its effect on a phenotype requires additional evidence.

The fraction or percentage of guanine and cytosine residues in a nucleic-acid sequence. Composition contributes to duplex stability but does not establish primer specificity.

A G protein is a protein switch whose activity depends on the bound guanine nucleotide. The three-subunit G proteins coupled to G protein-coupled receptors relay activation to other proteins; they are distinct from the receptors themselves. See the Alberts reference under G protein-coupled receptor.

Guanidinium is the resonance-stabilized cationic group found in protonated arginine side chains. Its N–H donors can make paired hydrogen bonds to the O6 and N7 acceptors on guanine.

A nucleoside comprising guanine attached to ribose. Its free $3^{\prime }$ hydroxyl group initiates group I intron RNA splicing; guanosine is incorporated into the excised intron, rather than acting as a regenerated catalyst.

A metabotropic receptor couples ligand recognition to intracellular signal transduction, which can regulate a separate ion channel, alter enzyme activity or change gene expression. In the usual neurotransmitter context, metabotropic receptors are G protein-coupled receptors: receptor activation is relayed by a G protein. There need not be a change in membrane potential at all. ^[ref]

Two routes should be distinguished. An activated G protein can regulate an ion channel locally at the cell membrane, without an obligatory diffusible second messenger. Alternatively, it can regulate an enzyme that changes the availability of a second messenger, such as cAMP or inositol trisphosphate. Downstream proteins then change channel opening, phosphorylation or other cellular activities. The receptor, G protein, second messenger and final target are distinct components; an activated G protein is a relay protein, rather than itself a small-molecule second messenger.^[ref]

Do not turn this useful chemical synapse classification into the claim that every receptor is a GPCR, or that every indirect response must pass through a G protein. For example, the insulin receptor is an enzyme-linked cell-surface receptor whose intracellular catalytic activity initiates protein phosphorylation. Intracellular receptors supply another category. “Metabotropic” commonly names the GPCR branch in neuroscience; the broader idea of biochemical coupling does not make all non-channel receptors structurally identical. ^[ref]

Separating recognition from execution allows signal amplification and branching. A persistently activated receptor can activate several relay proteins; a regulated enzyme can produce many second messenger molecules. As an explicitly invented count example, suppose one activated receptor activates four relay proteins during one interval, and each relay activates a distinct enzyme that produces fifteen second messenger molecules during the next interval. With sufficient substrates and no loss, this gives $4\times 15=60$ molecules. Those counts do not specify a physiological gain or response duration. A directly gated ion channel also passes many ions while open, so amplification is not an exclusive property of metabotropic receptors: always name the input and output being compared. ^[ref]

A particularly instructive electrophysiology experiment separated a chemical trigger from the ion channel carrying its eventual electric current. In 1987, Hirono, Ito and Sugiyama injected messenger RNA from rat brain into frog egg cells, then tested acquired responses to neurotensin and muscarinic acetylcholine stimulation. Under voltage clamp at $-60\mathrm{mV}$, neurotensin elicited transient, oscillatory inward electric currents after a delay of several tens of seconds. This delay refers to that response and preparation. Reversal potential measurements implicated increased permeability to chloride anions, rather than a pore specific to the applied ligand. Both neurotensin and acetylcholine responses were inhibited by pertussis toxin, which interferes with susceptible G proteins, and by intracellular EGTA, which binds free calcium. Injecting inositol trisphosphate mimicked the responses; a large prior dose of inositol trisphosphate reduced subsequent responses to either extracellular signal. These interventions implicate a shared intracellular relay and calcium-dependent channel opening.^[ref]

The informative comparison with the nicotinic acetylcholine receptor experiment is mechanistic: an extracellular ligand can produce an inward electric current either by directly gating the pore or by mobilizing an intracellular second messenger and stored calcium that activate another pore. Intracellular inositol trisphosphate bypasses reception, whereas EGTA tests the need for free calcium. However, neither inhibition nor mimicry alone identifies every molecular connection: a perturbation can affect several targets, and an injected mediator can activate a pathway without proving that it is the only natural route. The egg cell's endogenous machinery and the mixed messenger RNA preparation also limit extrapolation to an intact neuron. The experiment establishes functional coupling in its preparation, rather than the exact molecular composition or timing of every metabotropic receptor.

Even G protein-dependent channel regulation need not involve a lengthy second messenger cascade. Pfaffinger and colleagues reported in 1985 that acetylcholine-evoked potassium channel activation in embryonic chick atrial cells required intracellular GTP, a guanine-containing nucleotide, and was abolished by pertussis-toxin pretreatment. Changes in cyclic nucleotides, including cAMP, did not account for the channel response. The report discusses intrinsic activation delays of roughly $30$–$100\mathrm{ms}$, illustrating relatively rapid metabotropic receptor action. These observations support a G protein coupling step without cyclic-nucleotide second messengers; they do not by themselves exclude every possible intermediary or prove the specific subunit-channel interaction.^[ref]

A small timing model makes the extra steps visible without inventing an experimental trace. Let $u(t)$ jump from zero to one at $t=0$ and remain on. For a direct-gating model output $d$, one relay variable $x$, and its downstream output $y$, take zero initial conditions andAll outputs are normalized dimensionless variables; they are neither recorded electric currents nor fitted concentrations. With illustrative time constants ${\tau }_d=2\mathrm{ms}$, ${\tau }_x=20\mathrm{ms}$ and ${\tau }_y=100\mathrm{ms}$, their exponential function solutions for $t\ge 0$ areInitially $d^{\prime }(0)=1/{\tau }_d$, while $y^{\prime }(0)=0$ because its relay must first build up. Both outputs eventually approach one. Equal normalized final values do not imply equal biological amplitudes or absence of signal amplification; normalization deliberately removes that question.

The model's useful feature is the delayed buildup created by sequential relaxation, not its arbitrary numbers. Actual kinetics also depend on ligand delivery, relay deactivation, neurotransmitter clearance and the final target. A local G protein route can act promptly, while an ionotropic receptor can remain active during continued exposure or feed a longer-lasting intracellular response. The beauty of metabotropic receptor coupling is its flexibility: the same recognition event can adjust electrical responsiveness and other cellular tasks through different targets. To classify an observed response, combine timing with molecular identity and controlled perturbations.

A nucleic acid sequence records the ordered nucleobases of a nucleic acid strand, conventionally read from $5^{\prime }$ to $3^{\prime }$. The standard DNA alphabet is adenine A, cytosine C, guanine G, and thymine T; the standard RNA alphabet uses uracil U in place of thymine T. Each letter denotes the nucleobase attached to one nucleotide, so the six-letter DNA nucleic acid sequence $5^{\prime }\text{-}\mathrm{ACGTAC}\text{-}{3}^{\prime }$ describes six linked nucleotides. See NHGRI: representing DNA by a sequence of letters.

Order carries information that composition loses. Both ACGTAC and AACCGT contain two adenine, two cytosine, one guanine, and one thymine nucleobase, but their nucleic acid sequences differ at positions 2, 3, 4, 5, and 6. At position 2, for example, the first has cytosine and the second adenine. Reporting only the four counts cannot reconstruct either ordered record.

To obtain the partner of $5^{\prime }\text{-}\mathrm{ACGTAC}\text{-}{3}^{\prime }$, apply the DNA base pairing rules position by position: A selects T, C selects G, G selects C, T selects A, A selects T, and C selects G. Alignment gives $3^{\prime }\text{-}\mathrm{TGCATG}\text{-}{5}^{\prime }$. Its opposite end labels matter because the two are antiparallel nucleic acid strands. Reading the partner from its own $5^{\prime }$ end reverses the aligned lettering and yields the reverse complement, $5^{\prime }\text{-}\mathrm{GTACGT}\text{-}{3}^{\prime }$.

Reversal alone gives CATGCA; complementation alone gives aligned TGCATG. Neither operation alone gives the partner written in the usual $5^{\prime }$-to-$3^{\prime }$ convention. Applying the reverse complement operation twice returns ACGTAC: GTACGT complements to CATGCA, which reverses to ACGTAC. This convention makes a single reported strand sufficient to specify an ideally complementary DNA duplex, provided its direction is known.

At ordinary physiological pH, the arginine side chain is predominantly guanidinium. Its N–H groups are donors, and its positive charge can reinforce favorable electrostatic interactions. guanine exposes adjacent O6 and N7 acceptors in the major groove, allowing two guanidinium N–H donors to make a paired interaction without breaking Watson–Crick pairing. The diagram places this arginine contact on the exposed guanine edge, not between the paired bases.

adenine also exposes N7, but at the corresponding neighboring position it has the exocyclic N6 amino donor rather than a carbonyl acceptor. It therefore cannot make the same donor-to-two-acceptors match with guanidinium. A single N7 contact would be less discriminating. The classic structural recognition analysis explicitly proposed paired arginine–guanine contacts in the major groove.^[ref] protein geometry, hydration and phosphate contacts also affect sequence recognition; this local motif explains a preference rather than guaranteeing that every arginine-containing protein recognizes every G–C site.

Use the standard Watson–Crick edges, with the glycosidic attachments at purine N9 and pyrimidine N1. The A–T pair has two hydrogen bonds: adenine N6–H donates to thymine O4, and thymine N3–H donates to adenine N1. The C–G pair has three: cytosine N4–H donates to guanine O6, guanine N1–H donates to cytosine N3, and guanine N2–H donates to cytosine O2. The middle and right panels above show these atoms and the sugar-attachment directions.

The major-groove side is the edge toward purine N7 and toward thymine O4/its methyl group or cytosine N4. The minor-groove side is toward purine N3 and adenine C2–H or guanine N2, and toward pyrimidine O2 and the sugar attachments. Groove labels refer to the exposed outer edges of the paired bases; the hydrogen bonds joining the bases are not themselves empty grooves. The resulting patterns differ even when the bases remain paired inside duplex DNA.

DNA is a directional polymer with a sugar-phosphate backbone linked by phosphodiester bonds. Its deoxyribose sugars carry adenine, thymine, guanine or cytosine. In the usual cellular duplex, antiparallel strands form a double helix with complementary A–T and G–C pairing, stabilized by base stacking as well as hydrogen bonding. The sequence can vary without changing the basic backbone, allowing a large information capacity. Complementarity provides a template for copying and repair; the two strands are not merely two redundant lists stored without a copying mechanism. Major and minor grooves permit proteins to recognize sequence-dependent chemical patterns. Eukaryotic chromosomes package DNA around histones, while bacterial chromosomes are usually supercoiled and associated with other proteins; not all cellular DNA is nuclear or linear.

RNA uses ribose with a reactive 2-prime hydroxyl and usually uracil in place of thymine. It is often a single chain, but intramolecular complementary segments form stems, loops and elaborate three-dimensional folds. Messenger RNA conveys coding information to ribosomes; transfer RNA combines an anticodon with an amino-acid attachment site; ribosomal RNA has structural and catalytic roles in translation. Regulatory and catalytic RNAs show why it is inadequate to describe RNA solely as a transient DNA copy. RNA's local pairing and folding let sequence encode both information and molecular machinery. DNA's chemical stability and duplex repair make it especially suitable for long-term inheritance.

Replication is highly faithful but imperfect. Heritable sequence changes, recombination and gene duplication create variation, and selection, Genetic drift and population history determine its spread. Thus the same chemistry explains both continuity of lineages and their divergence; mutation is not directed toward what an organism needs. Conserved genes, homologous sequences and the widely shared genetic code support common ancestry. RNA catalysis makes an early RNA-based stage of evolution a plausible hypothesis, not a direct historical observation or a deduction that all modern RNA preceded DNA in a demonstrated sequence. Structure constrains function and evolutionary possibilities without uniquely determining a species' history.^[ref]

Ribonucleic acid, or RNA, is a nucleotide polymer whose nucleotide sequence can both carry information and organize a working molecular shape. Its backbone contains alternating ribose sugars and phosphate groups connected through covalent bonds. Its usual nucleobases are adenine A, uracil U, guanine G, and cytosine C. Thus RNA uses U where DNA uses T; its ribose also differs chemically from DNA's deoxyribose. See NHGRI: RNA.

In transcription, an enzyme uses one DNA strand as a template for RNA. For example, the DNA template $3^{\prime }\text{-}\mathrm{TACGG}\text{-}{5}^{\prime }$ specifies $5^{\prime }\text{-}\mathrm{AUGCC}\text{-}{3}^{\prime }$ in RNA. The base pairing rules now include A–U rather than A–T, while G–C remains. This RNA matches the other DNA strand's $5^{\prime }$-to-$3^{\prime }$ lettering after T is replaced by U. Identifying the template strand matters: replacing T by U in an arbitrarily chosen DNA strand is not a general method for deriving the RNA product. See Alberts et al., Molecular Biology of the Cell: from DNA to RNA.

RNA is often a single strand, but single-stranded does not mean straight or entirely unpaired. Complementary regions within one RNA molecule can undergo base pairing, bringing distant positions together. Consider the illustrative twelve-nucleotide nucleotide sequence $5^{\prime }\text{-}\mathrm{GCAUUUUUAUGC}\text{-}{3}^{\prime }$. Pair positions 1 with 12, 2 with 11, 3 with 10, and 4 with 9. The resulting stem-loop has four base pairs: G–C, C–G, A–U, U–A. Positions 5–8 remain unpaired in the loop. Both arms belong to the same continuous backbone, and the paired arms have the directions of antiparallel nucleic acid strands.

The diagram proposes a possible pairing pattern, not a prediction that this short RNA must adopt that shape under every condition. Actual RNA folding depends on additional interactions, temperature, ions, and associated proteins; the full three-dimensional arrangement contains more information than the drawn base pairs. The example's purpose is to show how a linear nucleotide sequence can specify contacts within itself.

Different RNA roles make this flexibility useful. Messenger RNA carries a message that a ribosome reads during translation to build a protein. Transfer RNA helps match that message to amino acid building blocks; see NHGRI: transfer RNA. Ribosomal RNA forms structural and catalytic parts of the ribosome. Other RNA participates in gene regulatory networks, changing how strongly genes are expressed. Many RNA molecules therefore function without being translated into proteins; for some genes, functional RNA is the product itself.

Most strikingly, some folded RNA acts as an enzyme: these catalytic RNA molecules are ribozymes. The ribosome's bond-forming active site is built from RNA, although the complete ribosome also contains proteins. Structural evidence appears in Nissen et al., The structural basis of ribosome activity in peptide bond synthesis, Science 289, 920–930 (2000). RNA therefore connects a readable record with physical recognition, regulation, and catalysis: the same kind of polymer can be a message, an adaptor, or part of molecular machinery.