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Back-splicing is an RNA splicing arrangement in which a downstream splice donor site joins an upstream splice acceptor site. In a common circularization event, the retained segment becomes circular RNA with a new junction joining a later part of the precursor to an earlier part. The donor is conventionally called a $5^{\prime }$ splice site and the acceptor a $3^{\prime }$ splice site; those labels describe splice-boundary roles, not the order of their coordinates along the precursor.

For a precursor with ordered exons $1\to 2\to 3\to 4$, a circle containing exons 2 and 3 has the ordinary internal junction $2\to 3$ and a circularizing junction $3\to 2$. A sequence read spanning $3\mid 2$ thus reports a different adjacency from the reference transcript order. The example specifies a possible connectivity, not a prediction that every such precursor forms this circle.

A 2013 study combined RNA sequencing with and without RNase R treatment. Reads crossing back-splice junctions were enriched among the more resistant RNA population, and complementary tests supported circular products.^[ref] A non-colinear junction read alone can also raise questions about mapping, another RNA arrangement or genomic structure. Agreement between junction evidence and a test of molecular connectivity makes the circular interpretation stronger; it does not by itself establish the circle's biological function.

A circular RNA has a covalently closed RNA backbone with no free $5^{\prime }$ or $3^{\prime }$ end. The closure concerns the chain of phosphodiester bonds, not whether a drawing happens to look round. A linear RNA folded into a stem-loop still has two ends. Circular RNAs can arise through RNA ligation; some cellular examples are generated by back-splicing of transcribed sequences.^[ref][ref]

One internal cut in the circle produces a single linear chain. One internal cut in an already linear chain produces two fragments, assuming no other links hold the pieces together. This connectivity difference explains why enzymes requiring a free end can distinguish the molecules, although circularity does not make an RNA immune to every cleavage process.

A 2013 study combined RNA sequencing with and without treatment by RNase R, an enzyme acting from RNA ends. Enrichment of back-splicing junction reads, together with additional validation, supported a substantial population of endogenous circular molecules.^[ref] Combining chemical resistance with junction evidence is more informative than classifying a molecule solely by its apparent shape or persistence after one treatment.

RNA ligation joins compatible RNA termini by forming a new phosphodiester bond. Joining two fragments extends the connected chain; joining the two ends of one fragment produces circular RNA. The end chemistry matters: an ATP-dependent RNA ligase can join a $3^{\prime }$ hydroxyl group to a $5^{\prime }$ phosphate, whereas RtcB uses a different combination of terminal groups.^[ref][ref]

An unmodified linear four-nucleotide fragment has three backbone links, and a six-nucleotide fragment has five. One intermolecular ligation gives ten nucleotides connected by $3+5+1=9$ links in a linear chain. Circularizing that chain adds one more backbone link while retaining ten nucleotides. Counting chain units, connecting bonds and separate molecules therefore answers three different questions.

A 1972 experiment made end joining measurable using radioactively labelled RNA. After reaction with a purified RNA ligase, labelled terminal phosphate became resistant to alkaline phosphatase. Additional tests with enzymes attacking from chain ends supported formation of circular poly(A) under the studied conditions.^[ref] A single protected label would show a chemical change at that end; the complementary end-accessibility tests made the circular-product interpretation stronger.

RNase R is a bacterial exoribonuclease that degrades RNA from the3′ end toward the5′ end. It hydrolyzes successive phosphodiester bonds rather than requiring an internal cut at each step. Its ability to digest structured RNA distinguishes it from enzymes restricted mainly to accessible single-stranded regions.^[ref]

The direction matters. For a sufficiently accessible linear RNA whose final three nucleotides are written5′–…GCA–3′, degradation encounters the terminal A before the preceding C. This notation illustrates direction, not a claim that an isolated three-nucleotide fragment is an efficient substrate. An intact circular RNA has no free end for this entry route; a strand break creates ends and changes the substrate's topology.

A2016 mechanistic study compared designed single-stranded and duplex RNA substrates with altered enzyme variants. It examined the S1 RNA-binding domain, intrinsic RNA helicase activity, ATP binding and conformational changes measured through tryptophan fluorescence. Those comparisons separated RNA binding, strand separation and phosphodiester cleavage rather than treating disappearance of an RNA band as a complete mechanistic explanation.^[ref]

Substrate structure and end accessibility therefore matter when interpreting resistance to digestion. Surviving RNA is an observation under the chosen reaction conditions; by itself it does not establish the exact structure or topology of every surviving molecule.

Robert Silber is a researcher credited with early characterization of RNA ligases. His full name and New York University medicine affiliation appear on the original 1972 article, although the indexed citation abbreviates the byline as R. Silber.^[ref]

Silber and collaborators investigated an RNA ligase from phage-infected Escherichia coli. The purified enzyme converted labelled terminal phosphate groups into internal phosphodiester bonds, requiring ATP and magnesium. Resistance to alkaline phosphatase, together with tests using exonucleases, supported joining the two ends of an individual RNA chain to produce circular RNA under the studied conditions.^[ref]

The experiment illustrates why several chemical observations are needed to establish molecular topology. A label no longer accessible at a chain end suggests a change in its chemical setting, but that result alone does not determine whether two different molecules joined or one molecule closed on itself.

An original counting model makes the ambiguity precise. Begin with $100$ linear chains, each with two free ends, giving $200$ ends. Closing $40$ chains into separate rings leaves $60$ linear chains, $120$ free ends and $100$ total molecules. Alternatively, joining $40$ disjoint pairs into longer linear chains leaves $60$ molecules, again with $120$ free ends. The same reduction in end count can therefore accompany different molecular products. These are invented counts, not the paper's measurements; additional chemical and size information is needed to distinguish the products.

The historical inference combined the protected label with evidence that the product lacked accessible opposite ends. It was an inference about the tested RNA substrate and assay, not a claim that every RNA ligase reaction always creates a circle.^[ref]

William Richard Jeck is a researcher credited with identifying and validating endogenous circular RNA species. His Duke institutional profile gives the full name, and the corresponding institutional publication list directly links him to the 2013 circular-RNA paper published under William R. Jeck.^[ref]

Jeck and collaborators compared RNA sequencing results from samples treated with RNase R and corresponding untreated samples. The approach enriched for molecules resistant to degradation of linear RNA and detected junctions inconsistent with ordinary linear exon order. Complementary assays tested the circular interpretation. The paper also reported an association with complementary Alu elements in bordering introns; that association alone is not a complete reaction mechanism for every circle.^[ref]

A numerical example clarifies what enrichment means. Imagine an original mixture of $10$ circular and $990$ linear RNA molecules, so circles comprise $1\%$. After a hypothetical treatment, suppose $8$ circular and $2$ linear molecules remain. Circles now comprise $8/(8+2)=80\%$ of the recovered mixture, an $80$-fold increase in their fraction, even though their absolute count decreased by $20\%$. These invented counts assume that molecules are counted comparably and are not measurements from the paper. They show why a large proportional enrichment must not be interpreted as treatment having created more circles.

The same care applies to a candidate back-splicing junction: identifying an unusual sequence order and proving that its source molecule is circular are separate steps. Enzyme resistance, junction evidence and independent product characterization address different alternatives. Jeck's credited contribution is therefore more informative than a catalogue of surprising sequencing reads: it illustrates how computational discovery and biochemical validation can constrain one another without making every resistant transcript a proven circular RNA.