GC Skew¶

Measure the signed excess of guanine over cytosine on an oriented sequence segment as their count difference divided by their nonzero count total.

Version: v2 · 2026-10-03 · History
Domain-specific #: 13267
Domain group: Natural Sciences
Origin domain: Biology & Ecology
Subdomain: Nucleotide Composition Analysis → Biology & Ecology

Core Idea¶

GC skew measures whether one specified strand segment contains relatively more guanine (G) or cytosine (C). Count both bases in that segment, then compute \((G-C)/(G+C)\) if \(G+C>0\). A positive number means G excess, a negative number C excess, and zero a balance. The value is undefined when the segment contains neither G nor C. Strand orientation and sign convention matter: Lobry's original study used \((C-G)/(C+G)\), the negative of the convention used here.[^{ref-989b8d01384d][}ref-ab52304982b9][^ref-4a6f27a2e757]

Scope of Application¶

Successive local values can be compared along a chromosome or plasmid. Lobry found a compositional sign switch near the known E. coli replication-origin region. Picardeau, Lobry and Hinnebusch applied the measure to Borrelia burgdorferi linear plasmids and used cumulative profiles to nominate candidate internal origin regions, distinguishing those candidates from a functionally mapped chromosome origin. A running profile can help reveal a broad pattern, but no single skew peak proves where replication starts. Grigoriev's archaeal origin calls were predictions, not a universal archaeal rule.[^{ref-989b8d01384d][}ref-96dddeff9c66][^ref-ab52304982b9]

Clarity¶

GC skew is not GC content: content combines G and C relative to all bases, while skew compares G against C. It is not AT skew. It is also not always the same as “cumulative GC skew”: one method sums normalized window values, while another sums raw per-base G-minus-C excess. Such plots have different vertical quantities. Always state the strand, interval, numerator sign and cumulative method before comparing results.[^{ref-ab52304982b9][}ref-4a6f27a2e757]

Manages Complexity¶

One normalized number summarizes the direction and relative size of G/C imbalance within a segment, allowing many regions to be compared without rereading the entire sequence. A cumulative profile can make a persistent trend easier to see. Small windows retain local detail but fluctuate; larger windows or cumulative presentation stabilize broad structure at the cost of blurring or carrying forward local changes.[^{ref-ab52304982b9][}ref-4a6f27a2e757]

Abstract Reasoning¶

Start with the oriented strand and segment, count G and C, check the denominator, and report the sign convention. Then compare consecutive segments using the same frame. A sustained change can support an interpretation of strand-scale asymmetry or suggest a replication-boundary region, but selection, gene orientation, rearrangements, and assembly problems can also shape the graph. Independent evidence is needed before calling an extremum a functional origin. The formula is descriptive, not an explanation of its own cause.[^{ref-989b8d01384d][}ref-ab52304982b9][^ref-4a6f27a2e757]

Knowledge Transfer¶

The same base-count relation applies to E. coli chromosome windows and Borrelia plasmid segments despite different genomic architecture and evidential purposes. Live Ratio is the proposed strict presupposition because the local index divides a signed difference by a nonzero reference total. The named GC-skew abstraction remains genomic: the G/C base roles and strand orientation do not travel to arbitrary imbalance ratios. Lobry's C-minus-G wording and cumulative GC-skew terms remain convention-qualified, not unconditional aliases.[^{ref-989b8d01384d][}ref-96dddeff9c66]

[^ref-989b8d01384d]: J. R. Lobry, Asymmetric Substitution Patterns in the Two DNA Strands of Bacteria, Molecular Biology and Evolution 13(5):660–665 (1996). [^ref-ab52304982b9]: A. Grigoriev, Analyzing genomes with cumulative skew diagrams, Nucleic Acids Research 26(10):2286–2290 (1998), original publisher full text. [^ref-4a6f27a2e757]: B. Hubert, SkewDB, a comprehensive database of GC and 10 other skews for over 30,000 chromosomes and plasmids, Scientific Data 9:92 (2022), original author-hosted PDF. [^ref-96dddeff9c66]: M. Picardeau, J. R. Lobry and B. J. Hinnebusch, Analyzing DNA Strand Compositional Asymmetry to Identify Candidate Replication Origins of Borrelia burgdorferi Linear and Circular Plasmids, Genome Research 10:1594–1604 (2000).

Relationships to Other Abstractions¶

Current abstraction GC Skew Domain-specific

Parents (1) — more general patterns this builds on

GC Skew presupposes Ratio Prime

Normalized GC skew presupposes a nonzero-denominator ratio.

Hierarchy path (1) — routes to 1 parentless root

GC Skew → Ratio → Comparison → Self Checking

Neighborhood in Abstraction Space¶

GC Skew sits in a sparse region of the domain-specific corpus (83^rd percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.

Family — Empirical Measurement & Statistical Inference Methods (50 abstractions)

Nearest neighbors

Computed from structural-signature embeddings · 2026-10-08