GC Skew Plotter
Plot the GC skew, (G−C)/(G+C), along a DNA sequence in windows, together with the cumulative skew. Shifts in cumulative GC skew often mark the replication origin and terminus of bacterial genomes.
Paste a DNA sequence above to plot its GC skew.
How it works
GC skew measures the local imbalance between guanine and cytosine on one strand: (G − C) ÷ (G + C). It is computed here in non-overlapping windows along the sequence, and the cumulative skew — the running total of the per-window values — is plotted alongside it.
GC skew is biologically informative because the leading and lagging strands of a replicating genome accumulate different mutational biases. In many bacterial chromosomes the cumulative GC skew reaches a minimum at the replication origin and a maximum at the terminus, so the turning points of the cumulative curve help locate these features. Choose a window size appropriate to your sequence length; FASTA headers and non-letters are ignored.
Examples
- A GC-rich (G-heavy) window gives a positive skew; a C-heavy window gives a negative skew.
- The cumulative curve's minimum often marks the replication origin in bacteria.
- A balanced GGCC window has a skew of 0.
Frequently asked questions
- What is GC skew?
- GC skew is (G − C) ÷ (G + C) measured over a window of sequence. It captures the strand asymmetry between guanine and cytosine content.
- Why plot cumulative skew?
- The cumulative sum smooths out local noise and reveals large-scale trends. Its turning points frequently coincide with the replication origin and terminus in bacterial genomes.
- What window size should I use?
- It depends on sequence length — a few hundred to a few thousand bases for whole genomes, smaller for short sequences. The tool clamps the window to the sequence length if needed.
- Does it accept FASTA?
- Yes. FASTA headers, whitespace and numbers are stripped automatically before the skew is computed.