GC skew

GC skew refers to the asymmetric distribution of guanine (G) and cytosine (C) nucleotides along a DNA sequence. It is calculated as (G-C)/(G+C) for a specific region of DNA. A positive GC skew indicates an excess of guanine over cytosine, while a negative GC skew indicates an excess of cytosine over guanine.

GC skew is often observed in bacterial and archaeal genomes and is associated with several biological processes, most notably DNA replication and transcription. It is generally hypothesized that the leading strand of DNA replication exhibits a positive GC skew, while the lagging strand exhibits a negative GC skew. This is attributed to differences in the mutation rates between the two strands during replication, potentially due to the deamination of cytosine to uracil which is subsequently repaired to thymine during lagging strand synthesis.

Transcription can also influence GC skew. Actively transcribed genes tend to have a positive GC skew on the transcribed strand. This is potentially linked to the repair mechanisms that target transcription-coupled DNA damage.

GC skew analysis is a common technique in bioinformatics and genomics. It can be used to identify the origin and terminus of replication in bacterial genomes, predict the direction of transcription, and study the evolutionary dynamics of genomes. Shifts in GC skew along a genome can signify gene transfer events, rearrangements, or other evolutionary changes. The magnitude and pattern of GC skew can vary among different species and even among different regions of the same genome.