Fig. 4:

Purine skew is driven by the direction of transcription, not replication. a, Schematic illustrating regions considered in b. Thin arrows indicate direction of replication. Thick arrows represent coding sequences positioned on each replication strand. oriC, origin of replication. b, Top, purine content of 300-nt sequence fragments derived from any position on the leading strand (all; thick orange line), or sense to genes (sense; filled) (Methods). Bottom, same as top, but for the lagging strand of replication.