New strands, copied from the oldParent strands, base pairs and three end flangesB-DNA at true scale on the ruler (1 major tick = 1 nm): 2 nm wide, 0.34 nm per base pair, 10.5 base pairs per turn. Each purine (A, G) reaches further across than a pyrimidine (T, C), so every pair is the same width; A–T holds by two hydrogen bonds, G–C by three. The leading strand is copied right up to the fork, the lagging strand backwards in fragments (here 9 bp; really 100–200 nt in human cells) that leave nicks between them. The tour moves the fork about 1 bp a second, roughly 1,000× slower than in E. coli.
2 nm双螺旋直径helix diameter0.34 nm每碱基对上升rise per base pair10.5 bp每一圈base pairs per turn~1,000 bp/s大肠杆菌复制叉速度E. coli fork speed~50 bp/s人类复制叉速度human fork speed