Question

According to Watson and Crick's model, approximately how many base pairs are found per turn of the DNA helix?

Options

• ~15 base pairs per turn

• ~20 base pairs per turn

• ~5 base pairs per turn

• ~10 base pairs per turn

Answer 1

Watson and Crick's double helix model indicates that approximately 10 base pairs occupy each complete turn of the helix. With a pitch (vertical distance per turn) of 34 ångströms (Å) and a distance of 3.4 Å between adjacent base pairs, this gives 34 ÷ 3.4 = 10 base pairs per turn. This regular spacing is a key feature of DNA's uniform dimensions.