a1.2.6 - the DNA double helix

Question 1

describe the structure of a DNA double helix

Answer 1

• two polymers of DNA nucleotides, each with a sugar-phosphate backbone, run in antiparallel directions. complementary DNA nitrogenous bases (A-T, C-G) form hydrogen bonds between them, binding the two polymer stands (“double”) so that they wind around each other (“helix”)

Question 2

outline the complementary base pairing rule, including the type and number of bonds between bases

Answer 2

in DNA, the nitrogenous bases of two antiparallel strands form hydrogen bonds with each other. the complementary base pairing rule is that adenine only binds with thymine (with 2 H-bonds) and that guanine only binds with cytosine (with 3 H-bonds).

Question 3

define antiparallel in relation to DNA structure

Answer 3

• adjacent molecules are oriented parallel to each other but oriented in opposite directions.
• in DNA, one strand runs 5’ to 3’ and the complementary strand runs 3’ to 5’