Two chains of DNA twist around each other to form a DNA double helix with the phosphate-sugar backbone spiralling around the outside, and the bases pointing inwards with adenine base pairing to thymine and guanine to cytosine. The specificity of base pairing occurs because adenine and thymine align form two hydrogen bonds, whereas cytosine and guanine form three hydrogen bonds. The two strands in a double helix must therefore be complementary, with their sequence of bases matching such that the adenines of one strand are paired with the thymines of the other strand, and so on.:4.1 COUNT 5
QSN1 What is the backbone of a DNA double helix made of? ANS1 phosphate-sugar QSN2 What base pairs with thymine? ANS2 adenine QSN3 What base does guanine pair with? ANS3 cytosine QSN4 What causes the specificity of base pairing? ANS4 adenine and thymine align form two hydrogen bonds, whereas cytosine and guanine form three hydrogen bonds QSN5 Due to the cause of the specificity of base pairing, what must be true of the two strands in a double helix? ANS5 The two strands in a double helix must therefore be complementary