Question 1

Why are the two DNA strands antiparallel?

Accepted Answer

DNA strands have a directional sugar-phosphate backbone: each chain has a 5' end and a 3' end. In double-stranded DNA, the strands run in opposite directions, so one reads 5' to 3' while the complementary strand reads 3' to 5' in the same spatial direction.

Question 2

How do adenine and guanine differ from thymine and cytosine?

Accepted Answer

Adenine and guanine are purines, with two fused nitrogen-containing rings. Thymine and cytosine are pyrimidines, with one ring, so each complementary base pair contains one purine and one pyrimidine.

Question 3

How many hydrogen bonds are present in A–T and G–C base pairs?

Accepted Answer

An adenine-thymine pair has two hydrogen bonds between the bases. A guanine-cytosine pair has three hydrogen bonds. The hydrogen bonds form between complementary bases, while the sugar-phosphate backbones remain outside.

Question 4

What does Chargaff's rule state for double-stranded DNA?

Accepted Answer

The percentage of adenine equals the percentage of thymine, and the percentage of guanine equals the percentage of cytosine. Consequently, total purines equal total pyrimidines, so (A + G)/(T + C) = 1.

Question 5

What are the major and minor grooves in DNA?

Accepted Answer

These are two grooves that run along the length of the double helix between the sugar-phosphate backbones. Their different widths result from the geometry of the two backbones, and proteins can use these grooves to recognize specific DNA sequences.

DNA Double Helix — Watson–Crick Model & Chargaff's Rules

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