1. 21.3 DNA Double Helix
In the model shown, the sugar–phosphate backbone is represented by a ribbon with hydrogen bonds between complementary base pairs.
Learning Goal Describe the double helix of DNA.

2. DNA, Ratio of Bases Scientists determined that
adenine is paired (1:1) with thymine.
guanine is paired (1:1) with cytosine.
This relationship, known as Chargaff’s rules, can be summarized:
Number of purine molecules = Number of pyrimidine molecules
Adenine (A) = Thymine (T)
Guanine (G) = Cytosine (C)

3. Base Percentages in DNA of Selected Organisms

4. Complementary Base Pairs: A and T
DNA contains complementary base pairs in which adenine is always linked by two hydrogen bonds to thymine (AT).
Core Chemistry Skill Writing the Complementary DNA Strand

5. Complementary Base Pairs: G and C
DNA contains complementary base pairs in which guanine is always linked by three hydrogen bonds to cytosine (GC).

6. Double Helix of DNA The DNA structure is a double helix that
consists of two strands of nucleotides that form a double helix structure like a spiral staircase.
has two strands held together by the hydrogen bonds between the bases AT and GC.
has bases along one strand that complement the bases along the other.

7. Study Check
Write the complementary base sequence for the matching strand in the DNA section shown below.

8. Solution
Write the complementary base sequence for the matching strand in the DNA section shown below.