A DNA molecule is made of two strands. Each strand is a polymer of nucleotides. The two strands are twisted around each other to form a spiral shape called a double helix.
In each strand the sugar and phosphate groups join up to form the backbone on the outside. The bases point inwards towards the other strand. A base on one strand pairs with a base on the other strand: adenine always pairs with thymine, and cytosine always pairs with guanine. Weak bonds between the pairs hold the two strands together.
The two strands are the same length and every base has a partner. So if one strand has 30 bases, the other strand also has 30, and the double helix has 60 bases in total.
