When the dna ladder twists the way it normally does the shape of the molecule is called a double helix. The DNA is made of nucleotides consisting of a de-oxyribo sugar, a phosphate group and one of the four nitrogen base.
Adenine pairs with Thymine with 2 hydrogen bond and Guanine pairs with Cytosine with 3 hydrogen bond .This provided the clue the DNA molecule is a double helix , shaped like a twisted ladder.