DNA molecules are huge in size. On an average, a base pair of B-DNA with a thickness of.34 nm has a molecular weight of 660. The term kilobase pair (Kb = 1,000 base pairs) is common in the DNA structure. The number of base pairs and the length of DNA (contour length) varies from species to species.

DNA molecules are characteristically highly elongate structures. The Escherichia coli chromosome is a single molecule of double helical DNA, containing about 4 million base pairs. The DNA contains 4.6 million base pairs. Its contour length is about 3 mm and 1 mm in diameter.

The largest chromosomes of Drosophila melanogaster contain a single DNA molecule with 6.2 × 10⁷ base pairs.

For instance, the DNA from polyomavirus contains 4,600 base pairs and has a contour length of 1.6 μm. Furthermore, tropocollagen, the basic structural unit of collagen, has the shape of a rod, which is 3,000 Å in length and 15 Å in diametre.

DNA molecule from the T7 bacteriophage is, however, linear in shape.

In case of humans, the twenty-three haploid chromosomes have 2,900,000 KB with a total contour length of 990 mm.