Final answer:
Watson and Crick's model of DNA is described as a double helix, a structure comprising two antiparallel strands of nucleotides twisted around each other, with the external sugar and phosphate 'backbones' and internal base pairs resembling ladder steps.
Step-by-step explanation:
James Watson and Francis Crick, along with the crucial contributions from Rosalind Franklin, developed a model of DNA in which the two strands twist into the shape of a double helix. Watson and Crick proposed that the structure of DNA consists of two strands of nucleotides that twist around each other, suggesting that the correct shape DNA takes is that of a right-handed helix. The DNA double helix is composed of sugar and phosphate 'backbones' on the outside and the bases, which form hydrogen bonds with each other, on the inside, creating what resembles the steps of a ladder, known as base pairs. These two strands run in opposite orientations, known as antiparallel, where the 3' end of one strand faces the 5' end of the other.