Answer:sugar–phosphate groups; nitrogenous bases; antiparallel.
Step-by-step explanation:
A nucleotide consist of the sugar, phosphate group and the base. The structure of the DNA is similar to spiral staircase. The sugar-phosphate comprises the handrail and the base jutting inside represent the steps of staircase. The sugar-phosphate run down outside of the DNA double helix, hence they forms the backbone of the DNA double helix.
Nitrogenous bases are the staircase linking the both handrails. The nitrogenous bases inside the double helix are components that are responsible for forming the hydrogen bonds between the two strands of the double helix.
DNA is a double helical structure, it has two strands running in opposite directions (leading strand 5' to 3', the lagging strand 3' to 5'), hence they are "antiparallel".