by 
The Curious Bag
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The DNA double helix resembles a staircase. Looking at the molecule vertically, two sides spiral upward with four types of stair halves--nitrogenous bases adenine, cytosine, guanine, and thymine. Because the size of the stairs must be uniform, adenine can only pair up with thymine, and guanine with cytosine.
Composition of Sides
The helical sides of the DNA molecule consist of alternating phosphate and sugar "residues."
Deoxyribose
The sugar combining with phosphate groups is deoxyribose: C...H₁₀O₄. Deoxyribose is a cyclic five-carbon sugar, which is called pentose.
Phosphate Groups
Deoxyribose is connected to a base (step) on one side and a phosphate group at top and bottom. The bond connecting the sugar with the deoxyribose is called a phosphodiester bond.
Open Spaces
The two spiral sides of the DNA have large spiral openings between them to allow access by other important body chemicals. The access is to the edges of the bases, or steps.
DNA Functionality
DNA contains the genetic instructions for every known living organism. As needed, it can be replicated. It controls many cellular functions.
Source:
Estrella Mountain Community College: "DNA and Molecular Genetics"
Resource:
The Biology Corner: "DNA--the Double Helix"
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