Analogy:DNA and spiral staircase.

DNA has the shape of a double helix, just like a spiral staircase.

There are two sides of double helix, just like the two sides of staircase.

There are basepairs in DNA, just like the steps in staircase.

                       
          Points where the analogy breaks:

• DNA is living whereas a staircase is a non living thing.
• A human cell's DNA is about 3 meters in length but the length of a staircase is not fixed.