* Train: A train is a complex structure made up of many individual cars connected together. Each car has a specific function, and they all work together to transport goods or people.

* Cars: Individual cars are the building blocks of a train.

Now, let's apply this to DNA:

* Molecule of DNA: A DNA molecule is a complex structure made up of many individual nucleotides connected together. Each nucleotide has a specific function, and they all work together to store and transmit genetic information.

* Nucleotides: Individual nucleotides are the building blocks of a DNA molecule.

So, the answer to your analogy is nucleotides.