Here's a breakdown:

* Chromosomes are thread-like structures found in the nucleus of cells. They contain the genetic information that is passed down from parents to offspring.

* DNA is a complex molecule that carries this genetic information. It's structured as a double helix, resembling a twisted ladder.

* Genes are segments of DNA that contain the instructions for building and maintaining an organism.

* Chromosomes are made up of tightly coiled DNA, along with proteins called histones that help organize and protect the DNA.

So, the long molecule in a chromosome is the DNA molecule, which is essential for carrying and transmitting genetic information.