Chromosomes are important because they contain DNA, the biochemical substance required for the molecular-level cellular messages known as gene expression. The DNA contained in chromosomes carries the instructions required for the functioning and development of all living organisms. In this respect, the chromosomes and the DNA contained within them serve as a means of storing biological information.
Each chromosome is comprised of a DNA molecule made up of nucleotides in a double-strand formation that resembles a ladder with a twist. This formation is called a double helix. Each nucleotide is made up of sugar, a phosphate and a base. There are four different bases: adenine, guanine, cytosine and thymine. The order in which these four bases pair with each other to create sequences of base-pairs is what determines the encoding of the genetic information, much like letters combine to form words. In this case, the "words" function as a set of instructions for the organism's cells.