A chromosome is formed from a single DNA molecule that contains many genes . The DNA molecule is highly condensed. The human DNA helix occupy too much space in the cell.
Small proteins are responsible for packing the DNA into units called nucleosomes .
When they are stained, the mitotic chromosomes have a banded structure that unambiguously identifies each chromosome of a karyotype. Each band contains millions of DNA nucleotide pairs which do not correspond to any functional structure.