 Genes are arranged linearly along long chains of DNA base @-@ pair sequences . In bacteria , each cell usually contains a single circular genophore , while eukaryotic organisms ( such as plants and animals ) have their DNA arranged in multiple linear chromosomes . These DNA strands are often extremely long ; the largest human chromosome , for example , is about 247 million base pairs in length . The DNA of a chromosome is associated with structural proteins that organize , compact and control access to the DNA , forming a material called chromatin ; in eukaryotes , chromatin is usually composed of nucleosomes , segments of DNA wound around cores of histone proteins . The full set of hereditary material in an organism ( usually the combined DNA sequences of all chromosomes ) is called the genome . 
