chromosome
As a cell prepares to divide, its DNA first forms one or more structures called chromosomes. A chromosome consists of DNA and protein molecules coiled into a definite shape. Chromosomes are circular in prokaryotes and rodlike in eukaryotes. You can see an example of a human chromosome in Figure below . The rest of the time, DNA looks like a tangled mass of strings. In this form, it would be very difficult to copy and divide.