A chromosome is a physically discrete portion of the genome, which carries many individual genes. Eukaryotic chromosomes are composed of chromatin, which is a mixture of DNA and protein. The protein, primarily histones, acts as a scaffold organizing the structure of the DNA, which is a long string (polymer) of nucleotides (adenine, cytosine, guanine, and thymine) Prokaryotic chromosomes are single, circular, and do not contain a histone scaffold (that is, they are not composed of chromatin. Eukaryotic chromosomes are multiple and linear. The name chromosome, meaning "colored body," is derived from the fact that in early studies of cellular structure the chromosomes could be easily stained with colored dyes and therefore showed up as colored bodies under the microscope.
The diagram of chromosome structure above shows how DNA is organized in a eukaryotic cell. Each of the small clusters of histones shown is a nucleosome. This figure gives some idea of the depth of complexity of a eukaryotic cell. For more information click on the links at right.
Image: Uta Mackensen, EMBO