ABSTRACT
The original concept of a gene was a region of DNA that encoded a single polypeptide product. By the 1960s, the gene was clearly defined as the region of DNA that gives rise to a single polypeptide. The genome of an organism encompasses all of the genes of that organism. Even in a bacterial cell such as Escherichia coli, the amount of DNA required is substantial and so this DNA must be packaged. The large amount of genomic DNA in a eukaryotic cell is tightly packaged in chromosomes contained within a specialized organelle, the nucleus. In the nucleus, each chromosome contains a single linear double-stranded DNA molecule. When chromosomes are gently 'decondensed', they have the appearance under the electron microscope of 'beads on a string'. The length of the packaged DNA molecule varies. When chromosomes are depleted of histones, they are seen to have a central fibrous 'protein scaffold' to which the DNA is attached in loops.
