7.3 The eukaryotic chromosome

Whilst the bulk of eukaryotic DNA is packaged by proteins different from those in the eubacterial chromosome, the principles of bending DNA and neutralising the negative charges in its backbone are shared. Eukaryotic cells have considerably larger genomes than do prokaryotes (in most cases over 1000 times the size of the E. coli genome – see Table 1) and the DNA is found in the form of long linear chromosomes which are partitioned from the rest of the cell, within the nucleus.

Several degrees of DNA packaging and compaction are found within the eukaryotic nucleus, primarily reflecting the transcriptional and structural status of the DNA. The extent of DNA packaging also varies according to the stage of the cell cycle. For example, immediately prior to cell division, the DNA is particularly compacted and forms highly condensed structures suitable for physical separation at mitosis. The different degrees of packaging will be discussed later, but for now we will focus on the major protein components of eukaryotic chromatin, the histones.