|MadSci Network: Molecular Biology|
In Eukaryotes (animals, plants, fungi, protists... everything but bacteria and archaea, which are Prokaryotes) DNA exists in the form of linear chromosomes which reside in the nucleus of the cell. The majority (up to 90%) of eukaryotic chromosomes is non-coding DNA, which takes up a lot of room - mammalian chromosomes average around 2.5 cm in length. So eukaryotic DNA is packaged into Chromatin consisting of the chromosomal DNA and lots of proteins which condense the DNA into the smallest possible volume. The basic unit of DNA packaging is the Nucleosome, a bobbin shaped complex of eight proteins called Histones around which the DNA is wound. A chromosome covered with nucleosomes looks like "beads on a string". Addition of another protein called H1 (for Histone 1) allows adjacent nucleosomes to join together to form a nucleosome helix, much like twisting a string of beads results in a shorter, thicker "bead spiral". Addition of more proteins holds the nucleosome helices together as short loops, and even further packaging holds the "strands of loops" together in bigger loops. (This final packaging is actually similar to prokaryotic chromosomes which are loosely packaged by gathering looped bits together to form a rosette called the nucleoid.) Nucleosomes are very important not only for DNA packaging, but also for determining which genes can be used by the cell. Nucleosomes can be displaced along the chromosome, or opened up to make genes accessible to the cell for transcription to make proteins. There are many many laboratories (including my own) that are studying how nucleosomes control genes and why some DNA is more densely packaged than the rest.
Try the links in the MadSci Library for more information on Molecular Biology.