The total amount of DNA in an organism (its genome) can be estimated by physical measurements. Three basic classes of DNA exist in higher organisms. The term “complexity” refers to the number of independent sequences in DNA. Eukaryotic DNA can be divided into several classes of complexity. About half of the total DNA in a mammal is found in the most complex fraction. This fraction of the genome codes for functional genes and corresponds to sequences that exist in only one copy per genome. About a fifth of the DNA is moderately repetitive and present on the order of hundreds to thousands of times per genome. This fraction includes some sequences that are transcribed from many copies of the same sequence. For example, the genes for ribosomal RNA reside in this fraction. The remainder of the DNA is highly repetitive and can occur on the order of millions of copies per genome. This DNA is not transcribed much at all and may include DNA that is involved in chromosome structure.
Each of the three fractions contain a number of sequences that are sometimes called “junk” and can represent, for example, viruses that found their way into DNA in the past but were inactivated, leading to the fact that these sequences remain in the genome, but never express themselves. All of this DNA must be highly condensed. The DNA in each chromosome is a single molecule, on the order of several centimeters in length; the total DNA in a eukaryotic cell is as much as three meters long. This DNA must be condensed so as to fit into a nucleus that is about 10 ‐5 meters (10νm) in diameter. The condensed structure of eukaryotic DNA is called chromatin.