The human genome consists of 46 molecules of DNA, called chromosomes. Half of these are inherited from each parent, with each pair of chromosomes encoding a different segment of the human genome in duplicate. DNA contains only four different nucleotides, but this allows storage of a tremendous amount of information. For example, in a chain of five nucleotides, there are four possibilities at each of the five positions, or 4 x 4 x 4 x 4 x 4 = 4^5 = 1024 possible combinations! The complete set of human DNA, or the human genome, is comprised of 3 billion base pairs: to make a human requires a very precise set of nucleotide “instructions.”
Each base pair is 0.34 nanometers long. This means that each human cell contains over one meter of DNA! One meter of DNA must be packaged into a cell that is only micrometers in size. To accomplish this, the DNA is wrapped around proteins called histones.
Histones are small, positively charged proteins. The negative charge of the DNA backbone facilitates interaction with the histones. Eight individual histones come together to form a histone octamer. A length of DNA about 150 base pairs long wraps around each octamer, forming a nucleosome. Like beads on a string, each nucleosome core is connected to the next by a linker of 20-60 base pairs of DNA. This is illustrated below:
The “beads-on-a-string” structure illustrated above is looped and twisted to be further compacted and packaged tightly into the nucleus.