DNA in eucaryotes is confined within a nucleus, which is bounded by a double-layer nuclear membrane or envelope that is pierced by pores. The cutaway of part of a cell on the previous page gives some impression of the structure of the nucleus and its pores.

The DNA is combined with histones, which are basic proteins that probably help to control the use and suppression of information on different parts of the DNA. DNA is further organized into packages known as chromosomes. During cell division, DNA goes through a complicated copying process that is beyond this discussion, but the enzymes for both the replication of DNA and the formation of messenger RNA are found inside the nucleus.

Other specialized organelles such as the nucleolus and the centrioles, which are outside the nucleus, are essential parts of the reproductive process, but are not of immediate concern to us in a discussion of cells as organized systems of chemical reactions.