Self-Sustaining Chemical Systems:
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.