Nucleus
The nucleus of the eukaryotic cells contains the DNA of the cell and the associated proteins that
form the chromosomes, and is separated from the cytoplasm by a double porous membrane.
The outer nuclear membrane has a complex internal structure, part of which is covered with
ribosomes, i.e., protein and RNA containing bodies which are the sites of protein synthesis. In
aerobic eukaryotes, chemical energy is trapped in the microchondria, organelles bounded by a
double membrane and containing an internal membrane structure in which are found both the
electron transport system and the enzymes associated with respiratory metabolism. Mitochondria
are absent in anaerobic eukaryotes. In photosynthetic eukaryotes, i.e., green algae, an organelle
similar to the mitochondrion, the chloroplast, is found. Both mitochondria and chloroplasts contain
small circular DNA molecules similar to those found in prokaryotes, ribosomes and other
components of protein synthesis.