The relationship between the three is so overly complicated
because of horizontal gene transfer, i.e. cells sharing parts of
their genome with others instead of the normal passing-down to
the next generation. This works between eukaryotes, archaea and
bacteria and causes the mixing of the three. An overview
(originally proposed by Woese et al): - Bacteria are simplest
considering cellular organisation. They have no nucleus or other
internal membranes and no cytoskeleton. Their DNA is generally
"junk-free"* (i.e. no introns), not bound by histones
("organising" proteins) and the organisation of regulatory
sequences is relatively straightforward, typically with one
'operator' sequence before a gene. - Eukaryotes are complex
cells in comparison. They have a nucleus and are full of
internal membranes (ER, golgi, vesicles,...). Their complex
cytoskeleton allows them to grow very large in comparison. Their
DNA is usually full of more or less "junk"*, organised into
tight bundles by histones and they tend to have a