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Two cell types… or three?
Prokaryotes/Eukaryotes OR Bacteria/Archaea/Eukarya
Suppose you'd never seen a house before. You start finding houses. There are one-room houses and there
are houses with lots of rooms (separate kitchen, bathroom, bedrooms, etc). You decide that all of the oneroom houses are "of a kind" because they have only one room with all activities taking place in that one
room. Then you look at all of these houses more closely. You discover that all of the houses with lots of
rooms are made of brick and mortar, with concrete foundations. Then you notice that of all the one-room
houses, most are made of sticks and straw and some are made of the same brick and mortar (with a concrete
foundation) as all of those big houses with lots of rooms.
Most everyone agrees with your earlier conclusion that the one-room houses are "of a kind." You begin to
question your earlier assumption. You consider that the houses made of brick (both the one-room and multiroom houses) are actually more "of a kind" because of the basic building materials used in their construction.
Your conclusion is surprising even to you, but the reasoning is sound.
Such is the case with the Bacteria, Archaea, and Eukarya. The bacteria represent those one-room houses
made of sticks. The Archaea and Eukarya represent the one- and multi-room brick houses, respectively.
You reason that the molecular underpinnings (the building materials in this analogy) are more significant
than the features. This is the reasoning behind the proposal to abandon the Prokaryote/Eukaryote dichotomy
in favor of a trifurcating system of organizing cell types. At a molecular level, the Eukarya and the Archaea
are more alike than either is to the Bacteria. The fact that Bacteria and Archaea are all one-celled organisms
is indeed significant, but not so much so as to lump them together in a common group. Especially given that
at a molecular level, the Archaea are more closely related to the Eukarya than either is to the Bacteria.
When Carl Woese discovered the Archaea in 1977, many scientists were in disbelief that these
"archaeabacteria" were significantly different from the others (Eubacteria). They were about the same size,
they lacked a nucleus and other membrane-bound organelles. They were "of a kind." Woese was looking –
FOR the first time - at the most significant of all molecular underpinnings, specifically, the sequence of
nucleotides in ribosomal RNA. Ribosomal RNA is intimately involved in the most significant role of any
cell… the translation of the genetic code (gene to protein). Scientists in Germany were looking at other
molecular features of these cells, and found fundamental differences. Evidence accumulated. In 1990,
Woese proposed the three DOMAINS of life, based on these 3 fundamentally different types of cells.* Not
based on outward appearances, but at the much more significant molecular components, and the ways these
molecules interacted. Nevertheless, the "Prokaryote" concept is not going away, and for this reason, you
must understand that when you encounter Prokaryotes in your text (and others), you must understand that it
is meant to include both the Bacteria (Eubacteria) and the Archaea (Archaeabacteria). *Every decade, the
Leeuwenhoek Medal is awarded to the microbiologist who has contributed the most to our understanding of
microbiology than any other. Woese was the 1990 recipient - the 12th of 13 going back to the 19th century.
Your text presents both of the classification systems. In chapter 6 there is no mention of a third type of
cell… only prokaryotes and eukaryotes. However, in chapter 18, section 5, the text first divides the
prokaryotes into Eubacteria and Archaeabacteria, then on page 481 the 3 Domain system (Bacteria, Archaea,
and Eukarya) is presented. Science should never hold on to concepts when evidence comes in that
contradicts the accepted understanding. However, it is apparently very difficult for some scientists to
abandon their understanding that they have held for a very long time. It is also inconvenient for textbook
publishers to re-write their books. In time, the Prokaryote concept will go away. It has been proven to be
false in that it explicitly describes Bacteria and Archaea as being in the same lineage (family) of cells. All
"prokaryote" really means is "not eukaryote."