This new positive definition is based on molecular data rather than morphological observations. The old format has two major groups, the prokaryotes and the eukaryotes.
The current textbook paradigm for biological diversity and evolution is based on what I will call the prokaryote/eukaryote model. This posits that there are two kinds of cells: prokaryotic, those without nuclei (specifically, without nuclear membranes) and eukaryotic, those with a classical membrane-bounded nucleus. The model further posits that the former gave rise to the latter.
This "textbook" definition states that prokaryotes appeared first, and slowly evolved into eukaryotes. However, modern molecular analysis does not support this.
Molecular-sequence comparisons, first of ribosomal RNA genes in the late 1970s and of many other genes since, replaced analyses based on morphological subjectivities (such as the presence or absence of a nuclear membrane) with credible maps of evolutionary relationships between genes. These sequence comparisons have rendered the prokaryote/eukaryote model obsolete.
Analysis of the rRNA sequences results in the following evolutionary tree:
This illustrates that eukaryotes and archaea are more closely related than archaea and (eu)bacteria. So putting archaea and bacteria together in a group does not make evolutionary sense when the molecular biology is taken into account.
The article concludes with a call to remove the term "prokaryote" from the textbooks.
I believe it is critical to shake loose from the prokaryote/eukaryote concept. It is outdated, a guesswork solution to an articulation of biological diversity and an incorrect model for the course of evolution. Because it has long been used by all texts of biology, it is hard to stop using the word, prokaryote. But the next time you are inclined to do so, think what you teach your students: a wrong idea.
Our textbook has chapters on prokaryotes. I will do my best to refer to them as arcaea and bacteria.
7 comments:
I wanted to know if the quiz on monday will have questions on the microscope. Is the quiz only on organelles, the structures, and its functions?
@ Lisa
The quiz on monday will only be on organelle structure and function. There will not be anything on microscopes.
I am just checking to make sure that I fully understanding what you are saying here. Originally prokaryotes where defined in relationship to eukaryotes. Sort of saying that there are 2 types of people in the world, those that love pizza (Eukaryotes) and those that do not (Prokaryotes). But that prokaryotes are not in contrast to eukaryotes. In fact prokaryotes have been broken up into bacteria and archaea, with archaea being more closely related to eukarya than its former "groupie" of bacteria.
I hope I got that correct! Thanks!
@ kelly h
That is exactly right. I especially like your analogy to people who like pizza.
Extra credit opportunity: If correctly draw the diagram from the post on the back of the quiz, you will get 2 points of extra credit. If you write the name of the author of the article I cited, you will receive 5 points.
Will the extra credit count for Ms. Drust's class as well?
@ k!r@n
Of course it will. As long as Ms. Drust is OK with it, which I assume she will be.
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