Slide 1
Prokaryotes are organisms that lack a nucleus and other
membrane-bound organelles. Organisms with a nucleus are referred to as
eukaryotes.
They are a diverse group of organisms that are structurally rather simple but have a wide array of metabolic modes.
There are three basic shapes - cocci which are spherical, bacilli which are rod shaped, and spirilla which have a spiral morphology.
Compared to eukarytoes, the prokaryotes have a small amount of DNA. The
smallest known bacteria, the Mycoplasmas, have only about 500 genes.
The
largest bacteria have about 5000 genes. The simplest eukaryotic genomes
have about 5000 genes and the largest eukaryote genome belongs to Trichomonas
vaginalis - a single celled organism that causes urinary tract
infections. It has 60,000 genes while humans have only 20,000
genes. It has been a paradox to biologists why the number of genes is
not related to organismal complexity.
Slide 2
The prokaryotes are divided into two main groups - the bacteria
and the archaea. Domain bacteria contains prokaryotes whose cell walls
contain a unique substance known as peptidoglycan. Peptidoglycan is a
polymer of sugars and proteins that plays a structural role in bacteria.
Another distinguishing characteristic of the bacteria is that their genes lack introns. Introns are non-coding regions of genes. In both eukaryotes and archaea - genes have intervening sequences that are not transcribed or translated into protein. The bacteria do not possess these introns.
Slide 3
Domain Archaea is comprised of individuals that are characterized
by lacking peptidoglycan, producing unique cell membrane lipds, and possessing
introns in some of their genes.
The Archaea used to be considered extremophiles - organisms that can live in extreme environments like the highly saline environment of these San Francisco bay salt ponds, the extremely hot environment of this Yellowstone National Park hot spring, and the high sulfur high temperature environment at the bottom of the ocean near this ocean vent.
However, we now know that there are Archaea in non-extreme environments like soil and water. Some even inhabit the human gut.
Slide 4
Originally, the prokaryotic organisms were all grouped together
into a single kingdom - the Monera. You may have learned this
classification system in a previous biology course. Life used to be
classified using a 5 kingdom system - the plants, animals, fungi,
single-celled protists, and the monera - which included all of the bacteria
and archaea.
We now recognize that this system does not adequately reflect evolutionary relatedness between these groups. We now use a 3 domain system of classification. This system recognizes that all eukaryotic organisms are descended from a common ancestor. It also recognizes that the bacteria and the archaea are not closely related and share only superficial similarities.
Slide 5
The 3 domain system recognizes that the archaea and the eukaryotes
are more closely related to each other than either is to the bacteria.
We read evolutionary trees like these by noting the distance between branches. The closer a branching point - the closer the evolutionary relatedness. These trees reflect our current understanding that the prokaryotes are diverse and the bacteria and archaea are not closely related.