PROKARYOTE and EUKARYOTE
What Is Prokaryote?
Prokaryotes are mostly unicellular organisms without a nucleus, in contrast to eukaryotes, organisms that have cell nuclei and may be variously unicellular or multicellular. The difference between prokaryote and eukaryote cell structure is the most important in the living world. Most prokaryotes are bacteria, and the two terms are often treated as synonyms. However, Woese has proposed dividing them into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) on the supposition that these have separate origins. This controversial arrangement is called the three-domain system.
The name prokaryote comes from the Greek "PROS" meaning "before" and "karyon" meaning "nut", referring to the nucleus. Prokaryotes also lack cytoskeletons and membrane-bound cell compartments such as vacuoles, endoplasmic reticulum, mitochondria and chloroplasts. In eukaryotes, the latter perform various metabolic processes and are believed to have been derived from endosymbiotic bacteria. In prokaryotes similar processes occur within the cell membrane, and endosymbionts are extremely rare. They are usually much smaller than eukaryotic cells.
Prokaryotes have a single chromosome, contained within a nucleoid region rather than a membrane-bound nucleus, but may also have various small circular pieces of DNA called plasmids spread throughout the cell. Reproduction is exclusively asexual, through binary fission, where the chromosome is duplicated and attaches to the cell membrane, and then the cell divides in two.
______________________________________________________________________________________________
What is Eukaryote?
Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. They include the animals, plants, and fungi, which are mostly multicellular, as well as various other groups called protists, many of which are unicellular. In contrast, other organisms such as bacteria lack nuclei and other complex cell structures, and are called prokaryotes. The eukaryotes share a common origin, and are often treated formally as a superkingdom, empire, or domain. The name comes from the Greek eus or true and karyon or nut, referring to the nucleus.
It is generally accepted that the first living cells were some form of prokaryote, and they are known as fossils from over 3.5 billion years ago. Some have suggested structures within a Martian meteorite should be interpreted as fossil prokaryotes, but this is extremely doubtful.
Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. They include the animals, plants, and fungi, which are mostly multicellular, as well as various other groups called protists, many of which are unicellular. In contrast, other organisms such as bacteria lack nuclei and other complex cell structures, and are called prokaryotes. The eukaryotes share a common origin, and are often treated formally as a superkingdom, empire, or domain. The name comes from the Greek eus or true and karyon or nut, referring to the nucleus.
It is generally accepted that the first living cells were some form of prokaryote, and they are known as fossils from over 3.5 billion years ago. Some have suggested structures within a Martian meteorite should be interpreted as fossil prokaryotes, but this is extremely doubtful.
The Endosymbiosis Theory postulates that
- The mitochondria of eukaryotes evolved from aerobic bacteria (probably related to the rickettsias) living within their host cell.
- The chloroplasts of red algae, green algae, and plants evolved from endosymbiotic cyanobacteria.
- Both mitochondria and chloroplasts can arise only from preexisting mitochondria and chloroplasts. They cannot be formed in a cell that lacks them because nuclear genes encode only some of the proteins of which they are made.
- Both mitochondria and chloroplasts have their own genome, and it resembles that of bacteria not that of the nuclear genome.
- Both genomes consist of a single circular molecule of DNA.
- There are no histones associated with the DNA.
- Both mitochondria and chloroplasts have their own protein-synthesizing machinery, and it more closely resembles that of bacteria than that found in the cytoplasm of eukaryotes.
- The first amino acid of their transcripts is always fMet as it is in bacteria (not methionine [Met] that is the first amino acid in eukaryotic proteins).
- A number of antibiotics (e.g., streptomycin) that act by blocking protein synthesis in bacteria also block protein synthesis within mitochondria and chloroplasts. They do not interfere with protein synthesis in the cytoplasm of the eukaryotes.
- Conversely, inhibitors (e.g., diphtheria toxin) of protein synthesis by eukaryotic ribosomes do not — sensibly enough — have any effect on bacterial protein synthesis nor on protein synthesis within mitochondria and chloroplasts.
- The antibiotic rifampicin, which inhibits the RNA polymerase of bacteria, also inhibits the RNA polymerase within mitochondria. It has no such effect on the RNA polymerase within the eukaryotic nucleus.