Prokaryotic Cells
http://click4biology.info/c4b/2/cell2.2.htmA number of years ago two Norwegian biologists obtained a sample of soil from a nearby piece woodland. On returning to the laboratory they began to identify known species of
prokaryote and also to classify any unknown species in their sample. By the time the two norwegians had finished they had doubled the entire catalogue of known prokaryotic species. The introduction to prokaryotes provided here is however based on perhaps the best know prokaryote Escherichia coli (E.
coli). Notice that image of E. coli to the left shows a straight rod shaped cell. Other types of bacteria show different morphology some of these variations are shown in the table below.
There are two major kinds of prokaryotes:
Bacteria
As you may have read earlier in this unit, biologists now estimate that each human being carries nearly 20 times more bacterial, or prokaryotic, cells in his or her body than human, or eukaryotic, cells. If that statistic overwhelms you, rest assured that most of these bacteria are trying to help, and not hurt, you. Numerically, at minimum, there are 20 times more prokaryotic cells on Earth than there are eukaryotic cells. This is only a minimum estimate because there are trillions of trillions of
bacterial cells that are not associated with eukaryotic organisms. In addition, all Archaea are also prokaryotic. As is the case for bacteria, it is unknown how many Archaean cells are on Earth, but the number is sure to be astronomical. In all, eukaryotic cells make up only a very small fraction of the total number of cells on Earth. So who runs this place, anyway?
A diagram of the ultrastructure of Escherichia coli (E. coli) as an example of
a prokaryote
The general size of a prokaryotic cell is about 1-2 um.
Note the absence of membrane bound organelles
There is no true nucleus with a nuclear membrane
The ribosome's are smaller than eukaryotic cells
The slime capsule is used as a means of attachment to a surface
Only flagellate bacteria have the flagellum Plasmids are very small circular pieces of
DNA that maybe transferred from one bacteria to another.
Cell Wall: Made of a murein (not cellulose),
which is a glycoprotein or peptidoglycan (i.e. a protein/carbohydrate complex). There are two kinds of bacterial cell wall, which are identified by the Gram Stain technique when observed under the microscope. Gram positive bacteria stain purple, while Gram negative bacteria stain pink. The technique is still used today to identify and classify bacteria. We now know that the different staining is due to two types of cell wall
Plasma membrane: Controls the entry and exit of substances, pumping some of them in by active transport.
Prokaryotic cells can have multiple plasma membranes. Prokaryotes known as "gram-negative bacteria," for example, often have two plasma membranes with a space between them known as the periplasm. As in all cells, the plasma membrane in prokaryotic cells is responsible for controlling what gets into and out of the cell. A series of proteins stuck in the membrane (poor fellas) also aid prokaryotic cells in communicating with the surrounding environment. Among other things, this communication can include sending and receiving chemical signals from other bacteria and interacting with the cells of eukaryotic organisms during the process of infection. Infection is the kind of thing that you don't want
prokaryotic and eukaryotic. Because this cellular component is so important and so common, it is addressed in great detail in its own In Depth subsection.
Cytoplasm: Contains all the enzymes needed for all metabolic reactions, since there are no organelles.
The cytoplasm in prokaryotic cells is a gel-like, yet fluid, substance in which all of the other cellular components are suspended. Jello for cells. It is very similar to the eukaryotic cytoplasm, except that it does not contain organelles. Recently, biologists have discovered that prokaryotic cells have a complex and functional cytoskeleton1 similar to that seen in
eukaryotic cells. The cytoskeleton helps prokaryotic cells divide and helps the cell maintain its plump, round shape. As is the case in eukaryotic cells, the cytoskeleton is the framework along which particles in the cell, including proteins, ribosomes, and small rings of DNA called plasmids, move around.
Ribosome: The smaller (70 S) type are all free in the cytoplasm, not attached to
membranes (like RER). They are used in protein synthesis which is part of gene expression.
Nucleoid: Is the region of the cytoplasm that contains DNA. It is not surrounded by a nuclear membrane. DNA is always a closed loop (i.e. a circular), and not associated with any proteins to form chromatin.
Flagella: These long thread like attachments are generally considered to be for movement. They have an internal protein structure that allows the flagella to be actively moved as a form of propulsion. The presence of flagella tends to be associated with the pathogenicity of the bacterium. The flagella is about 20nm in diameter. This structure should not be
confused with the eUkaryotic flagella seen in protoctista.
Pilli: These thread like projections are usually more numerous than the flagella. They are associated with different types of attachment. In some cases they are involved in the transfer of DNA in a process called conjugation or alternatively as a means of preventing phagocytosis.
Slime Capsule: A thick polysaccharide layer outside of the cell wall, like the glycocalyx of eukaryotes. Used for sticking cells together, as a food reserve, as protection against desiccation and chemicals, and as protection against phagocytosis. In some species the capsules of many cells in a colony fuse together forming a mass of sticky cells called a biofilm. Dental plaque is an example of a biofilm.
Plasmids:
Extra-nucleoid DNA of up to 400 kilobase pairs. Plasmids can self-replicate particularly before binary fission.
They are associated with conjunction which is horizontal gene transfer.
It is normal to find at least one anti-biotic resistance gene within a plasmid. This should not be confused with medical phenomena but rather is an ecological response to other antibacterial compounds produced by other microbes. Commonly fungi will produce anti-bacterial compounds which will prevent the bacteria replicating and competing with the bacteria for a resource.
1
Conjugation
Direct contact between bacterial cells in which plasmid DNA is transferred between a donor cell and a recipient cell.
There is no equal contribution to this process, no fertilisation and no zygote formation. It cannot therefore be regarded as sexual reproduction.
Prokaryotic Genetic Material
All prokaryotic cells contain large quantities of genetic material in the form of DNA and RNA. Because prokaryotic cells, by definition, do not have a nucleus, the single large circular strand of DNA containing most of the genes needed for cell growth, survival, and reproduction is found in the cytoplasm.
The DNA tends to look like a mess of string in the middle of the cell:
Usually, the DNA is spread throughout the entire cell, where it is readily accessible to be transcribed into messenger RNA (mRNA) that is immediately translated by ribosomes into protein. Sometimes, when biologists prepare prokaryotic cells for viewing under a microscope, the DNA will condense in one part of the cell producing a darkened area called anucleoid.
As in eukaryotic cells, the prokaryotic chromosome is intimately associated with special proteins involved in maintaining the chromosomal
structure and regulating gene expression. In addition to a single large piece of chromosomal DNA, many
prokaryotic cells also contain small pieces of DNA called plasmids. These circular rings of DNA are replicated independently of the chromosome and can be transferred from one prokaryotic cell to another through pili, which are small projections of the cell membrane that can form physical channels with the pili of adjacent cells.
The transfer of plasmids between one cell and another is often referred to as "bacterial sex." Sounds dirty. The genes forantibiotic resistance, or the gradual ineffectiveness of antibiotics in populations, are often carried on plasmids. If these plasmids get transferred from resistant cells to nonresistant cells, bacterial infection in populations can become much harder to control. For example, it was recently learned that the superbug MRSA, or multidrug-resistant Staphylococcus aureus, received some of its drug-resistance genes on plasmids.
Prokaryotic cells are often viewed as "simpler" or "less complex" than eukaryotic cells. In some ways, this is true: prokaryotic cells usually have fewer visible structures, and the structures they do
have are smaller than those seen in eukaryotic cells. Don’t be fooled, however, into thinking that just
Biologists are now learning that bacteria are able to communicate and collaborate with one another on a level of complexity that rivals any communication system ever developed by humans.
Prokaryotes showed you, Facebook and Twitter. In addition, some Archaean cells are able to thrive in environments so hostile that no eukaryotic cell or organism would survive for more than a few seconds.6 Try living in a hot spring, salt lake, deep Earth, or a volcano!
Prokaryotic cells are also able to pull off stuff that eukaryotic cells could only dream of, in part because of their increased simplicity. Being bigger and more complex is not always better. These cells and organisms are just as adapted to their local conditions as any eukaryote, and in that
sense, are just as “evolved” as any other living organism on Earth.
Brain Snack
One kind of bacterial communication, also known as quorum sensing, is where small chemical signals are used to count how many bacteria there are.
The structures in electron micrographs of E. coli
1. Note the double membrane of this E. coli . This feature means that the cells do not retain the dark blue stain used in microscopy. They are therefore known as Gram-negative this contrast with Gram-positive single membrane bacteria.
2. There is some evidence in the image of pilli which are the surrounding light grey masses.
3. In the cytoplasm of the bacterium there are no visible organelles which is consistent with how we expect a prokaryote cell to appear.
4. The nucleoid region is not seen well in this particular image but is clearer in the next image.
The prokaryotic cells divide by binary fission
Prokaryotic cells divide by binary fission.
This is an asexual method of reproduction in which a 'parental' cell divides into two smaller but equally sized cells.
The cells are genetically identical and form the basis of a reproductive clone.
The process of binary fission takes place in four stage:
(a). Reproduction signal: The cell receives a signal, of internal or external origin that initiates the cell division.
E.coli replicates about once every 40 minutes when incubated at 37o C. If however we increase the
concentration of carbohydrate nutrients that the cell is supplied with then the division time can be reduced to 20 minutes. There is a suggestion here that an
external signal (nutrient concentration) is acting as the reproductive signal.
(b). Replication of DNA: bacterial cells have a single condensed loop of DNA. This is copied by a process known as semi-conservative replication to produce two copies of the DNA molecule one for each of the
daughter cells
The replication begins at a single point (ori)on the loop of DNA. The process proceeds around the loop until two loop have been produced, each a copy of the original. The process finishes at a single point on the loop of DNA called the ter position.
(c). Segregation of DNA: One DNA loop will be provided for each of the daughter cells.
As the new loops form the ori site becomes attached to some contractile proteins that pull the two ori sites, and therefore the loops, to opposite ends of the cell. This is an active process that requires the bacteria to use energy for the segregation.
(d). Cytokinesis: Cell separation.
This occurs once the DNA loop replication and segregation is complete. The DNA completes a process of condensing whilst the plasma membrane begins to form a 'waist' or constriction in the middle of the cell. As the plasma membrane begins to pinch and constrict the
