Click here to. Cell Biology. The World of Cells. Prokaryotic Cells. Prokaryotic cells are the simplest systems that exhibit all of the signs of life. Despite their small size, inside each cell there is the complete chemical and biochemical machinery necessary for growth, reproduction and the acquisition and utilization of energy. Prokaryotes have a large array of abilities.
Some of them live in the absence of oxygen, some live in extreme conditions of heat or cold, others at the bottom of oceans where the only source of energy is hot hydrogen sulfide bubbling up from the core of the earth. Common Features. Energy comes in many forms, and various types of prokaryotic cells are adept at using almost all of them.
In large measure the actual structure of one of these cells reflects the way in which it acquires energy, but, despite their diversity, all prokaryotic cells have the following features in common. A Cell Wall. Prokaryotic cells walls give structural integrity and shape to the cell and serve to anchor the whip-like flagellae see below.
A Plasma Membrane. Just inside the cell wall, the plasma membrane is a selective barrier which regulates the passage of materials to from the cell. It is through this membrane that a cell must exchange food molecules, gases and other vital ingredients. There are more than different forms of peptidoglycan. S-layer surface layer proteins are also present on the outside of cell walls of both archaea and bacteria. Bacteria are divided into two major groups: gram-positive and gram-negative, based on their reaction to gram staining.
Note that all gram-positive bacteria belong to one phylum; bacteria in the other phyla Proteobacteria, Chlamydias, Spirochetes, Cyanobacteria, and others are gram-negative. The gram-staining method is named after its inventor, Danish scientist Hans Christian Gram — The different bacterial responses to the staining procedure are ultimately due to cell wall structure. Gram-positive organisms typically lack the outer membrane found in gram-negative organisms.
Up to 90 percent of the cell wall in gram-positive bacteria is composed of peptidoglycan, with most of the rest composed of acidic substances called teichoic acids.
Teichoic acids may be covalently linked to lipids in the plasma membrane to form lipoteichoic acids. Lipoteichoic acids anchor the cell wall to the cell membrane. Gram-negative bacteria have a relatively thin cell wall composed of a few layers of peptidoglycan only 10 percent of the total cell wall , surrounded by an outer envelope containing lipopolysaccharides LPS and lipoproteins.
This outer envelope is sometimes referred to as a second lipid bilayer. The chemistry of this outer envelope is very different, however, from that of the typical lipid bilayer that forms plasma membranes. Gram-positive and gram-negative bacteria : Bacteria are divided into two major groups: gram-positive and gram-negative. Both groups have a cell wall composed of peptidoglycan: in gram-positive bacteria, the wall is thick, whereas in gram-negative bacteria, the wall is thin.
In gram-negative bacteria, the cell wall is surrounded by an outer membrane that contains lipopolysaccharides and lipoproteins. Porins, proteins in this cell membrane, allow substances to pass through the outer membrane of gram-negative bacteria.
In gram-positive bacteria, lipoteichoic acid anchors the cell wall to the cell membrane. Prokaryotes reproduce asexually by binary fission; they can also exchange genetic material by transformation, transduction, and conjugation. Reproduction in prokaryotes is asexual and usually takes place by binary fission. The DNA of a prokaryote exists as as a single, circular chromosome. Prokaryotes do not undergo mitosis; rather the chromosome is replicated and the two resulting copies separate from one another, due to the growth of the cell.
The prokaryote, now enlarged, is pinched inward at its equator and the two resulting cells, which are clones, separate. Binary fission does not provide an opportunity for genetic recombination or genetic diversity, but prokaryotes can share genes by three other mechanisms. Modes of prokaryote reproduction : Besides binary fission, there are three other mechanisms by which prokaryotes can exchange DNA.
In a transformation, the cell takes up prokaryotic DNA directly from the environment. In b transduction, a bacteriophage injects DNA into the cell that contains a small fragment of DNA from a different prokaryote. In c conjugation, DNA is transferred from one cell to another via a mating bridge that connects the two cells after the pilus draws the two bacteria close enough to form the bridge.
In transformation, the prokaryote takes in DNA found in its environment that is shed by other prokaryotes. This is a method of asexual reproduction that is similar in its end result to mitosis—two daughter cells result, each with the same number of chromosomes as the parent cell. However, when bacteria undergo binary fission, no mitotic spindle forms. Although plasmids can occur in a variety of sizes ranging from around a thousand base pairs to hundreds of thousands , they usually only have a small number of genes.
Antibiotic resistance is a trait that is frequently attributed to genes on plasmids. When a prokaryotic cell with a plasmid divides, the daughter cells each receive a copy of the plasmid, along with its regular chromosome. See more from our free eBook library. An article from Khan Academy about binary fission in bacteria. Eukaryotic Chromosomes. Eukaryotic vs. Prokaryotic Chromosomes.
DNA Structure.
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