Hermine E. Smikle
Microbes have a much longer evolutionary history than plants and animals, and therefore have had more time to evolve into diverse forms
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. Since they were here first, they had unchallenged access to all the sites both on the surface and in the interior of the Earth. These microbes have been challenged to survive under cataclysmic conditions that are unknown to animals and plants, because plants and animals have only been around for a relatively short span of evolutionary time.
Micro organisms have proven their ability to withstand challenges that have never been experienced by man.
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They are responsible for transforming the atmosphere through their chemical activities and thus making the Earth habitable for plants and animals. The catastrophic impacts that were experienced on the Earth during the early formation of life did not completely obliterate emerging life, microorganisms survived this period. After this traumatic period micro organisms multiplied and occupied most of Earth's environments, from the bottom of the ocean to the temperate regions and the polar ice. Different adaptations to these various places contribute to the diversity of traits developed by the micro-organisms. There are micro organisms that can withstand radiation 3000 times that which humans are able to withstand.
Bacteria
Bacteria are considered to be the smallest free living micro organisms, and are an ancient life form. Bacteria can reach and occupy all habitats that support life. They can live in fluids and some are airborne. They have a variety of shapes that enable them to occupy diverse habitats. Most bacteria have simple shapes, such as rods, spheres, or spirals. The average size of bacteria is about 1-5 m long and 1-2 m in diameter. They have rigid cell walls, this rigidity of the walls is necessary because the high concentration of salts and other molecules packed into the small cell interior exert powerful outward pressure. The strong walls prevent the bacteria from exploding. The cell walls also give bacteria their shape
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The growth of bacteria can be defined as an increase in size or mass. The cell increase in size then divides. In general growth in a bacteria population is the increase in mass, the increase in number through replication. There is therefore a distinction between increasing in size, the growth, and increasing number of cells through multiplication. Bacteria have the most rapid growth rate of any free living organism; under ideal conditions they can double in number every twenty minutes.
Bacteria reproduce asexually by binary fission. The first step is the replication of the chromosomes, secondly the cell walls begin to elongate, and the two chromosomes separate. At that time a septum begins to form between the two cells. After the septum is formed, the two cells separate.
Bacteria usually have only one or two chromosomes. This lack of complexity gives them the ability to mutate their genomes rapidly and add new DNA segments acquired from other micro-organisms. They have shorter generation time and this allows them to evolve rapidly such as becoming resistant to the antibiotics humans used against them to treat disease. Not all the changes in bacteria are caused by mutations. Bacteria can acquire DNA from other micro-organisms and from other species and incorporate it into their genomes. This process is called horizontal gene transfer. This horizontal gene transfer has played an important role in their evolution.
Viruses
Viruses are smaller than bacteria, and are generally too small to be seen under an ordinary light microscope. One other feature of viruses when compared to bacteria and other organisms is that their genomes are not necessarily composed of DNA. Some viruses have genomes composed of ribonucleic acid (RNA). The genomes of viruses are usually much smaller than those of bacteria. Viruses use the compounds in the cells they infect to reproduce. Their genes enable them to take over infected cells biosynthetic machinery for this purpose
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Viruses have very simple structures. The virus' genome is enclosed in a tightly packed protein coat called a capsid. The capsid can protect the genome while the virus is outside the cell it invades. A virus infects a cell by first attaching to the surface, and then releasing its genome into the cell's interior. This attachment of the virus to the target cell is very specific, because a virus can only attach to cells that have the right type of surface receptors for that virus. Viruses that attach to one type of cell may not attach to other types of cells. The second phase of the virus infection process is the take over of the biosynthetic machinery of the infected cell by the proteins produced by the virus' genes. The virus makes many copies of its genomes and its protein. These components are assembled into intact viruses, which then leave the infected cell. Viruses do not multiply by binary fission, but are assembled from component parts, the nucleic acid and protein.
The replication of a virus is done in five stages, the attachment, the internalization, the transcription of the genome, the viral assembly and the virus release. Viruses are described as, helical, icosahedral or complex in shape.