PROKARYOTIC CELLS
Viruses are the simplest form of life but they are not cellular. They rely on entry into cells to reproduce. They enter and use the cell's metabolic machinery to replicate their nucleic acid and to produce their protein coats. All types of cells have their own specific viruses. In general, viruses can only infect a single species, however, there are exceptions to this rule.
The simplest and most primitive of cells are the prokaryotic cells (pro means before and caryon refers to the nucleus). Prokaryotic cells are much smaller than eukaryotic cells. They are between 1 micron and 10 microns (a micron is one millionth of a meter) while eukaryotic cells are usually minimally 10 microns are most are larger. Prokaryotic cells do not a nucleus nor do they contain the other organelles characteristic of the more advanced eukaryotic cells. The ancestor of the first prokaryotic cells diverged into three lineages: the two main prokaryotic lineages are the eubacteria and the archaebacteria and the third was the prokaryotic ancestor of the present day eucalypts. The archaebacteria are more closely related to the eucalypts. The early ancestor of the archaebacteria also gave rise to the eukaryotic cell line.
The archaebacteria and eubacteria until very recently were put in the Kingdom Monera. But now each have been awarded kingdom status of their own: Kingdom Archaebacteria and Kingdom Eubacteria. Both kingdoms are composed exclusively of organisms with prokaryotic cells. However, the archaebacteria have more characteristics in common with eukaryotic cells because they share a common ancestor. Archaebacteria include the methanogens, halophiles, and thermophiles, organisms that live in very harsh environments. They are believed to resemble the very first cells. The eubacteria include several groups including the common bacteria and cyanobacteria (blue-green algae). The cyanobacteria are photosynthetic. Bacteria are heterotrophs and are important recyclers along with the fungi. Some of them are pathogenic but many are not. There are three basic shapes: rods (bacilli); spheres (cocci) and spirals (spirilli).
All cells including the prokaryotes, have a cell (plasma) membrane. Prokaryotes with few exceptions also have a cell wall containing a unique peptidoglycan, a biomolecule made of amino acids and sugars. This molecule resembles a hairnet which encloses the cell. Some prokaryotic cells, such as pneumococcus, also have a capsule overlying the cell wall. The capsule helps them escape being "eaten" by your phagocytic cells. Gram-negative and gram-positive are commonly used terms which refer to the different staining patterns among species of bacteria. The difference is due to the composition of the cell walls.
The interior of the cell contains the cytoplasm. The cytoplasm has a fluid base and is full of enzymes, metabolites and ribosomes. Ribosomes are structures composed of RNA and protein which are the protein factories. Ribosomes read the messenger RNA made from the DNA. All prokaryotic cells have a single, double-stranded (double helix), circular DNA molecule for their genetic material. This DNA is attached to the inner cell membrane where the DNA replicating machinery is located. The DNA replicates and forms two DNA molecules. (Cells may have more than one copy of the same DNA molecule.)
When the cell reproduces, new membrane is formed and the attached DNA molecules move apart. New cell wall material is synthesized which forms a separation between the two halves of the cell. Thus, the cytoplasm is divided between the two new cells. There are no "parents," each parent cell becomes two new cells. This process is called fission. Penicillin, an antibiotic, interferes with the synthesis of new cell wall material thereby preventing the bacterial cell from dividing.
The DNA is naked, it does not have proteins associated with it as eukaryotic DNA does. Although the DNA is concentrated in a small area called a nucleoid, it is not separated from the rest of the cytoplasm by a nuclear membrane such as is found in all eukaryotic cells.
Prokaryotic cells are simple compared to eukaryotic cells. Nevertheless, they are quite complex. Some of them have pili (singular, pilus) which are organelles of attachment that project from the cell wall. The sex pili are responsible for bacteria recognizing one another and the consequent formation of a conjugation tube which allows the transfer of DNA from a "male" cell to a "female" cell. In this way, recombination occurs and the resulting cell will contain a combination of genes from each parent cell. (Sex is any process of recombining genes to form new and unique combinations.) Bacteria may have flagella which allow them to move rapidly. The prokaryotic flagellum is structurally very different from the eukaryotic flagellum. It is simpler in composition and is attached to a mechanism in the cell wall which rotates much like the screw of a ship's propeller.