Biology 102 - General Biology

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.

A bacterium is a prokaryotic cell

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 but most are larger. Prokaryotic cells do not have 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 eukaryotes. The archaebacteria are more closely related to the eukaryotes since 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 and we will use this terminology. Some scientists have awarded kingdom status to each of the two groups: 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. Most other 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 captured and engulfed by the phagocytic white blood cells of your immune system. (The capsule was the difference between the "smooth" and "rough" strains of pneumococci mentioned in the lecture on the discovery of DNA as the genetic material.) 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 that are the protein factories of all cells. Ribosomes hold the messenger RNAs made from the DNA while the transfer RNAs read the codons and line of the amnio acids. 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 newly replicated 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, the peptidoglycan mentioned earlier, 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 as 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. Bacteria in your mouth attach to your teeth via their pili. Some bacteria attach to rocks via pili. Some bacteria have sex pili which 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.

Prokaryotes have some common characteristics. Compared to eukaryotic cells, they are small. Prokaryotic cells are usually between 1 and 10 microns, while eukaryotic cells are minimally 10 microns and usually larger. It was not until the invention of the electron microscope that the essential difference between prokaryotic cells and eukaryotic cells was appreciated. That is because the wavelengths of light used in the light microscope cannot resolve structures as small as bacteria. The electron microscope uses electrons instead of light and electrons have much smaller wavelengths which can allow us to see the details of cells. It was only when we had the electron microscope that we could clearly see the difference between the two cell types.

Prokaryotic cells do not have membrane bound organelles within them. For example, they do not have a nucleus. They have a single, circular molecule of naked DNA instead of the more complex chromosomes found in eucalypts. They usually divide by a simple process known as fission where the progeny are identical (except for mutations) to the parent. Like the eukaryotic cell, they have a cell membrane (plasma membrane) and cytoplasm with ribosomes (protein manufacturing sites) and enzymes to carry out all the metabolic reactions. Prokaryotes also usually have a special type of cell wall exterior to the cell membrane. This cell wall is composed of a peptidoglycan molecule which is like a hair net that envelopes the cell. Penicillin, the first antibiotic, interferes with the synthesis of new cell walls. A few bacteria, such as the mycoplasma, do not have this cell wall and live within host cells. In this sense the mycoplasma are like viruses because they are obligate intracellular parasites also.

The Kingdom Monera contains all the prokaryotic organisms, the Eubacteria and the Archaebacteria. This kingdom contains both autotrophs (self feeders) and heterotrophs (other feeders) and the metabolic diversity among the members of this kingdom is greater than in any other. Bacteria are the best known of the prokaryotes. While some bacteria are harmful, many are beneficial. Bacteria and fungi are the great recyclers. If it were not for them, the earth would be full of dead bodies.

Antibiotics are aimed at interfering with metabolic processes specific to prokaryotes. For example, some interfere specifically with prokaryotic protein synthesis. This is possible because prokaryotic ribosomes are different from eukaryotic ribosomes. Bacteria can become resistant to antibiotic action by acquiring changes in their DNA (mutations) or by acquiring plasmids (rather like viruses) which carry antibiotic resistance genes. Plasmids are small circular DNA molecules in the cytoplasm of the cell that get replicated along with the cell's DNA. Plasmids can be transferred between bacterial cells.