Prokaryotes have a single circular chromosome that may be 500 times the length of the cell.
The DNA has fewer associated proteins than that of eukaryotes. Proteins enable the DNA to be coiled and compacted so that it fits within the cell..
Sexual reproduction in Eukaryotes combines genes from two different individuals and thus promotes variation. Prokaryotes do not reproduce sexually but the processes listed below promote genetic recombination.
Conjugation A cell with DNA called F factor is able to replicate and transfer a copy if it's DNA to another cell without F factor through a tube called a sex pilus. F factor may exist as a plasmid or become integrated into the chromosome. If it is integrated into the chromosome, it may also transfer part of the chromosome. The sex pilus usually breaks before the entire DNA from the donor cell is transferred. The DNA that is transferred is used to replace similar genes in the recipient cell. Enzymes destroy the replaced genes.
Transformation - occurs when a bacterium picks up fragments of DNA released by dead bacteria or secreted by live bacteria.
Transduction is when bacteriophages (viruses) carry portions of bacterial DNA from one cell to another.
Mutation is a major source of variation in prokaryotes. A rapid mutation rate coupled with rapid reproduction promotes variation.
Some bacteria form endospores when environmental conditions become unfavorable.
Endospores are DNA and a portion of cytoplasm encased in a tough cell wall. They are resistant to extremes in temperature, drying, and harsh chemicals.
Traditionally, staining techniques, cell shape, mode of nutrition, and mode of cellular respiration have been used to classify prokaryotes but these techniques may not reveal evolutionary relatioinships. These characteristics are useful, however, for identifying certain kinds of prokaryotes.
Living organisms require organic compounds for food. Organic refers to molecules that contain carbon and hydrogen. Examples of organic nutrients are carbohydrates (sugars, starches), lipids, and proteins.
Autotrophs are organisms that make their own organic food. Heterotrophs consume food that is already present in the environment. For example, plants are autotrophs because they make their own food by photosynthesis. Animals are heterotrophs because they obtain their food by eating it.
There are two kinds of Autotrophic prokaryotes. Those that make organic food using energy from sunlight are photosynthetic. Autotrophs that make organic food using energy from inorganic chemicals are chemosynthetic.
The first photosynthetic prokaryotes to evolve did not produce oxygen.
The green sulfur bacteria and purple sulfur bacteria do not split water during photosynthesis. Instead, they split H2S; oxygen is therefore not released.
Photosynthetic prokaryotes have extensions of the plasma membrane called thylakoids. Many of the molecules needed in the reactions of photosynthesis are found within the thylakoid membrane.
Chemosynthetic prokaryotes obtain energy to make their organic food by oxidizing high-energy inorganic compounds (hydrogen gas, ammonia, nitrites, and sulfides) instead of consuming organic nutrients or using sunlight.
Many chemosynthetic prokaryotes are anaerobic; they are often found deep in the sediments of lakes and swamps.
Chemosynthetic prokaryotes form the basis of the food chain for some communities 2.5 km beneath the sea. Energy for these communities comes from hydrothermal (volcanic) vents. The hot water pouring out of these vents contains high concentrations of inorganic minerals such as sulfides that can be used as an energy source by the prokaryotes..
Heterotrophic prokaryotes feed on organic matter by secreting enzymes and absorbing the digested material.
Most heterotrophic prokaryotes are aerobic.
Three types of heterotrophs are described below. Each category is determined by the feeding mode.
Saprotrophic organisms are decomposers. They play a critical role in recycling (releasing) nutrients that are tied up in the bodies of dead organisms. Most heterotrophic prokaryotes fall into this category.
Parasites are organisms that live in close association with another species and one species benefits at the expense of the other. Usually, the smaller species resides within a larger species and derives its food from the larger organism. Normally, the larger organism is not killed.
Mutualistic organisms are those that live in close association with another species and both species benefit as a result of the association. For example, some nitrogen-fixing bacteria live in nodules on the roots of plants. They convert atmospheric nitrogen (N2) to a form that is usable by plants. Plants provide the bacteria with carbohydrates.
Obligate anaerobes are unable to grow in the presence of oxygen.
Facultative anaerobes can grow with or without oxygen.
Aerobic organisms require oxygen. Most bacteria are aerobic.
fix atmospheric nitrogen.
The only organisms capable of fixing nitrogen are bacteria, and this is primarily done by the cyanobacteria. The fixation of nitrogen by cyanobacteria may have allowed plants to invade the land during the Paleozoic.
Like plants, cyanobacteria have the photosynthetic pigment chlorophyll A and they use water as an electron donor during photosynthesis. When water molecules are split, oxygen is liberated. This process resulted in oxygen accumulating in the earths early atmosphere.