Prokaryote Growth and Reproduction This micrograph shows a bacillus bacteria (probably E. coli) undergoing binary fission. This is a form of asexual reproduction. During prokaryotic binary fission, as the parent cell elongates (x2) its DNA is replicated, then new plasma membrane and cell wall forms dividing the cell into two identical daughter cells. The two daughter cells are genetically identical to each other and the parent cell. The parent cell will no longer exist. The daughter cells will not grow. The advantage of this type of reproduction is that it is very efficient, does not require two parents or reproductive behavior, and can produce many offspring in a short time. The disadvantage is reduced genetic variation, the raw material of natural selection and evolution! The source of prokaryotic variation is mutation and horizontal transfer... 15
Prokaryote Genetic Recombination (Gene Shuffling) The process taking place in this micrograph is prokaryotic conjugation. This is not a form of reproduction because no new individuals are produced. The cell on the left has many more pili (AKA fimbriae) than the other. These cells are performing prokaryotic conjugation, a form of genetic recombination through horizontal transfer. They have formed a pili bridge across which a copy of a plasmid is being transferred from the cell on the left to the cell on the right. The genetic information transferred may include the gene to develop multiple pili and therefore conjugate with other individuals. It may also include genes for antibiotic resistance. Following conjugation, the two cells share more genes, but they remain genetically unique. Bacterial transformation is a mode of genetic recombination through horizontal transfer where a cell takes in genetic material from its surrounding environment. Bacterial transduction (at right) is a mode of genetic recombination through horizontal transfer involving bacteriophages. The advantage of horizontal transfer for prokaryotes is variation, for humans... genetic engineering. The disadvantage for humans is the transfer of antibiotic resistance. Carrying genes from another bacterium. 16
Prokaryote Survival & Dispersal s contain the cell s DNA along with a small amount of cytoplasm contained within cell wall material. s form when environmental conditions become unfavorable. The rest of the cell outside the endospore breaks down (lysis). s can remain dormant (alive, but metabolically inactive) for possibly millions of years! They germinate when environmental conditions become favorable. The primary purpose of endospores is to survive unfavorable conditions. The secondary purpose of endospores is dispersal by wind and/or water. 17
Importance of Bacteria Oxygen! (Cyanobacteria) Decomposers! Producers! (Cyanobacteria) Nutrient Cycling!... 18
Importance of Bacteria in the Nitrogen Cycle Rhizobium nodules on a legume. Nitrogen is important to all organisms because it makes up nucleic acids and proteins! The Earth's atmosphere is 78% nitrogen gas (N 2 ), which is not a form most living things can use. The only organisms able to incorporate atmospheric N 2 are bacteria! Nitrogen fixation is the process of transforming inorganic nitrogen (N 2 ) into a form photoautotrophs can use. Legumes are beans, peas and peanuts that have a mutualistic symbiotic relationship with nitrogen fixing bacteria, living in root nodules. Nitrogen fixation and nitrification by nitrifying bacteria are the only way nitrogen can be assimilated by photoautotrophs! 19