CHAPTER 27 NOTES 27 1 STRUCTUAL AND FUNCTIONAL ADAPTATIONS CONTRIBUTE TO PROKARYOTIC SUCCESS Most prokaryotes are unicellular They have a variety of shapes Despite being unicellular and small prokaryotes are well organized achieving all of an organism s life functions within a single cell CELL SURFACE STRUCTURES A key feature of nearly all Prokaryotes is the cell wall The cell wall maintains cell shape protects the cell and prevents it from bursting in a hypotonic environment In comparison to Eukaryotic Cell walls Prokaryotes use peptidoglycan polymer composed of modified sugars cross linked by short polypeptides instead of cellulose or chitin Peptidoglycan encloses the entire bacterium and anchors other molecules Gram Staining can be used to categorize bacterial species due to cell wall differences Gram Positive bacteria have simpler cell walls with large amounts of peptidoglycan while Gram Negative bacteria have less peptidoglycan and are more complex The lipid portions of lipopolysaccharides in the walls of many gram negative bacterium helps protect it from the body by being toxic causing fever or shock The outer membrane of gram negative bacteria helps protect it from the body s defenses Gram Negative bacteria are more resistant to antibiotics Antibiotics such as penicillin prevent the cross linking of peptidoglycan A capsule surrounds the cell wall of many prokaryotes and if it is not as well organized it is called a slime layer Capsules protect prokaryotes from various factors Another form of protection allows certain bacteria to form into endospores The original cell produces a copy of its chromosome and is surrounded by a structure Water is removed and metabolism halts It lyses and releases the endospore It can remain dormant for centuries Lastly fimbriae allow prokaryotes to stick to their substrate or one another by means of hair like appendages Fimbriea are shorter and more numerous than pili MOTILITY About half of all prokaryotes are capable of taxis directed movement away or towards stimulus Among the structures that allow prokaryotes to move are flagella Prokaryotic flagella are thinner and are not covered by an extension of the plasma membrane as opposed to Eukaryotic flagella Overall the structural and molecular comparisons indicate that flagella in bacteria archea and eukaryotes arose independently Evolutionary Origins of Bacterial Flagella The bacterial flagellum shown has three main parts motor hook and filament that are composed of 42 different kinds of proteins Just like the eye biologists believe that flagella may have started out as a simple structure and modified step wise over time Analysis shows that only half of the flagellum s proteins appear to be necessary to function This finding suggests that the bacterial flagellum evolved as other proteins were added to an ancestral secretory system Some proteins are homologous to proteins that secrete certain substances participate in ion transport or form This is an example of EXAPTATION the process in which existing structures take on new functions through descent with modification INTERNAL ORGANIZATION AND DNA Cells of prokaryotes are simpler than those of eukaryotes in both internal structure and physical arrangement of their DNA Prokaryotes lack complex compartmentalization associated with membrane bound organelles in Eukaryotes Prokaryotes have circular chromosomes while eukaryotes have linear chromosomes Prokaryotes also lack a nucleus their chromosomes are located in a NUCLEOID a region of the cytoplasm not enclosed by a membrane Some Prokaryotes may have smaller rings of independently replicating DNA molecules called plasmids DNA replication transcription and translation are fundamentally similar between Eukaryotes and Prokaryotes However they differ in ribosome size protein and RNA content REPRODUCTION Prokaryotes divide by binary fission into 2 identical cells which also divide They reproduce pretty rapidly producing a new generation every 20 minutes They are made up of three key features small size divide by binary fission short generational times 27 2 RAPID REPRODUCTION MUTATION AND GENETIC RECOMBINATION PROMOTE GENTIC DIVERSITY IN PROKARYOTES RAPID REPRODUCTION AND MUTATION Mutations though rare on a per gene basis can increase genetic diversity quickly in species with short generation times and large populations This can result in rapid evolution This shows that Prokaryotes are highly evolved GENETIC RECOMBINATION Genetic recombination can lead to diversity as DNA is combined from 2 sources In Eukaryotes this occurs in Meiosis and fertilization but this does not occur for Prokaryotes Instead there are 3 different mechanisms that can bring Prokaryotic DNA from different sources They are TRANSFORMATION TRANSDUCTION and CONJUGATION Transformation and Transduction In Transformation the genotype and possibly phenotype of a prokaryotic cell are altered by uptake of foreign DNA from its surroundings For example Streptococcus pneumonaiae can be transformed into pneumonia causing cells if exposed to pathogenic DNA When this occurs the cell becomes a recombinant Researchers once believed this was rare but it turns out many bacteria have cell surface proteins that recognize DNA from closely related species In Transduction phages bacteriophages carry prokaryotic genes from one host cell to another In most cases transduction results from accidents that occur during the phage replicative cycle reference diagram on pg 573 However a virus can attach to another prokaryotic cell and inject prokaryotic DNA acquired from the first cell donor If some of the DNA is incorporated into the recipient cell s chromosome by crossing over a recombinant is formed Conjugation and Plasmids In Conjugation DNA is transferred between two prokaryotic cells that are temporarily joined In bacteria this transfer is always one way The way this occurs is a pilus attaches to the recipient of a donor cell The pilus retracts and the 2 cells are pulled together Then a mating bridge occurs transfer DNA The ability to form pili and donate DNA is due to an F factor fertility The F factor can exist either as a plasmid or as a segment of DNA within the bacterial chromosome F factor as Plasmid called F plasmid Functions as DNA donors Cells lacking F factor are designated as F and receive DNA The F condition is transferable in the sense that an F cell converts and F cell to F if a copy of the entire F plasmid is transferred F factor in the