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Prokaryotes They’re almost everywhere Prokaryotes were the first organism and persist today as the most numerous and pervasive of all living things. Archaea and Bacteria are the two main branches of prokaryotic evolution. Prokaryotes account for two of three domains (super kingdoms) of life. Molecular systematics suggests that archaebacteria (domain Archaea) may be more closely related to eukaryotes (domain Eukarya) than to eubacteria (domain Bacteria). Diverse Adaptations of Form and Function Prokaryotes are generally single celled organisms, although some occur as aggregates, colonies, or simple multi-cellular forms. Diverse Adaptations of Form and Function Prokaryotes are generally single celled organisms, although some occur as aggregates, colonies, or simple multi- cellular forms. The three most common prokaryotic shapes are spherical (cocci) , rod shaped (bacilli), and helical forms. Diverse Adaptations of Form and Function Prokaryotes are generally single celled organisms, although some occur as aggregates, colonies, or simple multi-cellular forms. The three most common prokaryotic shapes are spherical (cocci) , rod shaped (bacilli), and helical forms. Nearly all prokaryotes have external cell walls, which protect and shape the cell and prevent osmotic bursting. Cell walls of eubacteria typically contain the polymer peptidoglycan. Gram-positive and gram- negative bacteria differ in the structure of their walls and other surface layers. ADAPTATIONS Many species secrete sticky substances that form capsules. Some have surface appendages called pili outside the cell wall. Both structures help the cells adhere to one another, and some pili are specialized for conjugation. Motile bacteria propel themselves by flagella, use flagella-like filaments positioned inside the cell wall (spirochetes) or glide on slime secretions. Prokaryotic cells are not compartmentalized by endomembranes. However, invaginations of the plasma membrane may provide internal membrane surface for specialized functions. Domain Domain Domain Bacteria Archaea Eukarya (Eubacteria) (Archaebacteria) (Eukaryotes) Universal ancestor A Comparison of the Three Domains of Life Domain Charactaristic Bacteria Archaea Eukarya Nuclear Absent Absent Present Envelope Membrabce- Absent Absent Present enclosed organelles Peptidoglycan Present Absent Absent in cell wall Membrane Unbranched Some branced Unbranched lipids hydrocarbons hybrocarbons hybrocarbons Characteristic Bacteria Archaea Eukarya RNA One kind Several Kinds Several kinds polymerase Initiator amino Formyl- Methionine Methionine acid for start methionine of protein synthesis Introns Absent Present in Present (noncoding some genes parts of genes) Antibiotic Growth inhibites Not inhibited Not inhibited sensitivity by streptomycin by these by these and antibiotics antibiotics chloramphenicol DNA The prokaryotic genome consists of a single circular DNA molecule in a nucleoid region unbounded by a membrane. Many species also possess smaller separate rings of DNA called plasmids, which code for special metabolic pathways and resistance to antibiotics. REPRODUCTION Bacteria reproduce asexually. The type of cell division by which these prokaryotes reproduce is called binary fission. During binary fission, each dividing daughter cell receives a copy of the single parental chromosome. VARIATION Genetic variation occurs in prokaryotes through mutation and gene transfer. All major types of nutrition and metabolism evolved among prokaryotes. The evolution of prokaryotic metabolism was both cause and effect of changing environments on Earth. IMPACT TODAY Prokaryotes continue to have an enormous ecological impact. Prokaryotes, along with fungi, are decomposers that recycle chemical elements in ecosystems. Some prokaryotes live with other species in symbiotic relationships. PROKARYOTES TODAY Some parasitic prokaryotes are pathogenic, causing disease in the host. Bacteria have been put to work in laboratories, sewage treatment plants, and the food and drug industry. EUKARYOTES Eukaryotes originated by symbiosis among prokaryotes. The first eukaryotes to evolve from prokaryotic ancestors were probably unicellular. The primal eukaryotes were ancestral to plants, fungi, and animals, the eukaryotic organisms most familiar to us. PROCESS I INVAGINATION OF THE PLASMA MEMBRANE. The nuclear envelope, endoplasmic reticulum, Golgi apparatus, and related structures, may have evolved from specialized invaginations (infoldings) of the prokaryotic plasma membrane. PROCESS II ENDOSYMBIOSIS-Dr. Lynn Margulis of the University of Massachusetts Eukaryotic cell arose as a result of prokaryotes taking up residence inside other prokaryotes. The term endosymbiont is used for a cell that lives within another cell, termed the host cell.
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