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Bacteria Chapter 27 All bacteria are prokaryotes - cells without a nuclear envelope. Bacteria are the most abundant organisms on Earth. The biomass of bacteria is significantly larger than all other organism groups combined. Prokaryotes are the most ancient of all living things The Structure of Prokaryotes 1. Prokaryotic cells are smaller in size (1-10 μm) than eukaryotic cells (10 - 100 μm). μ = "micro" 2. Most prokaryotes are all strictly unicellular. 3. Prokaryotes do not have a nucleus, nor do they have histone proteins. Their DNA, which is circular, is free in the cytoplasm. 4. Cell division in prokaryotes is a simple binary fission and does not involve spindles made up of microtubules. 5. Prokaryotes lack the ability to form zygotes by way of sexual reproduction (although some species may exchange genetic information.) 6. Prokaryotes lack membrane-bound organelles and lack a cytoskeleton. 7. Bacterial flagella are simple and composed of a single fiber of the protein flagellin.(Eukaryotes have the complex 9+2 arrangement of microtubules.) These flagella are for locomotion. 8. The metabolic processes in prokaryotes are much more varied than they are in eukaryotes. Only bacteria can fix atmospheric nitrogen. 9. There are no cytoplasmic organelles except ribosomes in prokaryotes. 10. Prokaryotic cells have a cell wall that is different in structure from the cell wall of eukaryotic plant and fungal cells. Most bacterial cell walls contain peptidoglycan (not cellulose or lignin). 11. Many prokaryotes have a sticky wall of polysaccharides (a capsule) surrounding the cell wall. 12. Many prokaryotes have hairlike appendages called fimbriae that they use to stick to their substrate or to one another. 13. Some prokaryotes have pili, appendages that pull two bacterial cells together and promote the exchange of DNA. Cell-Surface Structure of Bacteria Nearly all bacteria contain a cell wall to protect the cell, maintain cell shape, and prevent it from bursting in a hypotonic environment. Bacterial cell walls contain peptidoglycan, a polymer of nitrogen-enforced sugars (NAG and NAM) that are crosslinked with short polypeptides Archae have cell walls composed of polysaccharides and protein, but they lack peptidoglycan. The eubacteria that have peptidoglycan cell walls can be separated into two categories of bacteria: (1) Gram-positive bacteria have simple walls with a thick peptidoglycan layer. (2) Gram-negative bacteria have complex cell walls with 2 membranes surrounding a thin peptidoglycan layer. • Eubacteria having thick peptidoglycan cell walls readily absorb crystal violet - a deep purple cell dye - making them Gram positive . •Gram positive bacteria are highly susceptible to penicillin. •Penicillin affects the bacterium's ability to form cell walls by acting as an enzyme inhibitor. Without a regulatory cell wall, water rushes in and the bacterium lyses. •It is easy to see the medical benefit of the Gram stain. Gram positive cells may be treated with penicillin. •Other eubacteria have thin peptidoglycan cell walls that do not absorb crystal violet these bacteria are Gram negative. •These bacteria are not susceptible to penicillin. •The bacteria appear red because when they are counterstained (after crystal violet has been added and then rinsed), they absorb the red counterstain. Bacterial Motility •About 1/2 of bacteria demonstrate motility - purposeful movement in response to stimuli. •What is taxis? Positive taxis? Negative taxis? •What is kinesis? •The most common locomotory structure is the flagellum. •Flagella are found arranged in a multitude of ways on the bacterial body. •Bacterial and archael flagella operate in the same way, but are made of different proteins. •They both function differently that eukaryotic flagella. •This evidence supports the idea that flagella developed independently in all three domains. (ie. They are analogous structures!) The Internal Structure and Organization of Bacteria •Bacteria lack internal membranes and compartmentalization. •They can, however, infold their plasma membrane and use it for ATP synthesis via chemiosmosis. •The DNA is much shorter (fewer genes), circular, and largely lacks introns (some archae do have introns). •The DNA is restricted to an area called the nucleoid. There is no nuclear envelope. •In addition to their single, circular chromosome, some bacteria have accessory DNA called a plasmid. Each plasmid contains only a few genes. •Bacteria DO have ribosomes, although the ribosomes are different in structure than eukaryotic ribosomes. •Bacterial ribosomes are smaller than eukaryotic ribosomes, and have different protein and rRNA. •However, archae have rRNA that is more similar in structure to eukaryotes than eubacteria, and this is the principle basis for establishing closer relatedness between the archae and eukaryotes than between eubacteria and eukaryotes. Bacterial Reproduction •Bacteria reproduce by binary fission, not mitosis. •Optimal reproductive rates double populations every 1-3 hours. •Bacterial populations grow rapidly, exponentially. •Some bacteria can withstand hostile environmental conditions by forming spores around themselves. •During spore formation, the original bacterial cell produces a copy of its chromosome and surrounds it and a small bit of cytoplasm with a protective capsule. Most of the water is removed, and the metabolic machinery grinds to a halt. When the original cell dies, the spore is released. •Spores can remain viable for years, even decades and centuries. Genetic Recombination In Bacteria •Bacteria populations demonstrate substantial genetic variation because there are multiple mechanisms that can lead to high variation within a bacterial population. •The most important of these mechanisms are (1) mistake-making during replication and binary fission and the mutation of the naked bacterial DNA, and (2) bacterial recombintion. Rapid Reproduction and Mutation •If an organism group reproduces sexually, most recombination is due to the random union of gametes. •However, prokaryotes do not reproduce sexually. •Because bacteria reproduce so rapidly, even low mutation rates during replication can lead to substantial variation in a bacterial population. •This rapid recombination can lead to accelerated Bacterial Recombination •There are three mechanisms that occur naturally in bacterial populations that lead to genetic recombination combining DNA in new ways. 1. Bacterial Transformation 2. Bacterial Transduction 3. Bacterial Conjugation Bacterial Transformation •Bacterial transformation occurs when bacteria uptake foreign DNA from their surroundings. •Often, the DNA that they absorb from their surroundings is bacterial DNA from dead bacteria. •Bacteria are most likely to absorb DNA that comes from closely related species - they have cell surface proteins that recognize "similar species" DNA, and they absorb it differentially. •Once inside the cell, the "new DNA" can be incorporated into the host DNA by homologous DNA exchange. Bacterial Transduction •Phages (bacteria-infectious viruses) carry bacterial genes from one bacterium to another. •Transduction usually occurs when a virus incorporates a bit of bacterial DNA into its own DNA during replication. •The virus with the host cell DNA manages to transport this bit of bacterial DNA to a new host, but the host bacterium survives and incorporates the transducted DNA into its own genome. Bacterial Conjugation •During conjugation, DNA is exchanged between two bacterial cells (usually of the same species). •In this process, the movement of DNA is one-way. There is a donor bacterium and a recipient. •Exchange is facilitated by use of a conjugation pilus. •The pilus pulls the two bacterial cells together, and may serve as the conduit through which bacterial DNA moves from one cell to the other. •The ability to form pili and donate DNA results from a piece of DNA on the donor genome called the F factor. •The F factor can reside in either the bacterial DNA or the bacterium's plasmid. If the F factor is part of plasmid DNA, a DNA donating bacterium is designated as the F+ cell, and the recipient cell is the F- cell during conjugation. Following conjugation, the F- cell becomes an F+ cell. •If the F factor is found on the bacterial chromosome, the cell containing the F factor is called an Hfr cell. (Hfr = High frequency of recombination) •An Hfr cell serves as a donor during conjugation, and the recipient cell is still called an F- cell. •When DNA from the Hfr cell enters, it is spliced into the recipient DNA at its homologous site, replacing that portion of the recipient cell genome.