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Lecture 7

BIO153 Lecture 7.pdf

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Christoph Richter

2009 BIO153: Lecture 7 The Prokaryotic Domains February 2, 2009 Recall that prokaryotes show great diversity in terms of their metabolic pathways. From an evolutionary standpoint, it is interesting to note that different metabolic pathways are “scattered” among lineages (e.g., photosynthetic prokaryotes do not form a monophyletic group). This suggests that metabolic pathways are convergent in many groups (independent acquisition of a particular type of metabolism) Understanding evolutionary relationships among the prokaryotes is complicated by the phenomenon of lateral gene transfer (genes are transferred among distantly related lineages; therefore similar traits are not good indicators of close relationships!) Evidence for lateral gene transfer: 1. Often find similar DNA sequences among distantly related organisms 2. the G-C ratio (the percentage of GC nucleotide pairs in complementary strands of DNA, relative to the percentage of A-T nucleotide pairs) in a particular DNA sequence may be markedly different from the rest of the genome How does lateral gene transfer happen? 1 Recall the modes of genetic recombination in prokaryotes: 1. the transfer of plasmids (conjugation) 2. prokaryotes take up raw DNA from environment (transformation) - e.g. prokaryotes that live in hotsprings often pick up genes from cells in same environment 3. prokaryotes may acquire genes via virus infection (transduction) 4. parasitic prokaryotes may acquire host genes (various mechanisms): e.g.Chlamydia is an obligate intracellular parasite of eukaryotes that is related to free-living bacteria; in parasitic cells, the overall genome is reduced, but there has been acquisition of some genes from host (e.g. ATP/ADP transferases) Genetic diversity among prokaryotes: • on average, ~15% of genes in each prokaryotic genome studied to date are genes unique to that species 2 • the function of many prokaryotic genes is not known • e.g. in E. coli , one of the most intensively studied prokaryotic genomes, the function of ~30% of the genes is unknown! The Prokaryotic Domains: 1. Eubacteria (“true” bacteria) (*Some texts refer to the Eubacteria as “Bacteria”; this is somewhat ambiguous as others refer to all prokaryotes as bacteria. I will try to be consistent and use the term Eubacteria!) Unique features of Eubacteria The amount of peptidoglycan varies among Eubacteria: Gram positive: peptidoglycan retains stain (purple) 3 Gram negative: less peptidoglycan; less stain (pink) From an evolutionary perspective, one of the most significant groups among the Eubacteria are the cyanobacteria. • Formerly called “blue-green algae” because some have blue pigments (phycocyanin) that give them a bluish tinge; they are photosynthetic and contain chlorophyll a as do true algae and higher plants. However, they are prokaryotes and should not be called algae! • They are aquatic and often form large blooms (particularly in polluted waters; their presence is often a bioindicator of polluted conditions); these blooms may be toxic. • They perform oxygenic photosynthesis (produce O 2) – very important! • Some form lichen in a symbiosis with fungi (We will discuss this further when we talk about Fungi). • Photosynthesis occurs on lamellae (plate-like structures within the cell –this will be an important point in Lecture 8) • They perform nitrogen fixation as well as photosynthesis (very odd, given that photosynthesis produces oxygen, and nitrogen fixation is an anaerobic process!). N2 fixation occurs in specialized cells called heterocysts that are walled off from the photosynthetic cells. • They occur symbiotically in some 4 organisms; when they do, they function like chloroplasts. • Not all are blue-green in colour; they contain other pi
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