Friday, March 13, 2009
Bacteria "mix up"
On page 70, Carroll talks about how "Brock lumped all of his new species into the classification 'bacteria'" concerning his discovery of the hyperthermophiles. What characteristics do archaea exhibit that can be mistaken for bacteria? Also, how are bacteria and archaea different? Specifically describe some tests that can be done on these hyperthermophiles that would prove that they do not belong in the kingdom Bacteria. When creating tests, concentrate on how archaea are structurally different from bacteria (and all the other kingdoms for that matter).
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Archaea and Bacteria have many similarities such as they both lack a nuclear envelope and membrane-enclosed organelles. They have circular chromosomes and are both prokaryotic. They are similar on the basis of cell structure and metabolism. Like bacteria, Archaea multiply by binary fission and move primarily by mean of flagella. They have many morphological similarities that sometimes makes it difficult to tell the m apart but Achaea are actually more genetically similar to eukarya than bacteria is to either of them.
ReplyDeleteThere are several tests you could do to determine whether the organisms are bacteria or Archaea. First of all, Bacteria have peptidoglycan in their cell wall and Archaea do not. In membrane lipids, the lipids in bacteria are unbranched hydrocarbons and have ester bonds while in Archaea they have some branched hydrocarbons and ether bonds. Bacteria only have one kind of RNA polymerase while Archaea have several kinds. The initiator amino acid for start of protein synthesis for bacteria is Formylmethionine, while for Archaea it is Methionine. It is rare for bacteria to have introns while for Archaea they are more often present in genes. Bacteria’s, for the most part, growth is inhibited in response to antibiotics streptomycin and chloramphenicol while for Archaea, the growth is not inhibited. Also, Bacteria do not have histones associated with DNA while Archaea do. And lastly, no species of bacteria have the ability to grow at temperatures over 100˚C while some species of Archaea are able to do this. Also, Achaea do not use the glycolysis pathway to break down glucose and many do not have functional Kreb’s cycle pathways.
The main reason why bacteria and archaea are split into two different domains is that ribosomal RNA found in archaea (more specifically 16S rRNA) was more closely related to eukaryotic ribosomal RNA than bacterial ribosomal RNA. Other differences not already stated by Julia are that some bacteria are photosynthetic and have chlorophyll while no archaea are photosynthetic, archaea can go through methanogenesis (produce methane as a metabolic product in low-oxygen environments) while bacteria cannot, bacteria need transcription factors while archaea don’t, and bacteria can help in nitrification while archaea can’t.
ReplyDeleteThere are several ways that one could tell the difference between a bacterium and an archaea. One important way is to examine their habitat. Some archaea known as extremophiles can live in very extreme environments, such as ones with lots of heat (thermophiles), salt (halophiles), a very high pH (alkaliphiles), or a very low pH (acidophiles). Also, archaea can get their energy from lots of different sources, such as ammonia, hydrogen gas, and metal ions. One structural way to differentiate archaea from bacteria and all eukaryotes is by looking at their cell membranes. Bacteria and eukaryotes mainly have glycerol-ester lipids, while archaea mainly have glycerol-ether lipids. In addition, the glycerol groups of archaea and all other organisms are enantiomers, meaning that the spatial arrangement between atoms in archaea glycerol groups is the reverse of all others. This probably means that archaea use totally different enzymes to synthesize phospholipids. The lipid tails of the phospholipid membranes of archaea are also chemically different than those in all other organisms. Those in archaea are long chains with many side branches and ring structures; those in all other organisms are straight with no branches or rings. Also, archaea sometimes have only one phospholipid monolayer instead of a phospholipid bilayer seen in all other organisms, as the tails are fused into single molecules and have two polar heads. This might help make the membrane more rigid and give the archaea a selective advantage by being able to withstand harsher conditions.
Sources:
http://en.wikipedia.org/wiki/Archaea
http://www.earthlife.net/prokaryotes/welcome.html
They were originally classified with bacteria because they are both prokaryotes, but by comparing ribosomal RNA and the completely sequenced genomes of several modern species, researchers have concluded that archaea and bacteria have separate evolutionary histories and thus classified them in separate domains. Like bacteria, archaeans move using flagellum and they are most similar in structure to gram-positive bacteria. Neither archaeans nor bacteria have a nuclear envelop or membrane-enclosed organelles. Members of both domains have a circular chromosome, called a plasmid. As Julia mentioned, bacteria have peptidoglycan in their cell walls and unbranched hydrocarbons as membrane lipids while archaeans do not and archaeans also have several kinds of RNA polymerase and methionine as the initiator amino acid for the start of protein synthesis while bacteria have only one kind of RNA polymerase and use formyl-methionine as the initiator amino acid for the start of protein synthesis. Bacteria rarely have introns while introns are present in some archaean genes. When exposed to the antibiotics streptomycin and chloramphenicol, bacterial growth was inhibited and archaean growth was not. Histones associated with DNA are absent in bacteria and present in archaea while bacteria cannot grow in temperatures greater than 100 degrees C when some species of archaeans can. Archaea are asexual and divide by binary fission, fragmentation, or budding. Unlike bacteria, no known archaeans can produce spores.
ReplyDeleteArchaeans can be distinguished from bacteria by the absence of peptidoglycan in the cell walls, the presence of branched hydrocarbons in the membrane lipids, the presence of histones associated with DNA, and if growth is not inhibited by the antibiotics streptomycin and chloramphenicol. Archaea are genetically distinct from bacteria and eukaryotes, with up to 15% of the proteins encoded by any one archaeal genome being unique to the Archaea, although most of these unique genes have no known function. Of the remainder of the genes unique to archaea that have an identified function, most are involved in methanogenesis. Shared genes are those relating to transcription, translation, and nucleotide metabolism. Archaeans also have uniquely organized genomes by function, such as enzymes catalyzing steps in the same metabolic pathway, into unique operons, and large differences in tRNA genes and their aminoacyl tRNA synthetases.
Campbell Book
http://en.wikipedia.org/wiki/Archaea