细菌系统学小结.doc

Bacterial Systematics (bact-sys)1 Summary - Bacterial Systematics Bacterial Phylogeny - Genotypic Characteristics of Bacteria: Nucleic acids are universally distributed (RNA and chromosomal DNA) and they alone can be used as standards for wide-ranging comparisons. DNA-Analysis: DNA is far more stable than RNA (RNAase rapidly decomposes RNA). ? Base Composition (G-C Ratio): Among prokaryotic DNA the ratio of the nucleotide bases adenine (A)plus thiamine (T) to guanine (G) and cytosine (C) varies within the range of 23-78 [mol%];expressed in [mol%]; G+C ratios w/ differences >20-30% among species samples seem less ? 100closely related; determination of C-G ratio is required in order to establish new taxa.[A+T+G+C] Samples showing 10% - 20% differences should be assigned to the same genus, 5% is the maximumrange permissible for same species. A >G+C ratio requires a > temperature to break the DNA-doublehelix into single strands; G+C is held by 3 H-bonds, whereas A+T has only 2 H-bonds (see genetics).? DNA-Fingerprinting: Via Restriction Fragment Length Polymerization (requires little DNA), thegenetic probe is broken by endonucleases into certain fragments. The number and locations of theseunits / cuts are unique for each genome (includes pattern / groups of these fragments of different size).The cleaved DNA-fragments are amplified via Polymerase Chain Reaction, purified, separated by gel-electrophoresis (larger units by Pulse Field Gel Electrophoresis), and compared with already existingpatterns.Random Amplified Polymorphic DNA): Randomly selected primers are used to specify for itsexistence, and if present, to amplify that particular sequence to be analyzed in gel-electrophoresis. ? DNA Marker: Certain radio-labeled markers do have complementary base sequences of the DNA-sample under investigation, therefore can bind at the proper complementary site of the strand. ? DNA-Relatedness: The more genetically closely related two organisms are, the more nucleotide basesequenc