Microbiology Procedure

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Nucleic Acid Hybridization

	Nucleic Acid Hybridization - (DNA-DNA homology or DNA base sequence and RNA sequences). Similarity of base, composition represents only a limited basis for close genetic relatedness because even distant organisms can by chance have a similar composition. Moreover as groups diverge in evolution the DNA sequence changes long before the base composition. Homology of sequence (DNA - DNA homology) can be measured quantitatively in terms of the ability of DNA strands from two different sources to form molecular hybrids in vitro. Surveys among higher organisms have revealed close parallelism between the results of such hybridization and the phylogenetic relations concluded on other grounds.
Surveys among higher organisms have revealed close parallelism between the results of such hybridization and the phylogenetic relations concluded on other grounds. When DNA preparations from two related strains of bacteria were mixed and treated at 10 to 30°C below the Tm value. hybrid DNA molecules are formed. However DNA preparations from two unrelated bacteria could not hybridise.
This discovery of reassociation of single stranded DNA molecules (single strands formed due to treatment given at a temperature. below Tm value) from different biological sources to form hybrid duplexes laid the foundations of DNA-DNA homology in bacterial genetic relatedness. Among bacteria this is useful only within closely related groups because it quickly disappears in the wider range of variation. Since duplexes can also be formed between single-stranded DNA and complementary RNA strands analogous DNA-RNA reassociations can be performed. Ribosomal RNA hybridisation to DNA is useful for estimating more distant kinship among bacteria.
RNA sequences have proved more useful than DNA sequences because they involve relatively small segments of the chromosome: those that code the base sequence either of the tRNAs or of the rRNAs. An alternative is thus to sequence the smaller molecules (RNA or protein) derived from DNA. This technique is thus used for comparative sequencing of specific RNA molecules.
	The molecules are highly conserved in an evolutionary sense and provide a very useful measure of the phylogenetic relationships of the wider groupings of microorganisms. The chosen molecules are ribosomal RNA components, the 165 rRNA and the 55 rRNA. J-however, nucleic acid hybridisations have not yet provided a phylogenetic bacterial taxonomy to replace the classical Bergey's Determinative key.

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Phylogenetic Approach Adansonian Approach - Numerical Taxonomy Macromolecular Approach DNA Base Composition DNA Base Compositions of Representative Bacteria Constancy of G+C content in Strains of Bacteria Nucleic Acid Hybridization Classification of Bacteria from Bergey's Manual of Determinative Bacteriology

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