Afascinating property of DNA is the complementarity of the nucleotide bases in its two anti-parallel strands, with G always pairing with C and A always pairing with T. This does not involve strong covalent chemical bonds but weak hydrogen bonds. There are three hydrogen bonds between G-C pairs and two between A-T pairs, so strand separation is easier in AT-rich DNA than in GC-rich DNA. Mild heating breaks these hydrogen bonds and is one way to separate the two strands, called denaturation or dissociation. Reducing the temperature under the right salt conditions leads to renaturation (reassociation or reannealing) of the two strands by reconstitution of the hydrogen bonds. The rate of renaturation depends on the frequency of collision between complementary sequences, which depends on their concentration. The concentration and time required for renaturation determines the Cot value (concentration × time). If a high concentration of labeled probe DNA is used, hybridization to complementary nucleic acid sequences in the target preparation can be achieved in a reasonably short time. These properties of DNA are extremely important, because they make it possible to detect specific DNA sequences (such as genes) on a nitrocellulose filter (molecular hybridization) or in cytological preparations (in situ hybridization) by using labeled DNA or RNA probes.
KeywordsMetaphase Spread Pericentric Inversion Painting Probe Cytogenet Cell Spectral Karyotyping
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