Central Dogma of Molecular Genetics
Restriction-Modification Systems of Bacteria DNA Sequencing Southern and Northern Analysis Polymerase Chain Reaction (or PCR) |
DNA SequencingThese cloning techniques have been widely used to isolate many genes from nearly all species. Once these genes have been isolated what can they be used for?
The dideoxy-chain-termination DNA sequencing technique is a DNA polymerase-based technique. This technique is based on the ability of a specific nucleotide (dideoxy nucleotide) to terminate the DNA polymerase reaction. These nucleotides do not have a free 3'-OH group, an absolute requirement for DNA polymerase activity. Thus, any time this nucleotide is inserted into the growing chain DNA synthesis stops. Technically, four polymerase reactions are performed, each containing the four nucleotides dATP, dTTP, dCTP and dGTP. In addition the reactions contain a limited amount of the one of the four dideoxybases so that all possible terminations can occur. After the reactions are finished, the products from the four reactions are separated side-by-side on a polyacrylamide gel. Each of the fragments within a lane ends with the base corresponding to the dideoxy nucleotide used in the reaction. Thus by reading the four lanes from the bottom of the gel to the top, the sequence of the DNA can be obtained. Diagrammatic Representation of a Sequencing Reaction Products
5' A T C T A G G-----------3' ddC-----------5' ddC--------------------5' ddT-----------------------------5' ddA-------------------------------------5' ddG--------------------------------------------5' ddA----------------------------------------------------5' ddT-------------------------------------------------------------5'Diagrammatic Representation of the Sequencing Gel
G A C T ---------------------- __ Longest product (5' end of gene) __ __ __ __ __ __ Shortest product (3' end of gene) ---------------------- Copyright © 1997. Phillip McClean
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