by Darian Peters

Created on: February 20, 2009   Last Updated: March 03, 2009

Deoxyribonucleic acid, DNA, is the molecule of heredity that contains the genetic code for generating and maintaining an organism. To understand an organism, how it functions, how it develops, how it evolved, and what diseases may befall it, etc. it helps to know what the DNA says, and what genes are present in the genome of the organism. A technique is required to analyse the DNA molecule and identify the nucleotides present and the order that they appear in. This is the DNA sequencing problem and there are several techniques for solving it.

At the heart of the problem is the sequence of the nucleotide bases adenine, thymine, cytosine, and guanine that appear on the DNA strand. These appear in groups of three known as codons. Each codon codes for a particular amino acid and it is these amino acids that are the building blocks of the proteins that are fundamental to so many activities in the cells of the organism.

Early sequencing techniques, back in the early 1970s, relied on 2-dimensional chromatography. An improvement came in 1976-77, when Maxam and Gilbert provided a method that used chemical modification and was capable of cleaving the DNA at particular bases. This chemical sequencing method was first used to look at DNA-protein interactions as well as the structure of the nucleic acid and also epigenetic effects. But this complex, and potentially hazardous, method was to be superseded by the chain-termination method.

The new method, originated by Frederick Sanger, used dideoxynucleotide triphosphates as the DNA chain terminators, and this proved to be a more efficient and less dangerous method, both in terms of the fewer toxic chemicals required and the less radioactivity used in the process. This type of method made the whole process of DNA sequencing a lot more widely accessible to researchers from around the globe through the increase in simplicity. Suddenly a lot more work could be done a lot more cheaply and safely.

A further advance is called dye-terminator sequencing. This method uses the natural properties of fluorescent dyes to label the four different oxynucleotides. The key point being that each of the different dyes has a different wavelength of fluorescence and emission. This technique links particularly well with another advance, that of automated sequencing. This is an especially important advance for the massive sequencing projects that are now being attempted, with entire genomes of organisms being sequenced rather than just single genes or regions of a chromosome.

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