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Maxam-Gilbert and Sanger's Approach to Sequencing

This chapter has an understanding of the history of sequencing. The Maxam-Gilbert and Sangers method of sequencing are explained in detail. A brief be aware on pyro sequencing is also added.

Sequencing is a process where the collection of nucleotides is deciphered in a specific portion of DNA or RNA. This technique offers several advantages in daignosis. . First of all, in a PCR product, it helps him to find out when there is a mutation in the collection. A classical example of this is assessing RAS gene mutations. The RAS gene usually shows mutations in codon 12 and 13. It may also almost never show mutations in codon 61. Without sequencing, the dedication of an mutation is impossible. Sequencing is also useful to confirm the existence of an individual Nucleotide Polymorphism (SNP) or a spot mutation in cases where Restriction Span Fragmentation Polymorphism (RFLP) is equivocal. Scientists make use of it to characterize recently cloned cDNAs and to check the fidelity of an newly created mutation.


Prior to 1970's, there was no system to check a DNA sequence. The only path to hypothesize a series was to determine the amino acid series and retrospectively determine the nucleotide sequence based on the appropriate codons. Given the degeneracy of the hereditary code, this technique was essentially intelligent guesswork at its best (start to see the section on transcriptions and translation Section 5).

In middle 70's, Maxam-Gilbert and Sanger developed methods to accurately determine a DNA sequence. These methods were troublesome and frustrating. The automated sequencing method was a significant improvement over the prior methods. An analogy between them is most beneficial illustrated by checking the pleasure in traveling a Ford Model T and an S category Mercedes Benz.


What Maxam and Gilbert suggested to determine a nucleotide collection was quite simple. They took a terminally tagged DNA molecule and with the aid of chemical real estate agents, broke it at the things of attachment with adenine, guanine, cytosine and thymine. They then produced radioactive fragments extending from the labeled end to the position of that base. They ran the whole product on Polyacrylamide Gel electrophoresis (Webpage) which fixed the details of breakage. They then required an autoradiograph which produced four different cleavages specific for every single base.

The information on the chemicals used for lowering need not concern us here. Suffice to state that the technique developed was both hypersensitive and specific. It provided a good chemical distinction between the bases. Evaluation of collection on both strands provided satisfactory check. However, the issues associated with this technique were substantial. It took couple of days to sequence 200 - 300 bases. Moreover, there were several 'ifs' related to the procedure such as 'if the radioactive labeling process didn't work', 'if the cleavage reactions did not perform as expected', 'if the gel didn't set up properly', 'if the electrophoresis didn't work, if the gel were torn or otherwise destroyed during transfer', and 'if the X-ray film developer broke down through the development'. Even if everything worked properly, one would be prepared to get 200-300 bases of an confirmed DNA sequence every couple of days. The other associated problems were that the majority of radioactive material was used and hydrazine that was a substance used for chopping been a neurotoxin.


At a comparable time as Maxam-Gilbert DNA sequencing, had been developed, Fred Sanger developed an alternative solution method of DNA sequencing. Rather than using chemical type cleavage reactions, Sanger chosen a method regarding a kind of ribose sugar.

The process that Sanger used was predicated on a newspaper by Atkinson et al. Atkinson showed that whenever 2', 3'-dideoxythymidine triphosphate (ddTTP) was contained in to the growing oligonucleotide string in place of thymidylic acid (dT), the chain extension discontinued and termination occurred specifically at positions where dT should have been included. Sanger extended this system to other dideoxy nucleotides (ddCTP, ddATP, ddGTP) and thus using four different pipes with four different ddNTP's, he managed to terminate DNA sequence at places where nucleotides were supposed to be incorporated.

Fig 11. 1 - Inside the upper -panel, there can be an OH group at position 4. This allows the chain to elongate. In the low panel, there is an H atom which includes substituted the OH group. This does not allow the string to elongate and so the string terminates.

Fig 11. 1 illustrates this basic principle. The presence of 'H' group rather than the 'OH' group will not allow the string to elongate and therefore, the string terminates. To place this into practice, you can use four separate reactions. Each effect has all the components for a PCR but additionally to dNTPs, a tiny proportion of ddNTPs is also added. The four reactions have four different ddNTPs. The ddNTP concentrations are carefully adjusted so that they get incorporated in to the growing DNA strand randomly and infrequently. Due to this, the elongating string terminates randomly. When the whole product is run on a gel using independent lanes for each and every nucleotide, it is evident that the position of the rings corresponds to the position of the nucleotides. Thus, the gel can be read off and one can simply decode the collection. For further understanding of the particular gel picture would look like, please refer to fig 11. 2.

Fig 11. 2 - A good example of a collection obtained using the Sanger's method. Remember that the four lanes have been called GA, CT, A G and T C respectively. Which means that in the PCR response, as well as the normal dNTP's, there were also ddNTP's of Adenine, Thymine, Guanine and Cytosine Guanine, Cytosine, Adenine and Thymine in the respective lanes. When the ddNTP is included, there is a termination of the expansion. When the products are run on a gel, the termination of the series sometimes appears as a strap when tagged by autoradiography. As observed in the written text, it can be seen that the series can easily be read off of the gel

Box 8. 1

Sequencing is an activity where the collection of nucleotides in a specific part of DNA or RNA is obtained.

The Maxam and Gilbert method used the concept of taking a terminally tagged DNA molecule and breaking it at the adenine, guanine, cytosine and thymine residues with chemical substance agents. This was run on a PAGE and the things of breakage were fixed.

Sanger used a 2' 3' dideoxythymidine triphosphate. This was contained in the growing string and it prevented further expansion. Four different tagged nucleotides are being used and they terminate the DNA sequences at those places where in fact the nucleotides were said to be incorporated.


The development of manual sequencing methods by Maxam Gilbert and Sanger et al was a dramatic improvement over the prior methods which were mainly predicated on guesswork and good fortune. Although chemistry of both the methods was route breaking, it was difficult to sequence large portions of the genome.

Development of computerized sequencing methods by Hood made certain that sequencing was faster and much better to perform as compared to the manual sequencing methods. The foundation of automatic sequencing is labeling the product with some form of a fluorescent dye that can be detected using a detector system.

Logically speaking, only two components in the Polymerase Chain Response can be labeled: the primers and the dideoxy sequences. In the method defined by Hood, the primer was tagged with one of four different fluorescent dyes. Each tagged primer was located in another sequencing response with one of the four dideoxynucleotides (to terminate the reaction) and of course all four deoxynucleotides. After conclusion, all the four reactions were pooled and run jointly in single lane of the polyacrylamide sequencing gel. A four-color laser-induced-fluorescence detector was used to check the gel as the effect fragments migrated past. The fluorescence signature of each fragment was then delivered to a computer where the software was trained to perform 'base phoning' (a pc program for determining basics (nucleobase) series from a fluorescence "trace" data generated by an robotic DNA sequencer). This technique was commercialized in 1987 by the Applied Biosystems.

James M. Prober and acquaintances at DuPont needed the fluorescent sequencing solution to its next level by producing "a far more elegant method". Instead of fluorescence-labeled primers, they labeled the terminators themselves. The first 'dye set' was based on succinylfluorescein. Each ddNTP was tagged with another chemically tuned succinylfluorescein dye which could be distinguished by its fluorescent emission. All dye-labeled terminators were thrilled by an argon ion laser beam at 488nm to create peak emission that may be distinguished by a detector. This detection system intended that the sequencing response could now be carried out in one tube with all four terminators present and fragment resolution would require only one gel street. For record, it must be described that initially researchers used to perform PCR products on a gel and then 'read' the fluorescence produced. The launch of capillaries was a discovery in the development of computerized sequencing methods. Small capillaries with 50јm interior diameter dissipate temperature very efficiently because of the high surface to volume proportion. A capillary based mostly system can, therefore, be run with higher voltages. This lowers their operating time drastically. Fluorescence can be recognized through the capillary tubes. Thus, the capillary systems could be computerized as opposed to gel based mostly systems. A schematic diagram of sequencing is shown in Fig 11. 3.


Sequencing is performed on a short chain of nucleotides, which may be the PCR product or a cloned DNA collection. No more than 1000 bases can be sequenced effectively, a long way off from roughly 50 to 250 million bases that comprise a human chromosome. If one takes a PCR product, a primer of known collection is required for every sequencing response. Thus, one cannot take any piece of DNA and "just sequence it. " A known starting point, and thus some understanding of the sequence, must begin the response.

There are two ways of making DNA manageable and thus starting the cloning process. The simpler way would be to perform a PCR and collection the products. The next method would be to clone the DNA. In cloning, a DNA collection is created into a vector and many thousand copies are made when the vector replicates. With this section, we won't sophisticated on cloning as a preferred method because sequencing of PCR products is very simple and additionally used on clinical specimens.

Sequencing the PCR product - Following a short PCR reaction, it is necessary to verify that the reaction has worked and a product has actually created. This is done by working the product on the gel and confirming that the merchandise is of right size. Then, another PCR reaction is performed using either fluorescent primers or fluorescent nucleotides as discussed earlier. Protocol must be adjusted based on the device used and can not be elaborated further. Following the confirmation of a successful PCR, the products are purified.

There are several methods for purifying PCR products. These are ultrafiltration, ethanol precipitation, gel purification and enzymatic purification. In a functioning lab, it is however, highly recommended to work with commercial kits for purification. Several manufacturers such as Sigma and Genetix manufacture such kits and it is advisable to check out their set techniques. One should keep in mind that the basic goal of DNA purification system is to remove chromosomal DNA, proteins, enzymes, residual organic and natural chemicals, detergents, residual agarose if DNA was extracted form a gel, primers, unincorporated nucleotides, and salts from enzymatic reactions; The commercial package should be chosen keeping all this in mind.


Errors unveiled during production of the DNA template: Most errors are launched during DNA template production by PCR established protocols. One major cause is the intrinsic error rate in incorporation of nucleotides by the theromostable DNA polymerases. Even polymerases that have an inherent proof reading function can end up with PCR products comprising an assortment of different sequences.


INVASIVE CARCINOMA BREAST - Both BRCA 1 and BRCA 2 genes are regarded as mutated in families with high risk of breast tumors. These mutations are extremely rare in sporadic instances of breast cancer. The issue with BRCA gene mutations is that we now have a large number of mutations. Over 1500 mutations have been characterized till time. It really is quite impossible to develop a standardized test for the evaluation of most these mutations. Therefore, PCR followed by sequencing of specific parts of the gene remains the main method of tests.

RAS GENE MUTATIONS - The RAS gene is often mutated in cancers like colonic, lung, pancreatic, and thyroid cancers. Additionally it is commonly mutated in meanomas and several other tumours. RAS gene mutations were first reported in the 1980's. You will discover three mobile homologues of viral oncogenes. These are HRAS, KRAS and NRAS. The most frequent mutations that occur in the KRAS gene are the mutations at codon 12 and codon 13. Less commonly, mutations at codon 61 arise. KRAS mutations are usually tested by sequencing.

P53 MUTATIONS - Inactivating mutations in TP53 tumor suppressor genes are the most common genetic events in human malignancies. Majority of these arise from an individual point mutation in the portion encoding the DNA-binding domain name of TP53. These mutations render the mutant TP53 necessary protein unable to carry out its normal functions, i. e. , transcriptional transactivation of downstream concentrate on genes that control cell routine and apoptosis. Most mutations cluster in the TP53 DNA binding domains, which includes exons five through eight and spans roughly 180 codons or 540 nucleotides. Research of the p53 mutations is usually carried out by PCR of exons 5 to 8 followed by sequencing.


Large-scale re-sequencing of individuals genes has determined generally between 10 and 100 mutations in each tumor. The percentage of silent mutations is often quite high. However, careful research has resulted in the prediction that a limited variety of the newly identified mutations other than TP53, KRAS, etc. , are biologically significant. In future, it would appear that the PCR accompanied by sequencing is likely to play an increasingly important role in pathology.

SEQUENCING IN GENETIC DISORDERS - Two disorders will be handled in this section, Von Hippel Lindau disease and Connexin gene mutations in sensorineural deafness.

Von Hippel-Lindau (VHL) disease - It is a hereditary cancer syndrome caused by germline mutations in the VHL tumor suppressor gene. The VHL gene consists of three exons and encodes a mRNA of 4. 5 kb. Germline mutations were determined in the second option half of exon 1, in the first 1 / 2 of exon 3, and in a few part of exon 2. Missense, frameshift and nonsense mutations are known to occur along with deletions. Given the wide spectral range of mutations, the only real standardized way for screening process mutations is by sequencing.

Sensorineural deafness - Most hereditary reading reduction is inherited in a recessive manner, accounting for about 85% of non-syndromic reading reduction (NSHL). Deafness associated with DFNB1 locus on chromosome 13q11 is widespread in many elements of the planet. Two genes localised in this chromosomal region have been implicated in deafness. Included in these are connexin26 (Cx26, gene sign GJB2) and connexin 30 (Cx30, GJB6). The mutations in these regions are multiple and include missense, frameshift and nonsense mutations. Given the large number of mutations, sequencing has been used as the standard method for mutation evaluation.

SEQUENCING IN HEMATOLOGY - A couple of few instances of use of sequencing in hematology. Studies have pointed out sequencing as an adjunct analysis for clonality assessment in lymphomas. However, by and large, hematology does not use sequencing as an investigative modality; DNA and RNA based mostly PCRs are preferred. However is not being done, a possible use of sequencing in the analysis of Factor VIII mutations has been outlined.

The factor VIII gene is incredibly large (~ 180 kb) and structurally complicated (comprising of 26 exons). Immediate nucleotide sequence evaluation using programmed DNA sequencers is becoming more mainstream and confident results can be expected for male DNA (hemizygous). Sequencing should be interpreted cautiously for female DNA because heterozygosity may neglect to show in the sequencing data. Multiplex amplification of all of the essential parts of factor IX gene in a single PCR, followed by sequencing, presents a step forward and could be employed to factor VIII gene as well.

TROUBLESHOOTING DNA SEQUENCING: Although DNA sequencing usually works, there are times when it doesn't. This is extremely irritating just because a great deal of painstaking work has truly gone into planning the reaction. However, it must be stated that the causes of a failed sequencing are not many and usually the mistakes are amenable to correction. Some of these are given in desk 11. 1.

Table 11. 1 : DNA sequencing reaction failures



Degraded/ poor quality/ absent PCR product. The reasons for this are numerous and have been explained in detail in section 5.

Under these circumstances, it is best to repeat the PCR response.

Poor quality DNA. Very common when sequencing plasmid miniprep templates.

The best way of avoiding this issue is to not sequence plasmid DNA and sequence a PCR amplified fragment of the plasmid add. If this isn't possible, it is strongly recommended that a plasmid miniprep system is used. One tip is to perform a final ethanol precipitation on the kit purified plasmid DNA. This often solves problems with the grade of the template.

Loss of effect during clean-up. This is often a particular problem when using ethanol precipitation clean-up protocols.

This can be prevented by not using an ethanol precipitation protocol to clean the sequencing reaction. Commercial kits are for sale to cleaning up PCR products. These products work nicely but maybe expensive. However, when one considers the other bills mixed up in sequencing process, use of any commercial kit contributes only slightly to the entire process.

Bad water. The water used contains sequencing inhibitors.

Inhibitors can end up in lab normal water stocks that can get rid of DNA sequencing reactions. When there is an issue with water, it is advisable to throw out this particular and use a fresh stock - keep in mind drinking water is cheap.

Degradation of Taq DNA polymerase or dye tagged nucleotides.

If this is suspected, then it is a good idea to execute a control sequencing reaction before undertaking a large number of experimental reactions. Many problems can be prevented by keeping the chemicals in small aliquots and avoiding repeated freeze/thaw cycles.

Blocked capillary. The capillaries have to be maintained as per protocols.

Can be discovered by tracking track quality over a trace by track basis.

Box 8. 2

In robotic sequencing, the merchandise is tagged with a fluorescent dye that can be detected using a detector system.

Either the primers or the dideoxy sequences can be labeled.

A four coloring laser beam induced fluorescence detector detects the reactions fragments as they migrate past.

Capillary founded systems considerably reduce the run rate.

In robotic sequencing, a PCR is in the beginning run and the PCR product is either cloned and sequenced or sequenced without cloning.

The major source of error in direct DNA sequence examination is due to error introduced through the development of DNA template. An assortment of different sequences maybe produced.

Causes of failed DNA sequencing reactions are because of degraded/ poor quality/ absent PCR product, poor quality DNA, degradation of Taq or dye labeled nucleotides and obstructed capillaries.


Ever since Sanger brought out his solution to make sequencing the simple method it is today, workers have been looking for solutions to improve sequencing. The primary methods which are likely to be useful are sequencing by hybridization, parallel signature sequencing predicated on ligation and cleavage and pyrosequencing.

Pyrosequencing is a DNA sequencing approach that is dependant on the detection of the released pyrophosphate (PPi) during DNA synthesis. In a cascade of enzymatic reactions, obvious light is made that is proportional to the amount of included nucleotides.

Initially, there's a nucleic acid polymerization response where an inorganic PPi is released consequently of nucleotide incorporation by polymerase. The released PPi is subsequently changed into ATP by ATP sulfurylase, which gives the power to luciferase to oxidize luciferin and generate light. As the added nucleotide is known, the collection of the template can be motivated.

The result of the pyrosequencing reaction is as uses:

(NA)n + Nucleotide Polymerase (NA)n+1 + Pyrophosphate (PPi)

Pyrophosphate ATP Sulfurylase ATP

ATP + Luciferin + Air Luciferase AMP + Pyrophosphate + Oxyluciferin + CO2 + Light

It is to be remembered that dATP is a substrate for Luciferase. The addition of dATP±S was a significant improvement since dATP±S was found to be inert for luciferase, yet could be designed effectively by all DNA polymerases analyzed. The final step includes the addition of Apyrase. Apyrase, in the pyrosequencing effect system, effectively degrades the unincorporated nucleoside triphosphates to nucleoside diphosphates and consequently to nucleoside monophosphate.

The series of nucleotides in the effect is read as a pyrogram shown in fig 11. 4.

The problem in a pyrosequencing response is that the length of the sequences that can be analysed is usually quite small. Therefore, it is used mainly to verify the sequences that contain already been set up. It could also be used in the analysis of mane pin structures which may not be amenable to sequencing by standard methods.

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