The Maxam Gilberts Method Biology Essay

This chapter provides an apprehension of the history of sequencing. The Maxam-Gilbert and Sangers method of sequencing are explained in item. A short note on pyro sequencing is besides added.

Sequencing is a procedure by which the sequence of bases is deciphered in a peculiar part of Deoxyribonucleic acid or RNA. This method offers several advantages in daignosis.. Firstly, in a PCR merchandise, it helps him to find if there is a mutant in the sequence. A classical illustration of this is in measuring RAS cistron mutants. The RAS cistron normally shows mutants in codon 12 and 13. It may besides seldom show mutants in codon 61. Without sequencing, the finding of a mutant is impossible. Sequencing is besides utile to corroborate the presence of a Single Nucleotide Polymorphism ( SNP ) or a point mutant in instances where Restriction Length Fragmentation Polymorphism ( RFLP ) is ambiguous. Scientists use it to qualify freshly cloned complementary DNA and to look into the fidelity of a freshly created mutant.


Prior to 1970 ‘s, there was no system to look into a Deoxyribonucleic acid sequence. The lone manner to speculate a sequence was to find the amino acid sequence and retrospectively find the nucleotide sequence based on the appropriate codons. Given the degeneration of the familial codification, this system was basically intelligent guessing at its best ( see the chapter on written texts and interlingual rendition Chapter 5 ) .

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In mid 70 ‘s, Maxam-Gilbert and Sanger developed methods to accurately find a Deoxyribonucleic acid sequence. These methods were cumbrous and clip consuming. The machine-controlled sequencing method was a considerable betterment over the old methods. An analogy between them is best illustrated by comparing the joy in driving a Ford Model T and an S category Mercedes Benz.


What Maxam and Gilbert proposed to find a nucleotide sequence was rather simple. They took a terminally labeled DNA molecule and with the aid of chemical agents, broke it at the points of fond regard with A, G, C and T. They so produced radioactive fragments widening from the labelled terminal to the place of that base. They ran the full merchandise on Polyacrylamide Gel cataphoresis ( PAGE ) which resolved the points of breakage. They so took an autoradiograph which produced four different cleavages specific for each base.

The inside informations of the chemicals used for cutting demand non concern us here. Suffice to state that the method developed was both sensitive and specific. It provided a good chemical differentiation between the bases. Analysis of sequence on both the strands provided equal cheque. However, the jobs associated with this method were tremendous. It took few yearss to sequence 200 – 300 bases. Furthermore, there were several ‘ifs ‘ related to the process such as ‘if the radioactive labeling procedure did non work ‘ , ‘if the cleavage reactions did non execute as expected ‘ , ‘if the gel did non put up decently ‘ , ‘if the cataphoresis did non work, if the gel were torn or otherwise destroyed during transportation ‘ , and ‘if the X-ray movie developer broke down during the development ‘ . Even if everything worked absolutely, one would anticipate to acquire 200-300 bases of a confirmed Deoxyribonucleic acid sequence every few yearss. The other associated jobs were that a batch of radioactive stuff was used and hydrazine which was a chemical used for cutting happened to be a neurolysin.


At about the same clip as Maxam-Gilbert DNA sequencing, was being developed, Fred Sanger developed an alternate method of DNA sequencing. Rather than utilizing chemical cleavage reactions, Sanger opted for a method affecting a signifier of ribose sugars.

The rule that Sanger used was based on a paper by Atkinson et Al. Atkinson showed that when 2′,3′-dideoxythymidine triphosphate ( ddTTP ) was incorporated into the turning oligonucleotide concatenation in topographic point of thymidylic acid ( dT ) , the concatenation extension stopped and expiration occurred specifically at places where dT should hold been incorporated. Sanger extended this technique to other dideoxy bases ( ddCTP, ddATP, ddGTP ) and therefore utilizing four different tubings with four different ddNTP ‘s, he managed to end DNA sequence at topographic points where bases were supposed to be incorporated.

Fig 11.1 – In the upper panel, there is an OH group at place 4. This allows the concatenation to stretch. In the lower panel, there is an H atom which has replaced the OH group. This does non let the concatenation to stretch and therefore the concatenation terminates.

Fig 11.1 illustrates this rule. The presence of an ‘H ‘ group alternatively of the ‘OH ‘ group does non let the concatenation to stretch and therefore, the concatenation terminates. To set this into pattern, one can utilize four separate reactions. Each reaction has all the constituents for a PCR but in add-on to dNTPs, a little proportion of ddNTPs is besides added. The four reactions have four different ddNTPs. The ddNTP concentrations are carefully adjusted so that they get incorporated into the turning DNA strand indiscriminately and infrequently. Due to this, the stretching concatenation terminates indiscriminately. When the full merchandise is run on a gel utilizing separate lanes for each base, it is obvious that the place of the sets corresponds to the place of the bases. Therefore, the gel can be read away and one can easy decrypt the sequence. For farther apprehension of what the gel image would look like, delight mention to fig 11.2.

Fig 11.2 – An illustration of a sequence obtained utilizing the Sanger ‘s method. Note that the four lanes have been labeled as GA, CT, A G and T C severally. This means that in the PCR reaction, in add-on to the normal dNTP ‘s, there were besides ddNTP ‘s of Adenine, Thymine, Guanine and Cytosine Guanine, Cytosine, Adenine and Thymine in the several lanes. Equally shortly as the ddNTP is incorporated, there is a expiration of the extension. When the merchandises are run on a gel, the expiration of the sequence is seen as a set when labeled by autoradiography. As noted in the text, it can be seen that the sequence can easy be read off the gel

Box 8.1

Sequencing is a procedure by which the sequence of bases in a peculiar part of Deoxyribonucleic acid or RNA is obtained.

The Maxam and Gilbert method used the construct of taking a terminally labeled DNA molecule and interrupting it at the A, G, C and T residues with chemical agents. This was run on a Page and the points of breakage were resolved.

Sanger used a 2 ‘ 3 ‘ dideoxythymidine triphosphate. This was incorporated in the turning concatenation and it prevented farther extension. Four different labeled bases are used and they terminate the Deoxyribonucleic acid sequences at those topographic points where the bases were supposed to be incorporated.


The development of manual sequencing methods by Maxam Gilbert and Sanger et Al was a dramatic betterment over the old methods which were chiefly based on guessing and fortune. Though the chemical science of both the methods was way breakage, it was hard to sequence big parts of the genome.

Development of machine-controlled sequencing methods by Hood ensured that sequencing was faster and far simpler to execute every bit compared to the manual sequencing methods. The footing of machine-controlled sequencing is labeling the merchandise with some signifier of a fluorescent dye that can be detected utilizing a sensor system.

Logically talking, merely two constituents in the Polymerase Chain Reaction can be labeled: the primers and the dideoxy sequences. In the method described by Hood, the primer was labeled with one of four different fluorescent dyes. Each labeled primer was placed in a separate sequencing reaction with one of the four dideoxynucleotides ( to end the reaction ) and of class all four deoxynucleotides. After completion, all the four reactions were pooled and run together in individual lane of a polyacrylamide sequencing gel. A four-color laser-induced-fluorescence sensor was used to scan the gel as the reaction fragments migrated past. The fluorescence signature of each fragment was so sent to a computing machine where the package was trained to execute ‘base naming ‘ ( a computing machine plan for placing a base ( nucleobase ) sequence from a fluorescence “ hint ” informations generated by an machine-controlled DNA sequenator ) . This method was commercialized in 1987 by the Applied Biosystems.

James M. Prober and co-workers at DuPont took the fluorescent sequencing method to its following degree by developing “ a more elegant method ” . Alternatively of fluorescence-labeled primers, they labeled the eradicators themselves. The first ‘dye set ‘ was based on succinylfluorescein. Each ddNTP was labeled with a different chemically tuned succinylfluorescein dye which could be distinguished by its fluorescent emanation. All four dye-labeled eradicators were excited by an Ar ion optical maser at 488nm to bring forth peak emanation that could be distinguished by a sensor. This sensing system meant that the sequencing reaction could now be carried out in a individual tubing with all four eradicators present and fragment declaration would necessitate merely one gel lane. For record, it must be mentioned that ab initio research workers used to run PCR merchandises on a gel and so ‘read ‘ the fluorescence generated. The debut of capillaries was a discovery in the development of machine-controlled sequencing methods. Small capillaries with 50I?m interior diameter dissipate heat really expeditiously due to their high surface country to volume ratio. A capillary based system can, hence, be run with much higher electromotive forces. This lowers their running clip dramatically. Fluorescence can be detected through the capillary tubings. Therefore, the capillary systems could be automated as opposed to gel based systems. A conventional diagram of sequencing is shown in Fig 11.3.


Sequencing is performed on a short concatenation of bases, which can be either a PCR merchandise or a cloned DNA sequence. Merely about 1000 bases can be sequenced accurately, a far call from about 50 to 250 million bases that comprise a human chromosome. If one takes a PCR merchandise, a primer of known sequence is required for each sequencing reaction. Therefore, one can non take any piece of DNA and “ merely sequence it. ” A known starting point, and therefore some cognition of the sequence, is required to get down the reaction.

There are two ways of doing DNA manageable and therefore get downing the cloning procedure. The simpler manner would be to execute a PCR and sequence the merchandises. The 2nd method would be to clone the Deoxyribonucleic acid. In cloning, a Deoxyribonucleic acid sequence is introduced into a vector and several thousand transcripts are generated when the vector replicates. In this subdivision, we will non lucubrate on cloning as a preferable method because sequencing of PCR merchandises is simpler and more normally used on clinical specimens.

Fig 11.3 – A conventional diagram demoing the methodological analysis of machine-controlled sequencing. ( A ) The sequencing procedure utilizing fluorescent labeled bases. ( B ) Indicates the location where the gel has stopped because of the add-on of the ddNTP ‘s.

Sequencing the PCR merchandise – Following an initial PCR reaction, it is necessary to corroborate that the reaction has worked and a merchandise has really formed. This can be done by running the merchandise on a gel and corroborating that the merchandise is of right size. Then, a 2nd PCR reaction is performed utilizing either fluorescent primers or fluorescent bases as outlined earlier. Protocol needs to be adjusted based on the machine used and will non be elaborated further. After the verification of a successful PCR, the merchandises are purified.

There are several methods for sublimating PCR merchandises. These are ultrafiltration, ethanol precipitation, gel purification and enzymatic purification. In a functioning research lab, it is nevertheless, advisable to utilize commercial kits for purification. Several makers such as Sigma and Genetix industry such kits and it is advisable to follow their set processs. One should retrieve that the basic purpose of DNA purification system is to take chromosomal DNA, proteins, enzymes, residuary organic chemicals, detergents, residuary agarose if DNA was extracted signifier a gel, primers, unincorporated bases, and salts from enzymatic reactions ; The commercial kit should be chosen maintaining all this in head.


Mistakes introduced during production of the Deoxyribonucleic acid templet: Majority of mistakes are introduced during Deoxyribonucleic acid templet production by PCR based protocols. One major cause is the intrinsic mistake rate in incorporation of bases by the theromostable Deoxyribonucleic acid polymerases. Even polymerases which have an built-in cogent evidence reading map can stop up with PCR merchandises incorporating a mixture of different sequences.


INVASIVE CARCINOMA BREAST – Both the BRCA 1 and BRCA 2 cistrons are known to be mutated in households with high hazard of chest malignant neoplastic disease. These mutants are highly rare in sporadic instances of chest malignant neoplastic disease. The job with BRCA cistron mutants is that there are a big figure of mutants. Over 1500 mutants have been characterized boulder clay day of the month. It is rather impossible to develop a standardised trial for the rating of all these mutants. Therefore, PCR followed by sequencing of specific parts of the cistron remains the chief method of proving.

RAS GENE MUTATIONS – The RAS cistron is normally mutated in malignant neoplastic diseases like colonic, lung, pancreatic, and thyroid malignant neoplastic diseases. It is besides normally mutated in meanomas and several other tumors. RAS cistron mutants were foremost reported in the 1980 ‘s. There are three cellular homologues of viral transforming genes. These are HRAS, KRAS and NRAS. The most common mutants that occur in the KRAS cistron are the mutants at codon 12 and codon 13. Less normally, mutants at codon 61 occur. KRAS mutants are normally tested by sequencing.

P53 MUTATIONS – Demobilizing mutants in TP53 tumour suppresser cistrons are the most common familial events in human malignant neoplastic diseases. Majority of these arise from a individual point mutant in the section encoding the DNA-binding sphere of TP53. These mutants render the mutant TP53 protein unable to transport out its normal maps, i.e. , transcriptional transactivation of downstream mark cistrons that regulate cell rhythm and programmed cell death. Most mutants bunch in the TP53 DNA adhering sphere, which encompasses coding DNAs five through eight and spans about 180 codons or 540 bases. Analysis of the p53 mutants is normally carried out by PCR of coding DNAs 5 to 8 followed by sequencing.


Large-scale re-sequencing of human cistrons has identified by and large between 10 and 100 mutants in each tumour. The per centum of soundless mutants is frequently rather high. However, careful analysis has led to the anticipation that a limited figure of the freshly identified mutants other than TP53, KRAS, etc. , are biologically important. In future, it appears that the PCR followed by sequencing is likely to play an progressively of import function in pathology.

SEQUENCING IN GENETIC DISORDERS – Two upsets will be dealt with in this subdivision, Von Hippel Lindau disease and Connexin cistron mutants in sensorineural hearing loss.

Von Hippel-Lindau ( VHL ) disease – It is a familial malignant neoplastic disease syndrome caused by germline mutants in the VHL tumour suppresser cistron. The VHL cistron contains three coding DNAs and encodes a messenger RNA of 4.5 kilobit. Germline mutants were identified in the latter half of exon 1, in the first half of exon 3, and in some portion of exon 2. Missense, frameshift and nonsensical mutants are known to happen along with omissions. Given the broad spectrum of mutants, the lone standardised method for testing mutants is by sequencing.

Sensorineural deafness – Most familial hearing loss is inherited in a recessionary mode, accounting for about 85 % of non-syndromic hearing loss ( NSHL ) . Deafness associated with DFNB1 venue on chromosome 13q11 is prevailing in many parts of the universe. Two cistrons localised in this chromosomal part have been implicated in hearing loss. These include connexin26 ( Cx26, cistron symbol GJB2 ) and connexin 30 ( Cx30, GJB6 ) . The mutants in these parts are multiple and include missense, frameshift and nonsensical mutants. Given the big figure of mutants, sequencing has been adopted as the criterion method for mutant analysis.

SEQUENCING IN HEMATOLOGY – There are few cases of usage of sequencing in haematology. Surveies have mentioned sequencing as an accessory probe for clonality appraisal in lymphomas. However, by and big, haematology does non utilize sequencing as an fact-finding mode ; DNA and RNA based PCRs are preferred. Although non being done, a possible usage of sequencing in the analysis of Factor VIII mutants is being outlined.

The factor VIII cistron is highly big ( ~ 180 kilobit ) and structurally complex ( consisting of 26 coding DNAs ) . Direct nucleotide sequence analysis utilizing automatic Deoxyribonucleic acid sequenators is going more mainstream and confident consequences can be expected for male DNA ( hemizygous ) . Sequencing should be interpreted carefully for female DNA because heterozygosity may neglect to demo in the sequencing informations. Multiplex elaboration of all of the indispensable parts of factor IX cistron in a individual PCR, followed by sequencing, represents a measure frontward and could be applied to factor VIII cistron every bit good.

TROUBLESHOOTING Deoxyribonucleic acid SEQUENCING: Although DNA sequencing normally works, there are times when it does n’t. This can be highly annoying because a batch of painstaking work has gone into fixing the reaction. However, it must be mentioned that the causes of a failed sequencing are non many and normally the errors are conformable to rectification. Some of these are given in table 11.1.

Table 11.1: Deoxyribonucleic acid sequencing reaction failures



Degraded/ hapless quality/ absent PCR merchandise. The grounds for this are legion and have been explained in item in chapter 5.

Under these fortunes, it is best to reiterate the PCR reaction.

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

The best manner of avoiding this job is to non sequence plasmid DNA and sequence a PCR amplified fragment of the plasmid insert. If this is non possible, it is recommended that a plasmid miniprep kit is used. One tip is to execute a concluding ethyl alcohol precipitation on the kit purified plasmid DNA. This frequently solves jobs with the quality of the templet.

Loss of reaction during clean-up. This can be a peculiar job when utilizing ethanol precipitation clean-up protocols.

This can be avoided by non utilizing an ethanol precipitation protocol to clean up the sequencing reaction. Commercial kits are available for cleaning up PCR merchandises. These kits work really good but possibly expensive. However, when one considers the other disbursals involved in the sequencing procedure, usage of a commercial kit adds merely somewhat to the full procedure.

Bad H2O. The H2O used contains sequencing inhibitors.

Inhibitors can stop up in lab H2O stocks that can kill DNA sequencing reactions. If there is a job with H2O, it is best to throw out the H2O and utilize a fresh stock – remember H2O is inexpensive.

Degradation of Taq DNA polymerase or dye labeled bases.

If this is suspected, so it is advisable to execute a control sequencing reaction before set abouting a big figure of experimental reactions. Many jobs can be prevented by hive awaying the chemicals in little aliquots and avoiding repeated freeze/thaw rhythms.

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

Can be identified by tracking hint quality on a hint by trace footing.

Box 8.2

In machine-controlled sequencing, the merchandise is labeled with a fluorescent dye that can be detected utilizing a sensor system.

Either the primers or the dideoxy sequences can be labeled.

A four coloring material optical maser induced fluorescence sensor detects the reactions fragments as they migrate past.

Capillary based systems well cut down the tally rate.

In machine-controlled sequencing, a PCR is ab initio run and the PCR merchandise is either cloned and sequenced or sequenced without cloning.

The major beginning of mistake in direct DNA sequence analysis is because of mistake introduced during the production of DNA templet. A mixture of different sequences possibly produced.

Causes of failed DNA sequencing reactions are because of degraded/ hapless quality/ absent PCR merchandise, hapless quality DNA, debasement of Taq or dye labeled bases and out of use capillaries.


Ever since Sanger brought out his method to do sequencing the simple process it is today, workers have been looking for methods to better sequencing. The chief methods which are likely to be utile are sequencing by hybridisation, parallel signature sequencing based on ligation and cleavage and pyrosequencing.

Pyrosequencing is a Deoxyribonucleic acid sequencing technique that is based on the sensing of the released pyrophosphate ( PPi ) during Deoxyribonucleic acid synthesis. In a cascade of enzymatic reactions, seeable visible radiation is generated that is relative to the figure of integrated bases.

Initially, there is a nucleic acid polymerisation reaction in which an inorganic PPi is released as a consequence of nucleotide incorporation by polymerase. The released PPi is later converted to ATP by ATP sulfurylase, which provides the energy to luciferase to oxidise luciferin and generate visible radiation. Because the added base is known, the sequence of the templet can be determined.

The reaction of the pyrosequencing reaction is as follows:

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

Pyrophosphate ATP Sulfurylase ATP

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

It is to be remembered that dATP is a substrate for Luciferase. The add-on of dATPI±S was a considerable betterment since dATPI±S was found to be inert for luciferase, yet could be incorporated expeditiously by all DNA polymerases tested. The last measure includes the add-on of Apyrase. Apyrase, in the pyrosequencing reaction system, expeditiously degrades the unincorporated nucleoside triphosphates to nucleoside diphosphates and later to nucleoside monophosphate.

The sequence of bases in the reaction is read as a pyrogram shown in fig 11.4.

Fig 11.4. Pyrogram of the informations obtained from liquid stage pyrosequencing. Proportional signals are obtained for one, two, three and four base incorporations. Nucleotide add-on harmonizing to the order of bases is indicated below the pyrogram

The job in a pyrosequencing reaction is that the length of the sequences that can be analysed is normally rather little. Therefore, it is used chiefly to corroborate the sequences that have already been established. It may besides be used in the analysis of hair pin constructions which may non be conformable to sequencing by standard methods.


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