DNA Sequencing and Fragment Analysis

Posts tagged ‘capillary sequencer’

Sanger Sequencing: Historical Development of Automated DNA Sequencing

During the 1970s, Frederick Sanger developed a new technique allowing the base sequence of DNA to be determined. The design of his method is still very popular today. Sanger employed dideoxynucleic acids, ddNTPs, in addition to deoxynucleic acids together in the amplification of DNA during the Polymerase Chain Reaction (PCR). Instead of amplifying a section of DNA, the ddNTPS would cause amplification to terminate in a random number of amplified products. The ddNTPs were also radioactively labeled for detection. The end result of the Sanger chemistry would be a series of new DNA products with sizes increasing by a single base.

The PCR products were separated on a medium, polyacrylamide gel, with smaller products migrating faster than the larger products. The end result appeared like a ladder when detected.

The basics of the chemistry remain the same today. What has changed in Sanger sequencing is the method of separating the products and detection.

Original Base Labels were Radioactive

Before development of the fluorophore, Sanger labeled ddNTPs with a radioactive tag on the 5 prime end of the base. Because there was no method that could identify the different bases, a sample was amplified by PCR in four separate tubes. Each tube represented one of the four bases making up DNA. The amplified products were loaded separately into four lanes and allowed to migrate. Once complete, the gel was photographed by x-ray to view the result as shown in figure 1.

Fluorescent Labeled ddNTPs Replaced Radioactive Labels

Automation replaced the manual sequencing method with development of fluorescent labeled ddNTPs. Slab gels were still poured. However, all four bases were combined into a single reaction and loaded together. Samples would electrophorese and separate in the gel. Once the amplified products reached a region on the bottom of the plates, a laser would excite the fluorescent labels and color would be recorded by camera. Resulting images were collected by computer and analyzed as shown in figure 2.

Development of the automated chemistry allowed sequencing to be performed much faster. First, one lane on the gel was required for each sample. Second, sequence was recorded as electrophoresis was performed. The smaller more quickly migrating bands could be run completely through the gel as larger bands were recorded. Therefore more bases could be determined.

The problem that potentially occurred was in the plates. Because sequence results were recorded through the plates, the plates needed to be clean from debris and clear of any scratches. Although plates also needed to be clean for the radioactive method, it was more stringent for automated sequencing.

A significant amount of sequencing was performed using automated slab gel sequencing. But researchers still needed to pour plates and electrophoresis was performed for 12 hours or longer.

Capillary Sanger Sequencing

Capillary development occurred during the 1990s beginning with a single capillary machine, the ABI 310. The new technology eliminated any need for pouring gels. Instead, semi-liquid polymer was injected into the capillary before each run as shown in figure 3. Individual runs reduced the length of time required for electrophoresis to less than 3 hours.

The automated process changed very little from slab gel machines to capillary. But capillary sequencing was faster and more sensitive. It required much less DNA added to the PCR amplification. Better chemistries were developed in conjunction with automated sequencing. Capillary sequencers today characteristically perform 4 to 96 samples in a single run. The runs generally require 2 to 2 ½ hours of electrophoresis. Automated injection of samples allows hands-off operation through multiple sets of samples.

Frederick Sanger developed an important method for sequencing DNA. It allowed early completion of the human genome project, a genome with 3 billion bases. Although next-generation sequencing has expanded sequencing capabilities, Sanger sequencing is used for small sections of DNA often used in medical research.

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Double Sequence: A Common Problem in Sanger Sequencing

Sanger sequencing methodology requires template DNA to be relatively free from contaminating salts. Template quality is often determined electrophotometrically by loading the template on agarose gel. Agarose gel electrophoresis often shows the faster moving supercoiled DNA and slower moving nicked DNA found in a quality preparation. DNA samples produced by PCR amplification can be similarly tested by agarose gel electrophoresis. A good PCR product is viewed as a single clean band representing the desired length.

Another test is to measure absorbance spectrophotometrically at ultraviolet wavelengths 260 nm and 280 nm. A 260 nm / 280 nm ratio value of 1.8 is an indicator that the DNA is good.

Unfortunately, the result off the capillary sequencer shows double set of peaks. Because one set of peaks is directly over top the other, software cannot determine the correct bases to call as in figure 1. A result showing a double set of peaks is not as uncommon as many might believe.

What are the primary causes of double sequence?

Double Sequence Caused by Template Contamination

One of the early steps in plasmid DNA preparation is to select a desired colony and then inoculate liquid medium. In an overnight culture, the inoculated media would go through rapid bacterial cell division as the cells enter the log growth phase. Because the initial cells came from a single colony, it is believed that every cell is the identical product cloned from a single cell. However, this is not always the case. Colonies do not always come from a single cell. A colony could also be the product of multiple cells that failed to separate when spread on a plate. If the multiple cells are not identical, the sequence result could produce the double sequence shown in figure 1.

The region where double sequence begins is usually a restriction sight where a separate piece of DNA is inserted into a vector. One purpose of bacterial cloning is to produce copies of the inserted DNA. The clean sequence shown in figure 1 is vector sequence before the restriction sight. Despite differences in the two templates, the vector sequence is generally the same. The template contamination becomes apparent at the point where the new DNA has been inserted into the vector.

Often, the reverse or complimentary strand does not produce a double sequence and this is often confusing. If the DNA insert were relatively long, it would require sequencing the whole complimentary strand before double sequence is observed. The reason is simply because double sequence is caused by addition or deletion of an unknown number of bases at one restriction sight only. Therefore, both templates, the desired and contaminant have identical sequence in the complimentary direction.

PCR fragments could also produce double sequence. It is the result of errors in PCR amplification where two products are produced. The PCR fragments could produce two very similar sized products that appear as a single band once tested by agarose gel electrophoresis. Resolution using agarose gel is limited and does not separate DNA samples of very similar size.

Double Priming Results in Dual Amplification

Sanger sequencing is similar to PCR amplification. A primer anneals at the beginning of the region to be sequenced and Taq polymerase adds bases (dNTPs) in extension to produce and identical strand. A primer that matches two regions on either the insert or the plasmid could cause two separate amplifications simultaneously. The result of double priming is shown in figure 2.

Unlike template contamination, two separate products generally do not appear clean in the beginning. It does not require the primer to match identically to both annealing sights as long as the bases of the primer are similar with a match on the 3 prime ends. What happens if one annealing site is downstream from the other annealing site on the same strand of DNA? The downstream primer blocks bases extending from the first primer and the double sequence eventually becomes a single set of peaks.

Single Nucleotide Polymorphisms Could Also Cause Double Sequence

PCR amplification is used to determine potential heterozygous bases as single nucleotide polymorphisms (SNPs). It is one method for detecting mutations that could cause certain genetic diseases. The SNP likely could be observed as a base change in one of two identical alleles amplified together and is generally easy to spot from automated Sanger sequencing. A single base position is represented by two peaks to positively identify a SNP.

However, SNPs could also be the result of a base insertion or deletion (indel) as in figure 3. From the figure it is possible to follow two identical strands of sequence that are different by one base. Both alleles match until the GGCC region. However, one allele is missing the T-base and has shifted upstream by one base. The large clean peaks observed in the double peak region are simply the same base represented by two different products.

Preventing Double Sequencing in Sequencing Results

There are some cases where double sequence cannot be resolved as with PCR fragments that contain indels. However, certain steps could be taken to reduce conditions described in figure 1 and figure 2.

Plasmid preparations that produce double sequence at the point of insert can be prevented using a process called single colony isolation. Once the plasmid cells have been spread on a plate and grown, the select colony is isolated and spread on a second plate. Although the process requires additional time, it is worth the effort in cases when double sequence often occurs.

The process for preventing double sequence in PCR fragments is more complicated because PCR conditions need better optimization. It could be something as simple as reducing the amount of primer or number of cycles. Or require small adjustments in PCR conditions including temperature. Guidelines for PCR optimization will be a future topic.

Samples that produce double sequence as the result of double priming can be resolved by extending bases to the custom primer that was selected for sequencing. A better choice is to select a different primer even when double priming is caused from one of the possible universal primers present on a particular vector.

Double sequence result patterns are generally similar for each condition discussed here. Anyone is invited to present results similar to that presented. Questions are also welcome.

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