I’m kinda finding it a bit frustrating because the process of preparing extracted patient DNA for data collection has step reactions that take hours of wait time in-between them.
[I’m actually blogging now because I have to wait for 90 minutes for a PCR run to complete.]After the
Micro-array Scanner encounter, my next fascination is the Applied Biosystems (ABI) Prism 310 Sequencing machine which I am using to sequence the DNA from biopsies with the unknown type of muscular dystrophy.
[See related post- DNA Sequencing I ]This state-of-art machine replaces the traditional gel electrophoresis with capillary electrophoresis. Equipped with high performance computer technology, the device is capable of reading and storing the data and result with low percentage errors.
Polymerase Chain Reaction (PCR)The key to DNA sequencing is the PCR. The DNA that is extracted from patient muscle tissue has to be amplified – making it available in large amounts. Dystrophin, the gene that I am investigating in my project, is actually one of the longest (if not the longest) gene in the human genome. I am therefore using primers (both forward and reverse) to amplify only exons of interest in the DNA. 30 Primers were designed for this gene. Up to date, I have only sequenced exons using only 3 primers.
After running the PCR on the Thermocycler, the amplified DNA segments are run through a
gel electrophoresis reaction. Using a ladder key, a specific band is expected at a position that corresponds to the primer that was used. The specific number of base pairs is known since we designed the primers in the lab.
In this case, I used a primer that was designed to amplify an exon sequence of about 200 bp. Electrophoresis of the PCR product produced the following result [<-].
This result show that the PCR amplification of the particular exon was a success since the band is corresponds to the 200-300 bp bands in the ladder. I was instructed to commence to the next steps only after the confirming that the used primer worked. You just gotta love PCR and Gels!
ABI prism 310 Genetic Analyzer
The DNA undergoes a couple of protocols in order to prepare it for the sequencer. The steps include the Exonuclease, Sequencing, Ethanol/Sodium Acetate Precipitation and the Denaturation reactions.
The Analyser has a laser which
causes the applied dye on the four nucleotides (A-T-C-G) to illuminate different colors of the visible light spectrum. The colors are then recorded and sent to the computer for further analysis.
Sequencher® 4.9 is then used to align the output sequences (both reverse and forward) with their corresponding exon and primer sequences to make contigs.
This software has a chromatogram feature that allows for analysis of the quality of the output nucleotide sequences and detection of errors in the contig sequences. (see Real patient data below).
Click images to view them in larger version
Sadly, this is probably the last technique that I will be learning from the CRI laboratories for this summer. LG (Life‘s Good).