Preimplantation Genetic Diagnosis: Polymerase Chain Reaction

PGD, or preimplantation genetic diagnosis, is one of the few pre-pregnancy genetic testing methods that can help to decode genetic disorders at the embryonic stage. This essentially means that the IVF couple has a better chance of being implanted with healthier embryos, i.e. embryos free from genetic disorders. One of the testing methods used within PGD is Polymerase Chain Reaction.

Understanding Polymerase Chain Reaction

Polymerase chain reaction is mainly used for detecting single gene disorders. These are genetic disorders that are present in only one of the chromosomes. Each embryo receives 23 chromosomes each from the mother and father, for a total of 46 chromosomes. While some genetic disorders are easy to detect since they induce an anomaly across multiple chromosomes, some aren't as simple to identify, because they are present in just one chromosome.

Polymerase chain reaction, or PCR, is actually a DNA analysis procedure. PCR is capable of identifying a large variety of single-gene defects. This includes disorders caused by dominant and recessive genes. PCR does so by using a method that is called DNA amplification. Each of the existing DNA sequence is copied and used for analysis as explained below.

How Is PCR Done?

A DNA's unique structure is present in the form of its coiling, wherein it is composed of two strands arranged in the form of a double helix. PCR requires that these strands are separated from each other or uncoiled. This is done through denaturation.

Denaturing

The extracted DNA (deoxyribonucleic acid) is immersed in an enzymatic solution. This solution contains DNA polymerase enzyme, primers (nucleic acid compounds) and unattached nucleotide bases (DNA subunits). This solution is heated to a specific temperature. The heating furthers the bonding of the DNA strands, forcing them to uncoil.

Making DNA Copies

Upon cooling the solution, primers are able to bond to each of the uncoiled DNA strands. DNA polymerase helps in creating new DNA strands because it allows the nucleotide bases to link with the primers. This process is repeated several times until a sizeable number of DNA copies are prepared. PCR is quite fast in producing these DNA copies.

Possible Errors

Errors in PCR testing are possible. This can lead to a damaging misdiagnosis, because the embryo wrongly marked as carrying the genetic defect is destroyed. Each destroyed embryo puts forth time and cost considerations for the couple undergoing fertility treatment, since more embryos need to be created.

Preciseness of polymerase chain reaction results is heavily dependent on the quality of the testing environment. This means that the slightest of contaminants or an error by the laboratory technician can alter the results.

Polymerase chain reaction is prone to a typical kind of error-inducing process called Allele Dropout. The unique DNA sequence within a gene is critical to polymerase chain reaction. However, sometimes genes can have varying DNA sequences. Each sequence is called an allele. There is a risk that only one type of allele is identified and amplified by polymerase chain reaction.

Restrictions of DNA Samples

For PCR, a healthy and undamaged sample of DNA is critical. PGD testing uses methods like polar body extraction or blastomere cell extraction for removing a single cell from the embryo. However, these methods can adversely impact the chances of segregating DNA in a healthy, testable form due to technical limitations.

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