Polymerase chain reaction and its use in obstetrics

Abstract

Deoxyribonucleic acid (DNA) is composed of a deoxyribose backbone, the three position (3') of each deoxyribose being linked to the five position (5') of the next by a phosphodiester bond. At the two position each deoxyribose is linked to one of four nucleic acids; the purines adenine or guanine or the pyrimadines thymine or cytosine. Each DNA molecule is made up of two such strands in a double helix with the nucleic acid bases on the inside. The bases pair by hydrogen bonding, adenine (A) with thymine (T) and cytosine (C) with guanine (G). Deoxyribonucleic acid is replicated by separation of the two strands and synthesis by DNA polymerases of new complementary strands. With one notable exception, the reverse transcriptase produced by viruses, DNA polymerases always add new bases at the 3' end of the molecule. Ribonucleic acid (RNA) has a structure similar to that of DNA but is always single stranded. The backbone consists of ribose, and uracil is used in place of thymine.

The case for routine determination of chorionicity and zygosity in multiple pregnancy

“…A single Jeffreys probe is highly predictive of zygosity, with less than a 1:10 000 chance of a false negative result i.e., a monozygous pattern in a dizygous twin (Overton et al, 1995). The main disadvantage of this method is that it is expensive and results take 7-10 days.…”

“…The method is based on amplifying five polymorphic microsatellites with an accuracy comparable to Southern blotting. It has been estimated that the chance of PCR based method yielding falsely identical patterns in dizygous twins from placental and amniotic fluid samples is approximately 0·4 per cent (Overton et al, 1995).…”