PCR, gel electrophoresis and genetic screening

Last time

Before we start: last time, in Genetic engineering tools, recombinant proteins and GM organisms, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A photocopier for DNA

A crime scene yields a speck of blood, far too little D N A to analyse. The polymerase chain reaction copies a chosen stretch of D N A millions of times in a couple of hours. Gel electrophoresis then sorts the copies by size so they can be read, and genetic screening uses both to test people for disease alleles.

What goes into PCR

The sample D N A, containing the target sequence, goes into a tube. Two short primers are added, which are complementary to the ends of the target. Free nucleotides, A, T, C and G, are the building blocks. A heat stable D N A polymerase such as Taq polymerase, from a hot spring bacterium, builds the new strands. A buffer and magnesium ions keep conditions right for the enzyme.

The three steps

Denaturation: heat to about 95 degrees. The hydrogen bonds between the strands break, giving single strands. Annealing: cool to about 55 degrees. The primers bind to complementary sequences on the single strands. Extension: heat to about 72 degrees, the best temperature for Taq polymerase. It adds nucleotides to the primers, in the five prime to three prime direction. Taq polymerase is not denatured at 95 degrees, so it does not need to be added again each cycle.

Exponential copying

Each cycle doubles the number of copies. After ten cycles, one molecule becomes 2 to the power 10, which is 1024 copies. After thirty cycles, there are 2 to the power 30, over a billion copies: about 1.07 times ten to the nine. Since each cycle takes only a few minutes, this takes under two hours.

A past-paper example

May June 2026, Paper 44, question 8. The polymerase chain reaction amplifies selected regions of D N A. Name the substances that are added to the P C R machine at the beginning. D N A with the target sequence, primers, free nucleotides, and Taq polymerase. They are then cycled through three temperatures: denature, anneal and extend.