(a) The table below shows details of the Sanger technique of gene sequencing.
1. Label 4 test tubes labelled A, T, C and G. Into each test tube add: a sample of the DNA to be sequenced (containing millions of individual molecules), the 4 DNA nucleotides and the enzyme DNA polymerase.


In each test tube add a small amount of a special modified nucleotide * that cannot form a bond and so stops further synthesis of DNA. Tube A = A*, tube T = T*, tube C = C* and tube G = G*. The * nucleotides are present at about 1% of the concentration of the normal nucleotides.


Let the DNA polymerase synthesise many copies of the DNA sample. From time to time at random a * nucleotide will be added to the growing chain and synthesis of that chain will then stop. A range of DNA molecules will be synthesised ranging from full length to very short. The important point is that in tube A, all the fragments will stop at an A nucleotide. In tube T, all the fragments will stop at a T nucleotide , and so on.


The contents of the four tubes are now run side by side on an electrophoresis gel.

(i) Give the sequence of the bases in the DNA used in this example by interpreting the developed gel shown in stage 4.


(ii) Give the corresponding mRNA sequence


(b) Describe and explain how electrophoresis is used to separate the different fragments of DNA produced by the Sanger technique