A2 OCR Biology: 2.2.11 Sequencing and Copying DNA 2

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Emily Summers
Automated Sequencing
The reaction mixture contains DNA polymerase, many copies of the single stranded DNA
template, free DNA nucleotides and primers.
Some of the free nucleotides contain a fluorescent marker, which are modified and when
added to the growing chain the enzyme is thrown off and the strand cannot have any
further nucleotides added.
Each nucleotide has a different coloured marker.
The primer anneals at the 3' end of the template strand so DNA polymerase can
The DNA polymerase adds free nucleotides according to base pairing rules so the
strand grows
If an altered nucleotides is added the polymerase enzyme is thrown off and the
reaction stops on that template strand
As the reaction continues, many DNA molecules are synthesised. The fragments
can vary in size and in some of them the template strand has only one additional
nucleotide added before the polymerase is thrown off and in others the strand may
be completed. In every case the final added nucleotide is marked with a specific
As these strands run through the machine a laser reads the colour sequence


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