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LOCATING AND SEQUENCING GENES
Recombinant DNA technology has enabled us to diagnose and treat many genetic disorders.
In order for us to do this it is necessary to know exactly where a particular gene is located.
To achieve this we use labelled DNA probes and DNA hybridisation.
A DNA probe is a short, single stranded section of DNA that has some kind of a label
attached to it which can be easily identified. Most commonly used probes are:
Radioactively labelled probes made up of nucleotides with radioactive isotopes.
Can be visualised using photographic film.
Fluorescently labelled probes these emit light when illuminated with invisible
ultraviolet light. Probes can be made to fluoresce with different colours.
A DNA probe is made that has bases complementary to the portion of the DNA
sequence (gene) we want to find.
DNA strands are separated using high temperature.
The separated DNA strands are mixed with probe, which binds to the
complementary bases on one of the strands.
The site at which the probe binds can be identified by the radioactivity or fluoresce it
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Gel electrophoresis allows for the separation of DNA molecules based on their size
DNA samples are placed into wells at one end of the gel and covered in a buffer
An electric current is passed through the gel.
Each nucleotide in a molecule of DNA contains a negatively charged phosphate
group, so DNA is attracted to the anode.
The molecules diffuse through the gel and smaller lengths of DNA move faster than
larger ones. (smaller ones move the furthest).…read more
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DNA sequencing is the process of determining the exact order of the bases A, T, C and G
in a piece of DNA.
The DNA to be sequenced is provided in singlestranded form. This acts as a
template upon which a new DNA strand is synthesised.
DNA synthesis requires a supply of the four nucleotides, the enzyme DNA
polymerase and a primer (a short sequence annealed to the template which initiates
the new DNA strand).…read more