TOOL 1 – Polymerase Chain Reaction (PCR)
DNA replication
DNA is made up of two rows of
subunits that are joined together to form a double helix shape. Between the
rows of subunits A only binds with T and C only with G, so each subunit is
facing its partner. The bond between subunits is strong but temporary. DNA
replicates itself by breaking the subunit bonds and unzipping its two rows. New
subunits bind to its exposed partner and subunits that are next door neighbours
are glued together to form rows; thus, one DNA molecule becomes two. DNA
replication is controlled by special proteins called DNA polymerases.
An analogy might help to visualise
this: imagine a ladder is chopped in half lengthways, resulting in two halves
with the rungs of the ladder exposed. Both halves are fixed one rung at a time;
thus, one ladder becomes two
Original Copies
AATTCCGG AATTCCGG TTAAGGCC
TTAAGGCC TTAAGGCC + AATTGGCC
PCR is the replication of specific
bits of DNA and is done in a test tube. All the ingredients (DNA, subunits, DNA
polymerase, “primers”) are mixed together at different temperatures; initially
DNA is separated into two rows, primers bind to both rows and then DNA
polymerase adds each subunit to replicate the original DNA molecule.
The important stage in PCR is
binding of the “primers”; primers are very short bits of DNA with a specific
subunit sequence (e.g. ATCG and TAGC) designed to bind to a specific region in
the DNA to be copied. When two unique primers are used the region of DNA
between them will be copied.
ATCG GCTA
TCGGATAGCATTAAAAAAAAAAGGGGCCGTAAATTC
The PCR cycle is repeated a number
of times to generate lots of copies of the region. In this way repeats can be
copied in enough quantity to be seen; DNA of different lengths (e.g. AAAAAAAAAA
or AAAA) can be separated by size and visualised by running it through a solid
agarose gel.
Once a repeat (or group of repeats)
has been found to be present in sufferers the house to house searches can begin
in that area; the genes surrounding a repeat are sequenced. A gene is a
functional bit of DNA that generates a protein.
No comments:
Post a Comment