Gel
electrophoresis
separates DNA
fragments by size in a solid support medium (an agarose gel).
DNA
samples are pipetted into the sample wells, seen as dark slots at
the
top of the
picture.
Application of an electric current at the top (anodal,
negative)
end causes the negatively-charged
DNA [remember it's an acid] to migrate
(electrophorese) towards the bottom (cathodal, positive) end. The
rate
of
migration
is proportional to size: smaller fragments move more quickly, and
wind
up at the bottom of the gel.
DNA is visualized by including
in the gel an intercalating dye, ethidium
bromide. DNA fragments take up the
dye as they migrate through the gel.
Illumination with ultraviolet light causes the
intercalated dye to fluoresce with a pale pink colour.
Note that the larger fragments fluoresce more
intensely. Although each of the fragments of a single class of
molecule are present in equimolar
proportions, the smaller
fragments
include less mass of DNA,
take up less dye, and
therefore fluoresce less intensely. This is most evident in the
lane at
the
extreme
right, which shows a "ladder" set of DNA fragments of known size
that
can be
used to estimate the sizes of the other unknown fragments.
