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Agarose Gel Electrophoresis
of DNA
DNA is driven through the agarose matrix by electric current. Smaller
or more compact molecules pass through the matrix easier and migrate
farther than large molecules. All DNA has the same charge per unit
length and linear pieces migrate according to size. The range of
sizes separated in a gel is controlled by the % of agarose in the gel.
Resolution Versus Matrix Concentration |
Agarose
% in 1x TBE |
Useful for Range of
Linear dsDNA Molecules (kb) |
0.3 |
5 - 60 |
0.6 |
1 - 20 |
0.7 |
0.8 - 10 |
0.9 |
0.5 - 0.7 |
1.2 |
0.4 - 6 |
1.5 |
0.2 - 3 |
2.0 |
0.1 - 2 |
*Information from Molecular Biology LabFax, ed. T.
A. Brown, Academic Press, 1991 |
The mobility is proportional to the voltage applied at low voltage
but increasing voltage decreases the resolution of larger fragments
of DNA. A general guideline for agarose gels in 1xTBE is 5V/cm
maximum for resolving fragment lengths greater than 2 kb. The
distance between the electrodes serves as the length in the calculation. Higher
voltages increase the temperature of the gel causing increased band
width and distortion of the lanes. The agarose can also melt,
especially the low melting point agarose sometimes used when DNA
is to be recovered from the gel. The mobility is also influenced
by the choice of buffer systems. Besides the Tris Borate EDTA,
pH 8.3 (TBE) buffer used in our experiments, a Tris Acetate EDTA
buffer (TAE) is preferred by some. The TAE buffer shifts the
range of resolution toward higher fragment lengths. |
 Ethidium
Bromide |
The nucleic acids are visualized with ethidium bromide (EtBr). This
fluorescent dye, which contains a tricyclic planar group, intercalates
between stacked base pairs of nucleotides and, in this environment, fluoresces
when excited with ultraviolet light; the fixed position of the planar
group and its close proximity to the bases causes dye bound to DNA to
display increased fluorescent yield compared to free dye.
- UV radiation at 254 nm is absorbed by DNA and transmitted to the
dye, whereas radiation at 302 and 366 nm is absorbed by the bound dye
- energy is re-emitted at 590 nm in the red-orange region of the visible
spectrum
NOTE: most commercial UV light sources emit at 302 nm, which yields
slightly less fluorescence than at 254 nm but produces LESS nicking of
DNA.
We will include EtBr in the gel only. In this case the dye extends the
length of linear and relaxed circular DNA by about 15% (the molecules are
more rigid which decreases their mobility). Supercoiled DNA is positively
supercoiled by ethidium bromide. Thus, the mobility of supercoiled DNA
with respect to linear and relaxed circular DNA varies with the concentration
of ethidium bromide present during the run.
If a DNA sample is too dilute to measure
at 260 nm or is contaminated with other compounds
that absorb in the UV range, the amount of DNA present can be estimated
from the intensity of ethidium bromide fluorescence. Since the amount
of DNA in a solution is proportional to the fluorescence emitted by
ethidium bromide, the DNA quantity in an "unknown" solution
can be estimated by comparing its level of fluorescence with the intensity
of known amounts of DNA.
The molecular weight DNA markers used in our study
are Quick-Load®
1 kb DNA ladder (New England Biolabs, Catalog
#N0468S). The ladder (see figure) produces eleven
DNA fragments ranging in size from 500 to 10,000 base pairs
(see picture).
This ladder can be used to quantitate the amount
of DNA in a sample; the mass of DNA in each band in the ladder
has been calibrated. |
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The size of linear fragments of DNA is determined by comparison
to standards: the log10 (# base pairs) is plotted versus
distance migrated or Rf value {Helling R.B., Goodman H.M., and
Boyer H.W. 1974. Analysis of endonuclease R-EcoRI fragments of
DNA from lambdoid bacteriophages and other viruses by agarose-gel
electrophoresis. J. Virol.14: 1235-1244}. |
The loading buffer (LB) combined
with the DNA samples contains two tracking dyes, bromophenol
blue and xylene cyanol, for
visually monitoring electrophoresis and glycerol to
make the sample dense enough sink to the bottom of the well; the
stock solution is designed to be diluted about six fold in the sample.
Bromophenol blue runs about the same size as a linear double-stranded
DNA molecule of 300 base pairs in length in 1X TBE on a gel of 1%
agarose. In low percentage gels of 0.4% agarose, the dye can emulate
a 1000bp fragment. Remember not to run this dye off the bottom of
the gel when you are trying to analyze small fragments. Xylene cyanol
runs about the same as a linear double-stranded DNA molecule of 4kb
in a 1% agarose gel. |
Agarose Gel Electrophoresis: Pouring and Running Gels

CAUTION
Agarose
can become superheated and violently boil over. Exercise
caution when heating. Swirl flask occasionally during
heating. Heat until close inspection reveals that the agarose
is 100% dissolved. Undissolved agarose will
appear as little flecks that look like Lilliputian
contact lenses.
HEALTH
HAZARD
Ethidium bromide (EtBr) is a powerful mutagen and a probable
carcinogen; it's moderately toxic and should be handled
with care. Wear
gloves when handling contaminated
equipment or solutions containing ethidium bromide. Confine
the compound to the restricted area. After
running the gels, use paper towels and plastic
wrap to protect equipment and surfaces from
being contaminated.
Dispose of the gels in the Biohazard Waste
Box. Do NOT put paper towels, plastic wrap,
or gloves in this waste box--ONLY the gels.

1. |
Prepare gel tray as diagrammed using an
adjustable gel
caster. Level the tray using a bubble level. |
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2. |
Flasks of melted agarose (0.8 or 1%) in 1X
TBE have been prepared and held
at 50°C. Carefully pour melted agarose
(40-60 ml)
into a beaker. Remember the solution is hot! |
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3. |
Let the instructor know when you are ready to add the
EtBr to your gel. After the instructor adds EtBr to
the gel, gently swirl the agarose.
EtBr stock solution = 0.625 mg/ml; we use 1 drop (~25 μl) EtBr/60 ml agarose so final concentration of EtBr = 0.3 μg/ml |
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4. |
Immediately, pour the melted agarose into the level casting
tray. Use a pipet tip to push bubbles towards the
bottom of the gel. |
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5. |
Let the tray cool until gel is translucent (takes
at least 20 minutes). CLEAN UP ANY DRIPS
ON THE BENCH AND RINSE THE BEAKER WITH RO-H2O! |
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6. |
Prepare DNA samples.
For agarose gels it is advisable to load the same volume into
each well. Some samples may need to be diluted with water or
TE to achieve this.
- Thaw DNA at room temperature and pulse spin samples
- Add 6X loading buffer III (6X LB) to
each sample for final concentration of 1X
NOTE: 6X LB contains
0.25% bromophenol blue
0.25% xylene cyanol FF
30% glycerol (in water)
*from Sambrook, J., Fritsch, E.F., and Maniatis, T. (1989) Molecular
Cloning: A Laboratory Manual, Second Edition (Cold Spring
Harbor Labortory Press)
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7. |
Turn lever on the caster to break the seal. Carefully
remove comb and place casting tray into the electrophoresis
unit for running. Fill unit with 1X TBE buffer to
~ 1 mm above gel. Pour
buffer carefully onto the center of the gel to prevent the
gel from sliding off the tray. |
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8. |
Carefully load ALL of each sample into
different wells in the gel and record the order of the samples
in your notebook (record the location of each sample,
not just the ones that you loaded). Do
not press the tip into the bottom of the well while loading--allow
the sample to sink there.
The instructor or TA will
load 10 µl of Quick-Load®
1 kb DNA ladder. |
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9. |
Position the lid and connect the electrodes so that
the anode is at the bottom of the gel (“run to
red”).
PLEASE
NOTE: Banana plug fittings are not to be turned or twisted.
Only push on and pull off by grasping the plug not the cord
(or the entire lid for the boxes) without turning. |
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10. |
Turn on the power supply (switch is located on the right side). Select “voltage” and
set the voltage to 130 V using the raise/lower arrow
keys; after the voltage is set, press the RIGHT SELECT button
until DISPLAY is lit to monitor the actual voltage (v) and
current (ma) during the run.
Press RUN. The
500 bp standard will run just behind the dark blue dye front,
and smaller fragments that run ahead of the dye may not be
visible in this type of analysis.
CAUTION: Lethal voltages are present while
the power supply is "ON." Do not touch the gel or
buffer until the power is off and electrodes are disconnected. |
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11. |
STOP the run after ~ 45 min. Once
the voltage is “zero” turn off the power supply,
disconnect the electrodes, and carefully remove the lid. |
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12. |
WEAR GLOVES. Drain excess buffer into the electrophoresis
unit and place casting tray with gel onto a paper towel
and carefully carry to the photography area. DO NOT
spread EtBr outside the designated area!! From this
point forward, assume that your gloves are contaminated with
EtBr--do not touch anything that is not supposed to be contaminated.
Place gel onto a sheet of plastic wrap on the transilluminator. CAUTION:
The gel is still laden with EtBr and should be handled only with
gloved hands. Scrupulously avoid all skin contact with the gel. Do
not remove the gel from the designated EtBr bench. A waste container
is provided there. |
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13. |
Photograph the gel and compare the observed bands
to the standards. The
instructor or TA will take pictures for you using a UVP BioDoc-It™ System (components
are listed below). |
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Gel Documentation
ATTENTION: Avoid
doing anything that would unintentionally contaminate
the transilluminator or anything else with EtBr. For
instance, do NOT lay gels directly on the transilluminator,
but always on plastic wrap. Do NOT contaminate the
equipment (knob, cover, table, etc.)--REMOVE
your gloves FIRST.
- Position the gel in the center of the UV transilluminator
and smooth out any wrinkles in the plastic wrap
- With the cover in place, turn on the transilluminator
- Capture an image of the gel with your Smart phone or camera
Dispose
of the gels in the Biohazard Waste Box.
Do NOT put paper towels, plastic wrap, or gloves in this
waste box--ONLY the gels.
Estimation of DNA Amounts
Two methods are widely used to estimate DNA concentrations.
Absorbance at 260 nm:At 260 nm, an absorbance (A) of 1 unit corresponds to a concentration of
- 50 μg/ml for dsDNA
- 40 μg/ml for RNA
- 33 μg/ml for ssDNA
- 20-30 µg/ml for oligonucleotides
Although this method is quick and nondestructive and gives information
about the purity of the sample (e.g., presence of protein or
organic contaminants), reliable estimates are obtained only
with concentrations of at least 1 μg/ml. Additionally, this
method cannot distinguish between DNA and RNA.
Intensity of Ethidium Bromide Fluorescence:
If a DNA sample is too dilute to measure at 260 nm or is
contaminated with other compounds that absorb in the UV range,
the amount of DNA can be estimated from the intensity of
ethidium bromide fluorescence. Since the amount of DNA in
a solution is proportional to the fluorescence emitted by
ethidium bromide, the DNA quantity in an "unknown" solution
can be estimated by comparing its level of fluorescence with
the intensity of known amounts of DNA of similar size. |