Syllabus
Lab Resources
Pre-lab Preparation
Additional Resources
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BIOC 111 Day 2
Agarose Gel Analysis
of Plasmid DNA
OWL-Space Resources
Introduction
DNA work will continue with the following activities.
Today you will use analyze the plasmid DNA that you prepared
last time using agarose gel electrophoresis
Background
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. |
Experimental overview
The procedures for today involve gel analysis. On some
procedures you will work as an individual; on others
you will work as a team. Make
sure that you use the appropriate pipettor and set the
volume correctly—if you’re unsure, then
ask. Record all procedures and data in your
lab notebook, indicating “who” performed
a procedure step when you work as pairs; turn in copies
of notebook pages at the end of the laboratory session.
- Agarose gel electrophoresis of DNA (two teams will
share a gel)
SPECIAL NOTE: Record enough procedure details in your
notebook during lab today so that you can repeat these procedures using
your notebook as the ONLY resource. Write the methods in your
own words (i.e., do not just “copy” the steps
from the web page or handouts).
Agarose gel electrophoresis of DNA [0.8% agarose
in 1X Tris-borate-EDTA (TBE)]
PROTOCOL
- Thaw uncut control, single digest, and one of the
double digests at room temperature and
pulse spin samples
- Add 5 µl 6X loading buffer III (6X
LB) to each sample
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)]
- The
instructor or TA will load 10 µl of Quick-Load
1 kb DNA ladder (New
England Biolabs)
- Carefully load ALL of each sample
(30 µl) 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.
- 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 (or the entire lid for
the boxes) without turning.
- 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 electrodes
are disconnected.
- STOP the run after 45-50 min. Once
the voltage is “zero” turn off the power
supply, disconnect the electrodes, and carefully remove
the lid.
HEALTH HAZARD
- Wear gloves and place casting tray with gel
onto paper towels 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 with those gloves
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.
- The instructor or TA will take pictures
for you using the UVP BioDoc-It™ System (components
are listed below).
- 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.
ATTENTION: Avoid doing anything that would unintentionally
contaminate the transilluminator or camera with EtBr. For instance,
do NOT lay gels directly on the transilluminator, but always on plastic
wrap. Do NOT contaminate the equipment (door knob, camera, printer,
etc.)--REMOVE your gloves FIRST.
Gel Documentation with a UVP BioDoc-It™ System
(Ultra-Violet Products Ltd, Upland, CA)
System Components
- CCD Video Camera
- Zoom Lens
- UV Blocking Filter--the orange-colored filter absorbs UV and IR radiation
from the transilluminator and enhances the orange/pink bands of ethidium
bromide stained gels
- Transilluminator (302 nm)
- Darkroom Cabinet
- LCD Monitor
- Thermal Printer
Homework Assignments
Prepare item 1 of the homework assignments
in your laboratory notebook and turn in the duplicates
at the beginning of the next laboratory session. Complete
item 2 on Owlspace.
1. Analyze an agarose gel (attach either an enlarged
copy or the original picture of the gel to the homework pages)
Identify the restriction enzymes you used for the single and
double digests. Use the plasmid map and the picture of
your gel from today to obtain the following information:
- Look at the location of the digested DNA fragments
on the picture of the gel. What are the apparent
sizes of these bands for the single digest and for
the double digest in comparison to the standards on
the gel?
- Watch the class video in OWL-Space about "Agarose
Gel Analysis." Use the picture
of your gel from day 2 to hand
draw a DNA standard curve in your notebook. A
computer-generated plot is not appropriate for this
application. Plot log10 (# base
pairs) versus distance migrated from the well (i.e.,
top of the gel) and use the
graph to estimate the sizes of the restriction fragments
for each digest reaction. Construct
a table for the standards containing #bp, log10(#bp),
and distance migrated (cm or mm); construct a second
table that contains the restriction enzyme(s) used
in each digest reaction and the distance migrated for
each resulting DNA fragment. Show
all measurements and calculations. Do
these sizes agree with the apparent sizes that you
observed on the gel?
- Compare the experimental results (from analysis
of agarose gel) with the expected sizes (as
predicted from plasmid map). Do they agree?
If not, suggest possible reasons for the discrepancy.
2. Materials and Methods section from a published
research article (Literature Example): Look
up a published research article that uses molecular
biology techniques similar to those you have used in
this lab course.
- Bring a
copy of the FIRST page of that article and the Materials
and Methods section to class with you
- In the methods
section, circle or highlight methods that are
similar to the ones that you used in this lab course
Here
are some suggested journals to search: Protein
Expression and Purification, Journal of Molecular Biology,
Journal of Biological Chemistry, Proceedings of the
National Academy of Sciences.
3. Complete and submit the graphing quiz
Complete and submit the graphing quiz before your lab
day 3. It is a timed quiz (1 hour), so be prepared. You
are welcome to review the tutorial and the pre-quiz as
many times as you like before taking
the timed quiz. Do not "begin" if you are not ready to
complete the quiz--your work will be submitted automatically
after 1 hour (you can't "start" it and come back to it
later). Once you start the quiz you are to answer
the questions from memory without consulting either resource.
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