Molecular Probes

Section 8.4 — Nucleic Acid Detection and Quantitation in Electrophoretic Gels and Capillaries

Nucleic Acid Detection in Gels

The new generation of fluorescent nucleic acid gel stains from Molecular Probes — the SYBR Gold, SYBR Green I and SYBR Green II dyes — are by far the best high-sensitivity reagents for staining DNA () and RNA () in electrophoretic gels. These gel stains provide greater sensitivity with lower background fluorescence than the conventional gel stain, ethidium bromide. In addition, the SYBR Safe DNA gel stain showed very low mutagenic activity when tested by an independent, licensed testing laboratory, and it is not classified as hazardous waste or as a pollutant under U.S. Federal regulations (Product Highlight: SYBR Safe DNA Gel Stain).

SYBR Gold Nucleic Acid Gel Stain

Our Patented SYBR Gold nucleic acid gel stain (S11494, SYBR(R) Gold Nucleic Acid Gel Stain) is simply the most sensitive stain available for detecting DNA or RNA in gels using a standard 300 nm UV transilluminator and Polaroid 667 black-and-white print film. Although the SYBR Green I and SYBR Green II gel stains are still the standard by which gel stains are judged and are preferred for specific applications, certain characteristics of the SYBR Gold stain represent a further improvement over our SYBR Green I and SYBR Green II gel stains for routine gel analysis. The SYBR Gold nucleic acid gel stain provides:

• Maximum sensitivity. Upon binding to nucleic acids, the SYBR Gold stain exhibits a >1000-fold fluorescence enhancement and a quantum yield of ~0.6. By comparison, ethidium bromide exhibits <30-fold fluorescence enhancement upon binding nucleic acids and a quantum yield of ~0.15. Because of its superior fluorescence characteristics, the SYBR Gold stain is greater than 10-fold more sensitive than ethidium bromide for detecting DNA and RNA in gels using a 300 nm UV transilluminator and black-and-white photography. We routinely detect as little as 25 pg of dsDNA or 1 ng of RNA per band using a 300 nm UV transilluminator or a blue-light transilluminator — sensitivity levels even higher than those of silver staining. For detecting glyoxalated RNA with 300 nm transillumination, the SYBR Gold stain is 25–100 times more sensitive than ethidium bromide (Figure 8.70) and thus represents a significant advance for protocols requiring sensitive RNA detection. The SYBR Gold stain has also proven to be more sensitive than our SYBR Green II stain for detecting single-strand conformation polymorphism (SSCP) products.
• Rapid gel penetration. Staining gels with the SYBR Gold stain after electrophoresis followed by gel photography provides the optimal sensitivity. The SYBR Gold stain penetrates agarose gels faster, and stains thick and high-percentage gels better than any other post-electrophoresis stain.
• Versatility. The SYBR Gold stain is a universal nucleic acid gel stain that provides the highest sensitivity for dsDNA, ssDNA and RNA detection in many gel types, including high-percentage agarose, glyoxal/agarose, formaldehyde/agarose, native polyacrylamide– and urea–polyacrylamide gels. No wash step is required in order to achieve maximal sensitivities.
• Ease of use. As a result of the low intrinsic fluorescence of the unbound dye, gel staining with the SYBR Gold dye shows extremely low background fluorescence and does not require a destaining step, even when staining agarose/formaldehyde gels. After incubating the gel in the SYBR Gold staining solution for 10–40 minutes (depending on the thickness and percentage of the agarose or polyacrylamide gel) the golden-yellow–fluorescent DNA or RNA bands are ready to be photographed.
• Economy. The SYBR Gold stain is at least as sensitive as silver staining for detecting dsDNA in polyacrylamide gels (Figure 8.71). Also, the SYBR Gold stain is very competitively priced, making it an affordable alternative to labor-intensive silver staining.
• Compatibility with other molecular biology techniques. The presence of unbound SYBR Gold dye in stained gels at standard staining concentrations does not interfere with restriction endonuclease or ligase activity or with subsequent PCR reactions. SYBR Gold nucleic acid staining is compatible with both Northern and Southern blotting — the stain transfers with the DNA or RNA to the blot and is washed off during incubation in the prehybridization mix. The SYBR Gold stain is also easily removed from dsDNA by simple ethanol precipitation, leaving templates ready for subsequent manipulation or analysis.
• Instrument compatibility. Because the nucleic acid–bound SYBR Gold dye exhibits excitation maxima at both ~495 nm and ~300 nm (the emission maximum is ~537 nm) (Figure 8.72), it is compatible with a wide variety of instrumentation, ranging from UV epi- and transilluminators and blue-light transilluminators, to mercury-arc lamp and argon-ion laser–based gel scanners. Short-wavelength (254 nm) epi-illumination is not required to obtain high sensitivity with the SYBR Gold stain. For optimal sensitivity with black-and-white print film and UV illumination, SYBR Gold dye–stained gels should be photographed through the SYBR photographic filter (S7569, Photographic Filters for Fluorescent Dye–Stained Gels and Blots).

The SYBR Gold dye should prove invaluable in applications such as agarose/formaldehyde gel electrophoresis prior to Northern blot analysis, denaturing gradient gel electrophoresis (DGGE), single-strand conformation polymorphism (SSCP) studies (), as well as routine gel analysis. The high signals of the dye–DNA complex and the remarkable photostability observed with 300 nm transilluminators will make it easier to cut out low-abundance bands for subsequent manipulation, including subcloning, bandshift assays or other dsDNA template–based reactions.

SYBR Gold is the dye of choice for methylation-sensitive single-strand conformation analysis (MS-SSCA). It is also reportedly more sensitive and easier to use than silver staining in the telomeric repeat amplification protocol (TRAP). In the electrophoretic analysis of DNA forms in liposomes, the dye showed 40-fold greater sensitivity and more consistent staining between different isoforms of DNA compared with ethidium bromide. It has also been used to monitor the formation of crosslinked peptide–DNA complexes.

Each 500 µL vial of the SYBR Gold nucleic acid gel stain (S11494) contains sufficient reagent to stain at least 100 agarose or polyacrylamide minigels. The SYBR Gold nucleic acid stain is accompanied by detailed instructions for use of the dye in staining nucleic acids (SYBR(R) Gold Nucleic Acid Gel Stain); answers to frequently asked questions about all our SYBR dyes are available in a separate information sheet (SYBR(R) Nucleic Acid Gel Stains — Useful Tips).

SYBR Green I Nucleic Acid Gel Stain

Molecular Probes' Patented SYBR Green I nucleic acid gel stain (S7563, S7567, S7585; SYBR(R) Green I Nucleic Acid Gel Stain) is an extremely sensitive fluorescent stain for detecting nucleic acids in agarose and polyacrylamide gels. As with the SYBR Gold stain, this remarkable sensitivity can be attributed to a combination of unique dye characteristics. Our SYBR Green I stain exhibits exceptional affinity for DNA and a large fluorescence enhancement upon binding to DNA — at least an order of magnitude greater than that of ethidium bromide when detected by photography. Also, the fluorescence quantum yield of the SYBR Green I dye–DNA complex (~0.8) is over five times greater than that of the ethidium complex of DNA (~0.15). Furthermore, the SYBR Green I stain has been shown to be significantly less mutagenic than EtBr by Ames testing. The SYBR Green I stain is somewhat less sensitive than our SYBR Gold stain, but has some important characteristics that make it the preferred reagent for certain applications:

• Preferential DNA staining. The SYBR Green I nucleic acid stain has a much greater fluorescence enhancement when bound to dsDNA and oligonucleotides than when bound to RNA. With a standard 300 nm UV transilluminator and photographic detection, as little as 60 pg dsDNA per band can be detected with the SYBR Green I stain (Figure 8.74), whereas the SYBR Green I stain is not much more sensitive than ethidium bromide for staining RNA. This quality makes the SYBR Green I dye the ideal gel stain for applications in which RNA in the sample may obscure the results, such as when visualizing DNA fragmentation ladders from apoptotic cells (). Fluorescence of nucleic acid–bound SYBR Green I dye is also of sufficient sensitivity to allow detection and discrimination of viruses by flow cytometry.
• Sensitivity for oligonucleotide detection. We have determined that the SYBR Green I nucleic acid gel stain is nearly two orders of magnitude more sensitive than ethidium bromide for staining oligonucleotides in gels, provided that the gel is photographed according to instructions (SYBR(R) Green I Nucleic Acid Gel Stain). Using 254 nm epi-illumination, it is possible to detect 1–2 ng of a synthetic 24-mer on 5% polyacrylamide gels (Figure 8.76).
• Exceptionally low background. The SYBR Green I stain shows very low background fluorescence in the gel, making it the preferred dye for some laser-scanning instruments, where background fluorescence can produce unacceptable noise levels.
• Spectral compatibility with lasers and filter sets. The SYBR Green I stain has a UV-excitation peak of ~250 nm (Figure 8.77). Thus, higher sensitivity can be achieved with the SYBR Green I stain using 254 nm transillumination, as compared with the more common 300 nm transillumination. However, the visible excitation peak of SYBR Green I dye–stained nucleic acids near 497 nm is very close to the principal emission lines of many laser-scanning instruments. Because nucleic acid–bound SYBR Green I dye exhibits spectral characteristics (excitation/emission maxima ~497/520 nm) very similar to those of fluorescein (), it is compatible with the most common filter sets used in laser scanners.

Like the SYBR Gold stain, the SYBR Green I dye is very easy to use — the staining procedure can be completed in 10–40 minutes (somewhat longer for thicker gels), with no destaining step required prior to photography. Presence of typical staining concentrations of the SYBR Green I dye does not significantly inhibit the ability of several restriction endonucleases to cleave DNA. This property makes staining with the SYBR Green I dye potentially compatible with in-gel subcloning protocols. The SYBR Green I stain is also easily removed from dsDNA by simple ethanol precipitation. Answers to frequently asked questions about all our SYBR dyes are available in a separate information sheet (SYBR(R) Nucleic Acid Gel Stains — Useful Tips). For optimal sensitivity with black-and-white print film and UV illumination, gels stained with the SYBR Green dye should be photographed through the SYBR photographic filter (S7569, Photographic Filters for Fluorescent Dye–Stained Gels and Blots). Very good sensitivity is also achieved using a CCD camera.

The ultrasensitivity of the SYBR Green I dye makes it useful for detecting the products of DNA and RNA amplification reactions by gel electrophoresis, restriction mapping small amounts of DNA and detecting the products of bandshift and nuclease-protection assays. At least two independent laboratories found that use of the SYBR Green I stain allows the detection of DNA amplification products from low–target number PCR. Amplification products that are at the limit of detection using ethidium bromide are easily detected using the SYBR Green I dye. Reverse-transcription PCR (RT-PCR) reaction products have been detected with high sensitivity following gel electrophoresis and staining with the SYBR Green I dye, allowing the cycle number to be lowered, which reduces heteroduplex formation during amplification. The SYBR Green I stain was used to detect RT-PCR products amplified from B cells, Xenopus laevis embryos and smooth muscle cells. Using a laser scanner and the SYBR Green I stain, researchers have developed a high-throughput RT-PCR DNA profiling assay in multiwell agarose gels. The SYBR Green I dye was used to stain DNA in high-resolution gels capable of resolving 100–200 base-pair DNA fragments and differing by as few as two base pairs.

In other gel-based techniques, the SYBR Green I nucleic acid gel stain has enabled researchers to eliminate silver staining and frequently even radioactivity from their protocols. SYBR Green I dye staining was shown to be as sensitive as silver staining — as well as being more rapid, less laborious and less expensive — in a nonradioactive method for detecting hypervariable simple sequence repeats in electrophoretic gels. The SYBR Green I dye staining has completely replaced conventional silver staining techniques for routine identity testing in some forensics laboratories. In addition, the SYBR Green I stain is as sensitive as silver staining, but less expensive, for detecting STR (short tandem repeat) polymorphisms, and mitochondrial DNA deletions.

Use of the SYBR Green I stain also eliminates the need to label PCR products with radioisotopes in a kinetic PCR assay. Likewise, in a gel assay for detection of telomerase activity (telomeric repeat amplification protocol or TRAP) in human cells and tumors, SYBR Green I dye staining was found to be more sensitive than silver staining and gave results comparable to those achieved with a radioisotope-based TRAP assay. Moreover, unlike silver stains, the SYBR Green I stain does not label proteins carried over from the reaction mixture. SYBR Green I dye staining has also been shown to be as sensitive as ³H-labeled thymidine for detecting double-strand breaks in mammalian cells. It may be possible to further increase the sensitivity of some of these reported applications of the SYBR Green I stain by using the SYBR Gold nucleic acid gel stain (see above).

The high chemical stability of the SYBR Green I nucleic acid stain and the dye's selective sensitivity for detecting double-stranded products made in the presence of single-stranded oligonucleotide primers make the SYBR Green I stain the preferred dye for real-time quantitative analysis of PCR products in a solution assay (Section 8.3).

Each milliliter of our concentrated SYBR Green I nucleic acid gel stain (S7563, S7567, S7585) contains a sufficient amount of the reagent to stain at least 100 agarose or polyacrylamide minigels. Reuse of the staining solution can significantly increase the number of gels stained per vial. In some applications, such as preparative agarose gel electrophoresis, the amount of SYBR Green I dye used per gel can also be significantly reduced if the dye is added directly to the loading buffer. However, because the dye affects DNA mobility and dissociates from the smaller DNA fragments during electrophoresis, this method should not be used for size determination or for DNA fragments less than ~100 base pairs in length.

SYBR Green II RNA Gel Stain

Our Patented SYBR Green II RNA gel stain (S7564, S7568, S7586; SYBR(R) Green II RNA Gel Stain) is a highly sensitive dye for detecting RNA or ssDNA in agarose or polyacrylamide gels (Figure 8.78). Some outstanding features of the SYBR Green II RNA gel stain include its high binding affinity for RNA and its large fluorescence enhancement and exceptionally high quantum yield upon binding to RNA. Although it is not a specific stain for RNA, SYBR Green II dye exhibits a larger fluorescence quantum yield when bound to RNA (~0.54) than to dsDNA (~0.36). This property is unusual among nucleic acid stains; most show far greater quantum yields and fluorescence enhancements when bound to double-stranded nucleic acids. Moreover, the fluorescence quantum yield of the SYBR Green II complex of RNA is over seven times greater than that of the ethidium bromide–RNA complex (~0.07). The affinity of SYBR Green II RNA gel stain for RNA is also higher than that of ethidium bromide, and its fluorescence enhancement upon binding to RNA is well over an order of magnitude greater. Like the SYBR Green I stain, our SYBR Green II stain gives the greatest sensitivity on 254 nm transillumination or laser scanners. However, the best sensitivity for RNA detection using 300 nm transillumination is achieved with the SYBR Gold dye (see above). Other important properties of SYBR Green II RNA gel stain include:

• Sensitivity. Using 254 nm epi-illumination, Polaroid 667 black-and-white print film and a SYBR photographic filter (S7569, Photographic Filters for Fluorescent Dye–Stained Gels and Blots), we have been able to detect as little as 100 pg of ribosomal RNA (rRNA) per band on native 1% agarose gels and <1 ng rRNA per band on 5% polyacrylamide gels stained with SYBR Green II RNA gel stain. The detection limit of SYBR Green II dye–stained native gels excited with 300 nm transillumination is approximately 500 pg per band, as compared with about 1.5 ng for ethidium bromide–stained gels (Figure 8.78).
• Ease of use. Like the SYBR Green I and SYBR Gold stains, SYBR Green II RNA gel stain has a very low intrinsic fluorescence, eliminating the need to destain gels.
• Compatibility with urea and formaldehyde gels. Fluorescence of the SYBR Green II dye–RNA complexes does not appear to be quenched in the presence of urea or formaldehyde, so that denaturing gels do not have to be washed free of the denaturant prior to staining.
• Broad linear dynamic range. When used on a laser scanner, the SYBR Green II stain shows a dynamic range of over five orders of magnitude — far greater than the linear dynamic range of ethidium bromide — allowing more accurate quantitation of bands in the gel.
• Compatibility with Northern blots. Research at Molecular Probes has shown that SYBR Green II dye staining is also compatible with agarose/formaldehyde gels. The formaldehyde does not have to be removed prior to staining, and the sensitivity of SYBR Green II dye staining is 5–10 times better than that of ethidium bromide on these gels. In addition, staining agarose/formaldehyde gels with the SYBR Green II dye does not interfere with transfer of the RNA to filters or subsequent hybridization in Northern blot analysis, provided that 0.1% to 0.3% SDS is included in the prehybridization and hybridization buffers. Thus, SYBR Green II stain can be used to normalize the hybridization signal to the amount of RNA loaded on the gel.

SYBR Green II RNA gel stain facilitates the detection of viroid RNAs and other multicopy cellular RNA species. This gel stain has been used to visualize the migration behavior of 5S rRNA species after electrophoresis through a denaturing gradient gel, a method that was used to discriminate among different acidophile species in a mixed culture. SYBR Green II RNA gel stain should also improve the analysis of small aliquots from an RNA preparation, leaving the researcher with more material to carry out the primary experiment, be it Northern blotting, start-site mapping or cDNA preparation.

In addition to its use for detecting RNA, the SYBR Green II RNA gel stain is useful for single-strand conformation polymorphism (SSCP) analysis, which demands extremely sensitive detection techniques. Many of the nonradioisotopic SSCP methods currently in use, such as silver staining or chemiluminescence-mediated signal amplification, require long, complex procedures. An SSCP assay using precast polyacrylamide minigels and the SYBR Green II stain not only provides the precise temperature control required for the assay, but it is more rapid, less expensive and less labor-intensive than assays that use silver staining for detection. In another report, the SYBR Green II RNA gel stain was used to detect Ki-ras mutants by SSCP analysis and was reported to yield 10-fold better sensitivity than standard silver-staining techniques. The SYBR Green II stain is compatible with amplification by PCR — after SSCP analysis, the SYBR Green II dye–stained bands can be excised out of the gel and used in cycle-sequencing. The SYBR Green II nucleic acid stain also provides high-sensitivity staining for rRNA separated by high-resolution denaturing gradient electrophoresis (DGGE), making it possible to discriminate between closely related species of bacteria.

SYBR Green Nucleic Acid Gel Stains: Special Packaging and a Starter Kit

In addition to providing the SYBR Green nucleic acid gel stains packaged as 500 µL or 1 mL stock solutions in DMSO (S7563, S7564, S7567, S7568), we make both SYBR Green I and SYBR Green II available as a set of 20 individual vials, each containing 50 µL of the DMSO stock solution (S7585, S7586). This convenient packaging makes it easy to supply members of the laboratory with an aliquot of stock solution, or to share stock with other laboratories. Special packaging also minimizes potential losses due to contamination, spills and light exposure. Each milliliter of the concentrated gel stain provides sufficient reagent to prepare 10 liters of a staining solution. Although best results are obtained with freshly diluted dye, properly prepared staining solution can be stored for up to a week, if kept refrigerated and protected from light, and can be reused 2–3 times with little loss of signal. The SYBR Green nucleic acid stains are accompanied by detailed instructions for their use in staining gels (SYBR(R) Green I Nucleic Acid Gel Stain, SYBR(R) Green II RNA Gel Stain); answers to frequently asked questions about all the SYBR dyes are available in a separate information sheet (SYBR(R) Nucleic Acid Gel Stains — Useful Tips).

Our SYBR Green Nucleic Acid Gel Stain Starter Kit (S7580) is designed for laboratories that want to sample these products. The kit includes single 50 µL vials of both the SYBR Green I and SYBR Green II stains and a SYBR gel stain photographic filter, along with complete directions for their use (SYBR(R) Green I Nucleic Acid Gel Stain, SYBR(R) Green II RNA Gel Stain).

Our SYBR dyes are covered by Patents issued to Molecular Probes. These products are offered for research purposes only. Molecular Probes welcomes inquiries about Licensing these products for resale or other commercial uses, including fee-for-service activities.

SYBR Safe DNA Gel Stain

The SYBR Safe DNA gel stain provides sensitive DNA detection with significantly reduced mutagenicity, making it safer than ethidium bromide for staining DNA in agarose or acrylamide gels. Not only is the SYBR Safe stain less mutagenic than ethidium bromide, but the detection sensitivity of the SYBR Safe gel stain is comparable to that of ethidium bromide and 400 times greater than that of colorimetric stains for detecting DNA in electrophoretic gels. The SYBR Safe stain is provided as a premixed solution that can directly replace ethidium bromide in standard protocols; the SYBR Safe stain can either be cast into the gel or be used as a post-electrophoresis stain. DNA bands stained with the SYBR Safe DNA gel stain can be detected using a standard UV transilluminator, a visible-light transilluminator or a laser scanner. The SYBR Safe stain is also suitable for detecting RNA in gels. Bound to nucleic acids, the SYBR Safe stain has fluorescence excitation maxima at 280 and 502 nm, and an emission maximum at 530 nm (Figure 8.79). The SYBR Safe DNA gel stain offers:

• Increased safety. The SYBR Safe DNA gel stain has tested negative in three mammalian cell–based assays for genotoxicity, is less mutagenic than ethidium bromide in standard Ames tests and is not classified as hazardous waste under U.S. Federal regulations (Product Highlight: SYBR Safe DNA Gel Stain).
• Better performance. The SYBR Safe DNA gel stain is as sensitive as ethidium bromide and 400 times as sensitive as colorimetric stains for detecting DNA in electrophoretic gels.
• Convenience. The SYBR Safe stain is provided as a ready-to-use solution in 0.5X TBE or 1X TAE; it can be cast directly in the gel or used as a post-electrophoresis stain ().
• A quick staining protocol. Simply incubate the gel in staining solution for 30 minutes; no destaining is required.

Furthermore, Invitrogen scientists have demonstrated a vast improvement in cloning efficiency with DNA fragments isolated from agarose gels using the SYBR Safe stain and blue light versus the same DNA fragments isolated using ethidium bromide and UV light. In the experiment, a 1.25 kilobase gene was amplified by PCR. Seven equal amounts of the PCR product were electrophoresed on duplicate agarose gels; one gel was visualized with SYBR Safe stain and blue-light illumination, while the other gel was visualized with ethidium bromide and UV illumination. Bands were excised after defined exposure times, then purified using Invitrogen's PureLink Gel Extraction Kit. The purified product was then used in a Gateway BxP cloning reaction. A portion of each reaction product was transformed into OneShot TOP10 chemically competent bacteria; three serial dilutions were plated, and colonies were counted using an Alpha Innotech imaging system. The results showed an 80% reduction in transformation efficiency after only 30 seconds of exposure to ethidium bromide and UV light; after only 2 minutes of exposure, the number of transformants was nearly zero. In contrast, the transformation efficiency attained using SYBR Safe stain and blue light remained at virtually 100% of the control value throughout the entire 16-minute time course of the experiment.

The SYBR Safe DNA gel stain is supplied ready-to-use in two different sizes and in two different buffers. The 1 L unit size in 0.5X TBE or 1X TAE (S33100, S33111) provides sufficient reagent to stain ~20 minigels; the 4 L unit size in 0.5X TBE or 1X TAE (S33101, S33112) provides sufficient reagent to stain ~80 minigels and is supplied in a cube-shaped container with a removable spigot. We also offer a 400 µL volume of 10,000X concentrate (S33102). The SYBR Safe DNA Gel Stain Starter Kit (S33110) is a convenient packaging of the 1 L unit size of SYBR Safe stain in 0.5X TBE plus one SYBR Safe photographic filter (S37100, Photographic Filters for Fluorescent Dye–Stained Gels and Blots).

Ethidium Bromide

Ethidium bromide (EtBr, E1305, E3565, ) is the most commonly used dye for DNA and RNA detection in gels. It binds to single-, double- and triple-stranded DNA. Ethidium bromide has also been used to detect protein–DNA complex formation in bandshift assays and to observe single DNA molecules undergoing gel electrophoresis. In addition to the solid form (E1305), we supply ethidium bromide in a 10 mg/mL concentrated stock solution (E3565) for those wishing to avoid contact with the mutagenic powder.

Cyanine Monomers for Staining DNA in Electrophoretic Gels

Although the SYBR dyes are now the preferred gel stains, at least six of the monomeric cyanine dyes — TO-PRO-1, YO-PRO-1, BO-PRO-1, PO-PRO-1, JO-PRO-1 and LO-PRO-1 (Section 8.1, Table 8.2, Monomeric Cyanine Nucleic Acid Stains) — are also sensitive reagents for staining gels after electrophoresis and are compatible with UV trans- or epi-illumination or with laser-excited gel scanners. Their range of absorption maxima may make them superior to the SYBR dyes when using some lasers as excitation sources. We have determined that the limit of detection of dsDNA with some of these dyes is about 60 pg/band, using 254 nm epi-illumination and Polaroid 667 black-and-white print film photography.

Our TO-PRO-3 dye (T3605) can detect less than 0.1 ng/band DNA in an ultrathin-layer agarose gel–based electrophoretogram when excited by an inexpensive 640 nm red diode laser. Preloading of the gel buffer with the TO-PRO-3 dye has been recommended for this application when analyzing migrating allele-specific PCR fragments.

Cyanine and Ethidium Dimers for Staining DNA Prior to Electrophoresis

The extraordinary stability of our dimeric cyanine– and ethidium homodimer–nucleic acid complexes (Table 8.2) ensures that the dye–DNA association remains stable during electrophoresis. Thus, samples can be prestained with subsaturating nanomolar dye concentrations before electrophoresis (), thereby reducing the hazards inherent in handling large volumes of ethidium bromide staining solutions. The fluorescence intensities of both the EthD-1–DNA and TOTO-1–DNA complexes are directly proportional to the amount of DNA in a band; however, TOTO-1 dye staining has less effect on the electrophoretic mobility of DNA fragments than does EthD-1. Furthermore, unlike EthD-1–labeled DNA, in which up to two-thirds of the bound dye can be transferred to excess unlabeled DNA, the extent of transfer of TOTO-1 dye to unlabeled DNA is reported to be only about 15–20%, even when the TOTO-1–DNA complexes are incubated for up to 10 hours with a 100-fold excess of uncomplexed dsDNA. This property is valuable for multiplexed electrophoretic separations, particularly since our cyanine nucleic acid stains are available in so many visually distinct colors. If two DNA populations are stained with spectrally distinct cyanine dimer dyes and run in the same lane, simultaneous two-color detection can potentially eliminate errors caused by lane-to-lane variations in electrophoretic mobility. Binding of the TOTO-1 dye (T3600), YOYO-1 dye (Y3601) and ethidium homodimer-1 (E1169) to DNA initially results in inhomogeneous binding that yields double bands in DNA gel electrophoresis. These double bands can be avoided by incubating complexes for times long enough to allow binding to come to equilibrium or by heating samples to 50°C for at least two hours. Binding of our other dimeric nucleic acid stains (Table 8.2, Dimeric Cyanine Nucleic Acid Stains) does not seem to give this problem.

An extremely sensitive confocal laser–based gel scanner has been exploited in multiplexed electrophoretic separations to detect as little as four picograms per band of TOTO-1 dye– and YOYO-1 dye–stained dsDNA; although sophisticated equipment is required for achieving these low detection limits, such equipment is not essential for detecting somewhat larger quantities of these nucleic acid–dye complexes. Our TOTO-1 dye has been used to label DNA prior to electrophoresis in order to detect cystic fibrosis mutant alleles with a laser-excited fluorescence gel scanner, as well as to detect DNA amplification products on agarose gels with standard UV transillumination. The TOTO-1 dye has also been used to label nine DNA fragments of the dystrophin gene that were simultaneously generated using the polymerase chain reaction. The resolution obtained by gel electrophoresis of these labeled fragments compared favorably to that observed using fluorophore-labeled primers. The TOTO-3 and POPO-1 dyes (T3604, P3580; Section 8.1) have been similarly used to analyze DNA with a xenon lamp–based luminescence analyzer.

Ethidium homodimer-1 (EthD-1, E1169) has been used for fluorescence detection of 30–60 picograms DNA per band on polyacrylamide gels using a confocal laser–based scanning system. Ethidium homodimer-2 (EthD-2, E3599;