SDS-PAGE • Gel selection & preparation
SDS-PAGE gel preparation: gel choice, casting and quality checks
Start with the molecular-weight range that needs to be resolved. Choose the gel chemistry and pore range, use a validated manufacturer or laboratory recipe, and check physical quality before loading samples.
Why this matters
Choose the gel and check that it is ready to run
Gel preparation involves three linked issues: gel chemistry, separation range and physical quality. This page compares fixed and gradient gels, precast and hand-cast formats, and the checks to make before valuable samples are loaded.
Choose the gel for the target region
A percentage printed on a cassette is only meaningful together with the gel chemistry and molecular-weight region you need to resolve.
Target a narrow region or a broad range?
A suitable fixed-percentage gel can focus resolution on a narrower molecular-weight region. A gradient gel is useful when the experiment must retain a broad span of protein sizes on one gel.
Choose chemistry for the molecular-weight range
Traditional Tris-glycine is widely used; manufacturer systems also include Bis-Tris for broad-range separations, Tris-acetate formulations aimed at high-molecular-weight proteins and Tricine formulations that improve low-molecular-weight resolution. Use the separation chart for the exact product/system.
Do not translate a percentage range from one chemistry directly to another.
Precast or hand-cast?
Precast gels reduce casting variability and make product and lot documentation easier. Hand casting allows custom percentages and formats but adds polymerization, interface, well and batch-to-batch quality as experimental variables.
If hand-casting, use a validated recipe
Use the recipe for the exact acrylamide stock, crosslinker ratio, buffer system, gel volume and cassette format. Add polymerization initiators only when the setup is ready, and follow the reagent SDS and laboratory safety rules for unpolymerized acrylamide solutions.
How gel design affects separation
Fixed percentage
A homogeneous gel provides one pore-size regime. It is useful when the target region is known and the chosen chemistry provides sufficient separation around that region.
Gradient gel
Pore size decreases along the migration path, extending useful separation across a wider range and often sharpening zones as proteins encounter tighter sieving.
Stacking region
In a discontinuous Laemmli-style system, the stacking region narrows the starting zone before proteins enter the resolving gel. Damaged wells, poor stacking or an uneven interface can broaden samples early.
Resolving region
This is where size-dependent separation occurs. Gel chemistry, acrylamide concentration or gradient and running buffer operate as one system.
Hand-cast gel quality checks
1. Prepare the cassette and recipe before initiating polymerization
Do this
Clean and assemble plates/spacers, verify the casting seal and calculate the required volume from the validated recipe. Label the gel chemistry and intended percentage or gradient.
Check
The cassette is level, does not leak, and the reagent identities and concentrations match the recipe version being used.
Stop and fix when
The cassette leaks, the acrylamide stock/crosslinker ratio is uncertain, or the buffer/pH belongs to another gel system.
2. Cast the resolving region and create a clean interface
Do this
Prepare the resolving solution according to the validated recipe, add polymerization initiators at the specified step, mix without introducing bubbles and cast immediately.
Check
Polymerization completes as expected for that recipe and the top surface is flat enough to support an even stacking layer.
Stop and fix when
The gel stays soft or liquid, contains a large bubble in the sample path, pulls away from the plates or has a strongly sloped or irregular interface.
3. Cast the stacking region and form intact wells
Do this
Remove or rinse the resolving-gel overlay as appropriate for the recipe, cast the stacking solution and insert the comb without trapping bubbles at the tooth tips.
Check
The stacking region polymerizes and the comb can be removed without tearing the well bottoms or leaving obvious unpolymerized material.
Stop and fix when
Wells are torn, connected, bubble-filled or visibly different in depth across the gel.
4. Document or store the gel according to its chemistry
Do this
Record casting date, recipe or lot and any deviation. Use immediately or store only under conditions validated for that gel chemistry.
Check
Before loading, the gel remains hydrated, intact and free of obvious cracks, shrinkage, microbial contamination or damaged wells.
Stop and fix when
Storage history is unknown or the gel has changed physically enough that migration cannot be trusted.
Check these before loading
Common gel-preparation problems
Replace or recast a gel that is chemically or physically unsuitable before loading valuable samples.
Observation
The hand-cast gel does not polymerize as expected
Interpretation
Initiator activity, oxygen exposure, reagent identity/concentration, buffer chemistry or recipe execution is suspect.
Next action
Discard the failed gel, verify the recipe and fresh polymerization reagents, and repeat with documented reagent lots and conditions.
Avoid
Do not load valuable samples into a partially polymerized gel to see whether it works.
Observation
A bubble, crack or sloped interface intersects the target lanes
Interpretation
The physical path can create lane- or position-dependent migration artifacts.
Next action
Recast or replace the gel before loading experimental samples.
Avoid
Do not plan to correct a visible casting defect later during image analysis.
Observation
High-molecular-weight species are the main analytical target
Interpretation
A tight high-percentage gel can provide too much sieving in the region of interest.
Next action
Select a lower-percentage or appropriate gradient gel, or a chemistry validated for high-molecular-weight separation, using the manufacturer’s separation chart.
Avoid
Do not assume every low-percentage gel in a different chemistry gives equivalent migration.
Observation
Low-molecular-weight species must be separated from nearby small bands
Interpretation
The experiment needs stronger low-MW sieving and may benefit from a higher-percentage or low-MW-oriented chemistry such as a validated Tricine system.
Next action
Choose the gel from the product or system separation range and plan the endpoint so the low-MW region does not run off.
Avoid
Do not select a gel only because it worked for a much larger housekeeping protein.
Observation
Hand-cast gels vary noticeably from batch to batch
Interpretation
Casting or polymerization has become a major source of analytical variability.
Next action
Standardize recipe preparation, timing and acceptance checks; when reproducibility is more important than customization, compare a suitable precast system.
Avoid
Do not attribute every run-to-run shift to sample biology while gel preparation is changing.
Related SDS-PAGE guides
SDS-PAGE electrophoresis
Choose the gel and running system and check separation during the run.
Open →
SDS-PAGE troubleshooting
Use the ladder and lane pattern to distinguish casting, run and sample problems.
Open →
Complete Western blot protocol
Place gel preparation in the full sample-to-image workflow.
Open →
Experimental planning
Define target size, controls and a first pilot before using scarce samples.
Open →
Selection and methods references
Use these references for gel principles and system selection. For hand casting, final reagent volumes and polymerization conditions must come from the validated recipe actually used.
- Thermo Fisher — Overview of Electrophoresis
Explains polyacrylamide pore size, fixed-percentage versus gradient gels, discontinuous SDS-PAGE and representative buffer systems.
- Thermo Fisher — Precast Protein Gels
Compares Bis-Tris, Tris-glycine, Tris-acetate and Tricine systems and their intended applications.
- Laemmli, Nature (1970)
Classic paper for the widely used discontinuous SDS-PAGE approach.