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Agarose vs SDS-PAGE Gel Comparison Cheat Sheet
A printable reference covering agarose gels, SDS-PAGE, molecule separation, gel matrix, charge, size, and staining for grades 9-12.
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Agarose gel electrophoresis and SDS-PAGE are lab methods used to separate biological molecules in an electric field. This cheat sheet compares what each gel is used for, how the molecules move, and how results are interpreted. Students need this comparison because DNA, RNA, and proteins behave differently during electrophoresis. Knowing the differences helps you choose the correct gel and read band patterns accurately. Agarose gels usually separate DNA or RNA fragments by size because nucleic acids already have a negative charge. SDS-PAGE separates proteins mostly by size because SDS coats proteins with a uniform negative charge and helps unfold them. Smaller molecules move farther through both gel types, but the gel material, buffer, sample preparation, and staining methods differ. The most important idea is that gel electrophoresis uses a porous matrix and an electric field to sort molecules based on movement through the gel.
Key Facts
- Agarose gel electrophoresis is most often used to separate DNA or RNA fragments by length in base pairs.
- SDS-PAGE is most often used to separate proteins by size in kilodaltons after SDS gives them a mostly uniform negative charge.
- In electrophoresis, negatively charged molecules move toward the positive electrode, also called the anode.
- Smaller DNA fragments travel farther through an agarose gel than larger DNA fragments during the same run time.
- Smaller proteins usually travel farther through an SDS-PAGE gel than larger proteins because they pass more easily through the polyacrylamide matrix.
- Agarose concentration affects DNA separation, where higher percent agarose better separates smaller DNA fragments and lower percent agarose better separates larger fragments.
- Polyacrylamide gels have smaller, more uniform pores than agarose gels, making them better for resolving proteins and small biomolecules.
- A DNA ladder or protein marker is run beside samples so unknown band sizes can be estimated by comparison.
Vocabulary
- Agarose gel
- A porous gel made from agarose that is commonly used to separate DNA or RNA fragments by size.
- SDS-PAGE
- A protein separation method that uses sodium dodecyl sulfate and a polyacrylamide gel to separate proteins mainly by size.
- Electrophoresis
- A laboratory technique that moves charged molecules through a gel using an electric field.
- Molecular marker
- A mixture of known DNA fragment sizes or protein sizes used to estimate the size of unknown bands.
- Anode
- The positive electrode that attracts negatively charged molecules during electrophoresis.
- Gel matrix
- The porous material of a gel that slows molecules based on their size, shape, and interaction with the gel.
Common Mistakes to Avoid
- Using SDS-PAGE to directly compare DNA fragment sizes is wrong because SDS-PAGE is designed mainly for proteins, while agarose gels are usually used for DNA and RNA.
- Forgetting that DNA is already negatively charged is wrong because DNA moves toward the positive electrode without needing SDS.
- Assuming larger molecules travel farther is wrong because larger molecules are slowed more by the gel pores and usually remain closer to the wells.
- Comparing unknown bands without a ladder or marker is wrong because band position alone does not give an accurate molecule size.
- Thinking SDS-PAGE shows a protein's natural shape is wrong because SDS and heat often denature proteins, so separation is mainly by polypeptide size.
Practice Questions
- 1 A DNA sample has fragments of 200 bp, 800 bp, and 1500 bp. Which fragment should travel farthest in an agarose gel?
- 2 A protein marker shows bands at 25 kDa, 50 kDa, and 100 kDa. An unknown protein band runs closest to the 50 kDa marker. What is the best estimate of its size?
- 3 A student wants to separate PCR products that are 300 bp and 350 bp. Should the student use a lower or higher percent agarose gel for better separation of these small fragments?
- 4 Explain why SDS-PAGE can compare protein sizes more fairly than running folded proteins without SDS.