In gel electrophoresis, what is a DNA ladder used for?

• A. A porous membrane that transfers DNA from the gel during Southern blotting
• B. A set of DNA fragments of known sizes used as a molecular size standard to estimate the lengths of sample DNA bands
• C. A chemical dye that binds DNA to make bands fluorescent under UV light
• D. An enzyme mixture that cuts DNA at specific restriction sites before loading the gel

Answer: (B)

Verified step by step guidance

1. Understand that gel electrophoresis separates DNA fragments based on their size by applying an electric field to a gel matrix.
2. Recognize that to determine the size of unknown DNA fragments in the gel, a reference is needed for comparison.
3. Know that a DNA ladder consists of DNA fragments of known lengths, which are run alongside the sample DNA in the gel.
4. Use the DNA ladder bands as molecular size standards to estimate the sizes of the sample DNA bands by comparing their relative positions in the gel.
5. Conclude that the DNA ladder is essential for interpreting gel electrophoresis results because it provides a size scale for the DNA fragments.