In methods for analyzing DNA (e.g., agarose gel electrophoresis), what is the primary purpose of a DNA size standard (DNA ladder)?

• A. To specifically stain DNA so that bands fluoresce under UV light without any additional dye
• B. To ligate DNA fragments together by providing DNA ligase and compatible ends
• C. To increase the rate of DNA migration through the gel by raising the ionic strength of the buffer
• D. To estimate the sizes (in base pairs) of DNA fragments in a sample by comparison to fragments of known length

Answer: (D)

Verified step by step guidance

1. Understand that in agarose gel electrophoresis, DNA fragments are separated based on their size, with smaller fragments migrating faster through the gel matrix than larger ones.
2. Recognize that a DNA size standard, or DNA ladder, is a mixture of DNA fragments of known lengths (measured in base pairs) that are run alongside the sample DNA in the gel.
3. Use the DNA ladder as a reference to compare the migration distances of unknown DNA fragments in the sample, allowing estimation of their sizes by matching their positions to those of the ladder bands.
4. Note that the DNA ladder does not stain DNA or affect the migration speed directly; its role is purely as a molecular ruler for size estimation.
5. Conclude that the primary purpose of the DNA size standard is to provide a set of known fragment sizes to accurately estimate the sizes of unknown DNA fragments in the gel.