Final answer:
Bands in a gel paper appear brighter than others due to differing amounts of DNA; the brighter a band, the more DNA it contains. To estimate the amount of DNA in a sample, compare the brightness of the sample bands to that of a reference ladder with known DNA amount in each band.
Step-by-step explanation:
During agarose gel electrophoresis, some bands in a gel paper appear brighter than others because they contain more DNA. The brightness is proportional to the amount of DNA present in a band. When stained with a dye such as ethidium bromide, the DNA fluoresces under ultraviolet light, allowing us to determine the relative amount of DNA by comparing the brightness.
To approximate the amount of DNA in a sample on a gel, you can use a reference ladder. If the reference ladder has 1.0 micrograms DNA in each band, you would measure the brightness of your sample bands and compare them to the ladder. The DNA ladder serves as a calibration tool to estimate the DNA quantity by comparing the intensity of the sample bands to the intensity of the ladder bands with known DNA amounts.