Final answer:
A DNA ladder is a marker with known DNA fragment sizes used in gel electrophoresis to estimate the size of DNA fragments by comparison.
Step-by-step explanation:
A DNA ladder is a standard, or marker, used during gel electrophoresis to estimate the size of DNA fragments by comparing them against the known sizes of the ladder's DNA fragments. In a typical experiment, a DNA ladder is separated on a gel alongside samples of unknown DNA fragment sizes. By comparing the distance travelled by the DNA fragments in the gel, scientists can determine the approximate length of those fragments based on the known fragment sizes of the DNA ladder.
The ladder is composed of different DNA fragments of known lengths, and these fragments are created through various methods, including the use of dye-labelled dideoxynucleotides that terminate DNA synthesis at each nucleotide, ultimately generating fragments of every possible length for use as standards. During an experiment, the pattern formed by these fragments allows researchers to calculate the size of the DNA under study by locating its position relative to the ladder's bands.