How the gels are made
Every band, ladder included, sits where a straight semi-log line puts it, so a standard curve drawn from the gel’s own ladder reads back the size in the key. Each gel runs a little differently, so make a new curve for each one. Real gels bend at the ends of the range; these are drawn clean on purpose. Digest fragments always add up to the whole piece.
Sizing a band, step by step
In set DNA2AP, band X ran 27 gel mm on the drawn ruler, between the 500 bp band (25.5 gel mm) and the 400 bp band (31.5 gel mm). On semi-log paper the ladder makes a straight line, and reading across from 27 gives about 470 bp. Anything from 423 to 517 bp counts as right.
What each level includes
High school: sizing on a 100 bp ladder, linear DNA, three loci, and a first step that asks which two ladder bands a band falls between. AP Biology adds sizing on the 1 kb ladder, circular plasmids and four loci. Intro college has five loci and circular restriction maps. Restriction mapping can be added at any level.
Questions
How do you read a gel electrophoresis result?
The wells are at the top, by the negative electrode. DNA is negatively charged, so it runs down toward the positive electrode. Small fragments slip through the gel faster, so the bands farthest from the wells are the smallest. The ladder lane gives known sizes to compare with.
How do you find the size of a DNA band from the ladder?
Measure how far each ladder band ran, from the bottom of the well to the middle of the band. On semi-log paper, plot size on the log scale against distance and draw a straight line through the points. Measure the unknown band, go up to the line and read its size across.
How many fragments does a restriction enzyme make?
Count the cuts. A linear piece cut n times gives n + 1 fragments, because it has two ends. A circular plasmid cut n times gives n fragments, and one cut only opens the circle. Either way the fragments add up to the length you started with.
How do you tell from a gel who can be the father?
At each locus a child gets one band from the mother and one from the father. Cross off the child's bands that match the mother; the ones left over came from the father. A man missing them at two or more loci is excluded, because one mismatch can be a rare mutation. The gels here are practice with invented people, not real tests.
How do you know which suspect matches the crime-scene sample?
Every band in the crime-scene lane has to line up with a band in the suspect's lane, with no extra bands. Sharing a few bands means little, because unrelated people often share some.
Is gel electrophoresis on the AP Biology exam?
Yes. Topic 6.8 of the AP Biology Course and Exam Description (effective fall 2025) says gel electrophoresis separates DNA fragments by size and charge, with forensic identification as an example. It also puts the details of each technique beyond the exam, so semi-log sizing and restriction maps are extras for honors and college classes.
Sources, checked October 24, 2026: College Board, AP Biology Course and Exam Description, effective fall 2025, Topic 6.8 (PDF); Bio-Rad, Restriction Digestion and Analysis of Lambda DNA, manual 4006102 Rev D (PDF); S. M. Carr, Memorial University, Circular restriction mapping; National Institute of Justice, Parentage and relatedness (2023).