Genetic Fingerprinting Worksheet

All 12 labels on the diagram

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The set-up 8

Gel A slab of jelly-like agarose. Its pores work like a sieve.
Wells Small holes in the gel. Each DNA sample is pipetted into one.
Buffer A salty liquid that covers the gel. It carries the electric current.
Tank The box that holds the gel and the buffer.
Negative electrode The wire at the end where the wells are. DNA is pushed away from it.
Positive electrode The wire at the far end. DNA has a negative charge, so it is pulled this way.
Bands Each band is DNA fragments of one length. A stain makes them show.
Direction of travel DNA moves from the wells toward the positive electrode.

Reading a gel 4

DNA ladder Fragments of known lengths, run beside the samples. Bands are measured against it.
Long fragments Long pieces of DNA move slowly. They stay near the wells.
Short fragments Short pieces of DNA move fastest. They end up farthest from the wells.
Match for the sample Lane C has every band of the sample. No other lane does.

How this list was compiled: Every name and step checked on 2026-10-01 and kept only where two of these use it. (1) Wellcome Connecting Science, yourgenome, What is gel electrophoresis?, https://www.yourgenome.org/theme/what-is-gel-electrophoresis/ : a technique to separate charged molecules like DNA by size; a comb makes wells in an agarose gel; the gel is placed in an electrophoresis tank and covered with buffer, which conducts the current; a DNA marker, also called a size standard or DNA ladder, goes in the first well; DNA is negatively charged and moves toward the positive electrode; shorter strands move more quickly than longer ones; a stain shows the DNA as bands. (2) US National Human Genome Research Institute, Talking Glossary, Electrophoresis and DNA Fingerprinting, https://www.genome.gov/genetics-glossary/Electrophoresis and https://www.genome.gov/genetics-glossary/DNA-Fingerprinting : an electric current moves the molecules through a gel whose pores work like a sieve, smaller molecules moving faster than larger ones; standards of known sizes are run on the same gel; in DNA fingerprinting one aims to match two DNA fingerprints, a sample from a known person and one from an unknown person. (3) Wikipedia, Gel electrophoresis, Agarose gel electrophoresis and DNA profiling, text only, https://en.wikipedia.org/wiki/Gel_electrophoresis and https://en.wikipedia.org/wiki/DNA_profiling : wells, buffer, lanes, the DNA ladder, the negatively charged cathode and positively charged anode; DNA profiling is also called DNA fingerprinting and genetic fingerprinting and was developed by Alec Jeffreys in 1984. Licence: own work

The drawing: Own drawing from plain geometry; nothing traced. The band patterns are made up for teaching. Licence: own work

How it works

Genetic fingerprinting, also called DNA fingerprinting or DNA profiling, compares samples of DNA. The first drawing is the gel electrophoresis set-up; the second is a finished gel to read. Print either one blank or labeled.

Read the full guide

How the gel sorts DNA

Each sample goes into a well at one end of the gel, and buffer covers it. With the current on, DNA, which has a negative charge, moves toward the positive electrode. Short fragments slip through the gel fastest, so the fragments end up in order of length, as bands.

Reading a match

The DNA ladder holds fragments of known lengths, to measure the others against. Two samples match when every band of one sits level with a band of the other. The bands on this page are made up for practice.

To use it as a worksheet, press Print blank. Print labeled gives the answer key.

Questions and answers

What is genetic fingerprinting?

A laboratory method that compares DNA samples to see whether they match. It is also called DNA fingerprinting and DNA profiling. Alec Jeffreys developed it in 1984.

How does gel electrophoresis separate DNA?

An electric current moves the DNA through a gel whose pores work like a sieve. Smaller fragments move faster than larger ones, so after a while the fragments are spread out by size.

Why does DNA move toward the positive electrode?

DNA carries a negative charge, and opposite charges attract. So the wells are placed at the negative end and the DNA travels through the gel toward the positive end.

What is a DNA ladder?

A mix of DNA fragments of known lengths, also called a DNA marker or size standard. It runs in its own lane, and the bands of each sample are compared with it to estimate their lengths.