DNA Fingerprinting in Honors Biology: How to Read the Banding Pattern and Solve the Problem

Of all the topics in the Honors Biology biotechnology unit DNA fingerprinting is the one that is consistently covered by teachers. The good news is, once students understand it most of them genuinely enjoy it. There is something satisfying about looking at a set of banding patterns in a DNA fingerprinting problem and solving it like a puzzle. That is exactly the goal of this article, help you understand how to solve the problem.

What a DNA Fingerprint Actually Is

Every person's DNA is unique. While the vast majority of human DNA is identical across all people there are specific regions where patterns vary from person to person. Scientists use those variable regions to create a DNA fingerprint, a pattern of bands that is unique to each individual. No two people, with the exception of identical twins, have the same DNA and thus they have different patterns.

Before any of this analysis can happen scientists need enough DNA to work with. PCR amplifies a tiny sample into billions of copies in a matter of hours. Without that amplification step most real world DNA samples would be too small to produce a readable fingerprint. For more on PCR see the previous article in this series.

What the DNA fingerprinting Illustration Shows

When your teacher gives you a DNA fingerprinting problem it will typically show a series of vertical lanes, each representing a different DNA sample. One lane is labeled as the crime scene or unknown sample. The other lanes represent suspects. Each lane contains a series of dark horizontal bands at different positions.

Those bands represent fragments of DNA. Those fragments of DNA can be separated. Smaller fragments travel farther down the lane during the separation process, so bands near the bottom represent smaller pieces and bands near the top represent larger pieces. The position of each band is what matters.

How to Solve the Problem

This is where students start to enjoy it. The matching process is more straightforward than it looks.

Start with the crime scene lane. Look at each band and note its position from top to bottom. Then move to the first suspect lane and ask one question: does every band in this lane line up horizontally with a band in the crime scene lane? Not some of the bands. Every single one.

If any band in a suspect lane does not have a matching band at the same position in the crime scene lane that suspect is eliminated. It does not matter how many bands match. One mismatch rules them out.

Work through each suspect lane using the same process until you find the lane where every band aligns perfectly with the crime scene sample. That is your match, the DNA pattern is the same for that suspect and the DNA at the crime scene.

The Most Common Mistakes

Two things trip students up more than anything else. The first is assuming a partial match is enough. It is not. A DNA fingerprint either matches completely or it does not. The second mistake is counting bands instead of comparing positions. Always compare the position of the bands, not the number of bands.

How deeply your teacher tests DNA fingerprinting and which details they emphasize will vary. Some focus heavily on reading and interpreting the illustration. Others go deeper into the underlying process. Check your class materials to confirm their emphasis.

I offer one-on-one online tutoring tailored to your specific Honors Biology curriculum if you want to work through this or any other biotechnology topic with someone focused on exactly what your teacher is covering.

READ MORE

• PCR

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