DNA profiling: matching DNA in forensic science

0:00Blood is found at a crime scene, and there are three suspects. Whose DNA is in the blood?

0:07Some stretches of DNA have a short code repeated again and again in a row. These are called short tandem repeats.

0:15The number of repeats differs from person to person. Person 1 has 3 repeats, and Person 2 has 5.

0:24Across many of these stretches, the pattern is unique to each person, except identical twins, who share it.

0:32First, a method called PCR copies these stretches millions of times.

0:38Next, gel electrophoresis sorts the copies by size. The crime-scene sample goes into a well in a slab of gel.

0:47DNA has a negative charge, so an electric current pulls it toward the positive end.

0:55Shorter pieces, with fewer repeats, slip through the gel more easily and travel further.

1:01The sample ends up as a pattern of bands, called its DNA profile.

1:07Samples from Suspect A, Suspect B and Suspect C are tested the same way.

1:13Line up the bands. Suspects A and C share some bands with the crime scene, but not all.

1:21Suspect B matches every band, so the DNA very likely came from Suspect B.

1:28But a match does not prove guilt. It shows that Suspect B's DNA was there, not how or when it got there.

1:37DNA matching also helps free people who were wrongly convicted, identify human remains, and check the fish species on food labels.

1:46So matching band patterns show whose DNA was at the scene, not who committed the crime.