Video Transcript
Which of the following samples left at a crime scene would scientists typically not
be able to produce a DNA fingerprint from? (A) A blood splatter, (B) a single hair, (C) a shoe print, (D) a fingerprint on a
glass.
DNA is the biological macromolecule that stores our genetic information in the
nucleus of our cells. All the DNA that we have in our cell is called our genome. It contains all the information that makes us who we are.
On average 99.9 percent of our DNA is similar to any other human being on Earth. But still we are all different, unless we have a twin. This is because a tiny portion of our genome is extremely variable. That DNA makes us unique.
When our DNA is cut in fragments by specific enzymes called restriction enzymes, the
number and the size of the fragments is unique for each of us because of the highly
variable portion of our DNA. This can be visualized in a gel electrophoresis that separates the DNA fragments
based on their size, as shown here for one individual’s DNA. This way you can also compare the DNA fingerprints of different individuals, which is
very useful to solve a crime.
So to produce DNA fingerprints, scientists need first to find DNA on the crime
scene. And DNA can be found in cells containing a nucleus. So, let’s review our choice to find which sample does not typically contain cells
with a nucleus. Although red blood cells do not contain nuclei and DNA, many other cells like white
blood cells do have DNA that can be used to produce a DNA fingerprint. So this is not the answer to our question.
For DNA testing, the part scientists really want in a hair is the follicle, which is
the part at the base of human hair that contains DNA. So a single hair is potentially a sample that scientists can use for DNA
fingerprinting. We can eliminate this answer.
A shoe print is unlikely to contain cells shed from a person. The shoe print may be an informative clue on a crime scene, but it is not typically
what scientists use to produce a DNA fingerprint.
Let’s review our last choice to be sure of our answer. A fingerprint can be left on a glass. It’s often possible to find cells shed on the objects we touch. Thus, a fingerprint can be a source of cells that have a nucleus. In fact, DNA obtained from these shed skin cells is called “touch DNA” in
forensics. Thus, a fingerprint on a glass is likely to be used to produce a DNA fingerprint. We can thus eliminate this answer.
The correct answer to our question is thus answer (C) a shoe print.
