Video Transcript
In this video, we will outline what
DNA fingerprinting is and how it works. We will also discuss some uses of
DNA fingerprinting, and we will interpret some simple DNA fingerprints together.
For many years, the only way to
identify an unknown suspect was by finding and analyzing fingerprints left on a
crime scene because fingerprints are unique to each individual. Then, scientists discovered that
the DNA contained in our cells is unique to each of us as well. This gave police detectives a
modern tool to identify suspects and victims. This technology is also widely used
in paternity testing, helping individuals research their family tree, and in medical
research. Before we review some of the
applications of DNA fingerprinting technology, let’s first understand the principles
behind that technique.
DNA fingerprinting, or DNA
profiling, is creating a visual profile of someone’s DNA. This process was invented in the
1980s by Sir Alec Jeffreys. He famously used DNA fingerprinting
to solve a high-profile murder case at that time. The principles of DNA
fingerprinting have remained largely unchanged since then. However, the techniques used have
progressed a lot. So, what is the basis for DNA
profiling? Well, as you probably know, all of
our body cells have a nucleus, and each nucleus possesses all of our genetic
material, or DNA. Our DNA contains all of the
information to make us. The DNA in each of our cells is
typically the same as the DNA in all of our cells and remains the same in healthy
cells for all of our life.
DNA is a long, coiled, and
compacted molecule. When human DNA is uncompacted, it
spans about two meters in length. This DNA is divided into segments
called chromosomes. We inherit half of our DNA from our
biological father and the other half from our biological mother. Only identical twins have exactly
the same DNA. But the DNA between any two human
beings from anywhere on the planet is still similar at 99.9 percent. So, there’s only one-tenth of a
percent of our DNA that makes us different from each other. Let’s continue our investigation
into what makes one person’s DNA different from another.
If we took one piece of our DNA and
unpacked it and magnified it, we would see that this molecule is a double helix,
which contains a sequence of nucleotide bases represented here by the colored rods
inside the double helix. These bases are of four types, A,
T, C, and G, organized as complementary pairs. Each of our body cells, except our
gametes or sex cells, has six billion of these complementary base pairs. Some sequences of these nucleotide
bases are called genes. They encode information that can be
decrypted by the cell machinery to make functional units such as RNA molecules
and/or proteins.
All of these proteins produced from
our genes determine our traits and visible characteristics. But actually only one to two
percent of our DNA is coding genes. The majority of our DNA is actually
called noncoding DNA. It is called this because it
doesn’t directly code for the formation of functional molecules such as
proteins. Early geneticists even called this
DNA junk DNA as they thought it was probably useless. But forget about that. In fact, this DNA has very
important functions as well and cannot be considered useless. As you will see, it’s actually with
this noncoding DNA that we can really profile each individual. So, let’s learn a bit more about
this noncoding DNA.
Imagine that all the DNA contained
in a cell is represented by this pie. As we just said, only a tiny
portion of this pie is coding for RNA and/or proteins. Our genes make up about one to two
percent of the total. All of the rest is noncoding,
meaning it doesn’t produce RNA or proteins. Around 20 percent of the DNA still
has unidentified functions. We do know about 25 percent is
associated with genes, for example, to help regulate the way that genes are
expressed. The remaining 53 percent of DNA is
made up of repetitive sequences. These regions of repetitions found,
for example, at the extremities or ends of chromosomes called telomeres can play a
protective role as part of the DNA. In DNA fingerprinting, one type of
this repetitive DNA called short tandem repeats or STR can be particularly useful
because the number of repetitions of these short sequences can be highly variable
and unique to each individual.
In this diagram, we’re looking at
the same region of a chromosome in three different individuals. This region is known to contain two
sites, site 1 and site 2, where the sequence of nucleotides AGAT is repeated a
variable number of times. You can see that each individual
has a unique combination of repeats at these two sites. Now, imagine that our genome has
many other sites like this and each site has a highly variable number of
repeats. This creates billions of different
combinations. Thus, the DNA fingerprint is truly
unique in each individual. How is this variation possible
among individuals? Because these regions of DNA are so
repetitive, the machinery that duplicates it at the time that gametes are produced
is more prone to make errors and thereby either increase or reduce the number of
repeats of these sequences.
These mutations, that is, the
increase or decrease in the number of repeats, have essentially no consequences for
the offspring as they affect noncoding regions, so they are easily accumulated
during the production of gametes and then passed on from parents to children. As a result, these regions can vary
a lot among individuals and create a very unique fingerprint for each of us. When scientists became aware of the
uniqueness of these repetitive regions in each individual, they devised a way to
visualize these differences. And this process of visualization
revealed itself to be very useful in many applications, for example, in solving
crimes.
Let’s pretend we’re investigating a
crime scene. How can we use DNA
fingerprinting? First, we would extract DNA from
relevant biological tissue found on the crime scene. This could be obtained from cells
in a blood spatter, a hair, or even tiny amounts of skin left behind when someone
touched a surface. We can then amplify the DNA, which
means cause it to replicate and make many copies so that even if the initial sample
is very small, we’ll have enough to work with. We would also want to do the same
with the samples of DNA from our suspects, perhaps from a strand of their hair or a
sample of their skin cells. Then, we would add a restriction
enzyme to each vial.
A restriction enzyme is an enzyme
that breaks the DNA at the locations of certain sequences of nucleotides called
restriction sites. Because the enzyme will target
sites and areas of noncoding DNA, which will vary in the number of repeats between
the restriction sites, cutting DNA samples from multiple individuals at the same
restriction sites will result in DNA fragments of very different lengths. Now, we need to find a way to
separate all the fragments of DNA from each of our samples. For that, we need an
electrophoresis gel.
Gel electrophoresis is a technique
used by scientists to separate the DNA fragments from each sample based on their
size. In this process, DNA samples are
first loaded into wells at one end of the gel. These samples will include first a
DNA ladder or a standardized solution of DNA fragments that will make it easier to
interpret the other samples by comparison. We will also need to include a
sample from our crime scene to compare against our various suspects. And finally, we need to include
samples from each of our suspects. In this case, we’ve included three
suspects or suspects one, two, and three.
Then, an electric current is passed
through the gel. Because DNA fragments are
electrically charged, they will move through the gel toward the positively charged
end. However, due to the differences in
sizes of the DNA fragments, they will travel at different speeds inside the gel. Smaller fragments will move more
easily through the gel fibers and therefore more quickly, while larger fragments
will have more difficulty moving between the gel fibers and therefore will move more
slowly through the gel. After some time has passed, and at
least some of the DNA fragments have been able to move all the way to the end of the
gel, we can stop the current.
Contained in the gel is a special
type of dye that marks the DNA. A machine which is sensitive to the
DNA marker then creates an image of the results, similar to what we see here. The pattern of stripes left by each
DNA sample on the gel is what we call that individual’s DNA fingerprint. Each person has a unique DNA
fingerprint since the number of repeats in their repetitive DNA segments are unique
and therefore the sizes of the fragments from their DNA when it’s treated with the
restriction enzyme are unique as well.
Now, let’s look at the fingerprints
we see on this gel and compare them to the fingerprint of the blood we found at the
crime scene. Does it look like one of these
suspects is a match? Yes, suspect three matches. We can tell because this
individual’s DNA fingerprint has the exact same pattern of stripes as the sample
found at the crime scene. This suggests that this individual
was likely present at the crime scene.
DNA fingerprinting today is
commonly used in forensic science as a method of helping to determine a person’s
presence and likelihood of involvement in a crime. Newer genetic technologies mean
that we can get a DNA fingerprint from a very small sample of a person’s DNA. Another common use of DNA
fingerprinting is in determining biological parentage. Since we inherit half of our
genetic material from our biological mothers and half from our biological fathers,
half of our DNA fingerprint is likely to match that of our mothers and the other
half is likely to match that of our fathers.
So in order to determine paternity
using DNA fingerprinting, geneticists acquire three samples of DNA, one from the
mother, one from the child, and one from the potential father. They then follow the same steps
we’ve discussed previously to create a DNA fingerprint for each person and to
determine that this potential father is actually the likely father of this
child. First, the portions of the child’s
DNA fingerprint that are shared with the mother are identified and eliminated. Then, the remaining portions of the
child’s DNA fingerprint are compared against that of the potential father to see if
they could have been inherited from him. In this case, all of the remainder
of the child’s DNA fingerprint that doesn’t match the mother matches portions of the
DNA fingerprint of this potential father. So, it’s very likely that this man
is the biological father of this child.
Advances are being made all the
time in the field of DNA fingerprinting to make it even more reliable. A lot of care is taken when samples
are collected to avoid contamination with outside DNA. This is especially important for
samples being collected at a crime scene and is why the investigators that collect
the DNA often appear to be dressed as astronauts with all the protective clothing
they wear. Care is also taken to prevent the
degradation of the DNA samples before they’re analyzed. And at the point that the DNA
fingerprints are analyzed, the interpretation and comparison of the fingerprints is
aided by powerful computers in order to increase its accuracy. Today, DNA fingerprinting has
already been used to solve many crimes and to help many people find their parents or
ancestors.
Let’s see how much we’ve learned
about the applications of DNA fingerprinting by applying our knowledge to a practice
question.
Fill in the blank. DNA fingerprinting can be used to
identify closely related organisms, analyze samples found at crime scenes using
forensic technology, and blank. (A) Create recombinant DNA, (B)
determine the biological parents of a child, (C) treat genetic and hereditary
diseases, or (D) generate synthetic sections of DNA.
Every person with the exception of
identical twins has a unique genome. DNA fingerprinting is a technique
used to make a visual representation of a person’s unique genetic makeup. By visualizing and viewing a
person’s genetic makeup, we can find some valuable information. For instance, we can compare
samples of DNA to determine if a person was present at a crime scene. Or we can compare samples of DNA
from different species to investigate evolutionary relationships.
DNA fingerprinting can also be used
to settle issues of paternity in the case of a disputed lineage of a child. This is because the child will have
inherited half of their genetic material and therefore half their DNA fingerprint
from their mother and half from their father. By comparing the DNA fingerprint of
a child to that of the potential fathers, scientists can determine whether a male is
likely related to a child.
However, DNA fingerprinting is not
a way to create any type of genetic material or treat any genetic disease. And we do not use DNA
fingerprinting to generate synthetic sections of DNA, but only to analyze a sample
of DNA that we have taken. So the correct answer is (B). DNA fingerprinting can be used to
identify closely related organisms, analyze samples found at crime scenes using
forensic technology, and determine the biological parents of a child.
Let’s review the key points we have
discussed. The majority of DNA in humans does
not code for proteins. Instead, it is considered
noncoding. Noncoding regions are highly
variable between individuals. This contributes toward an
individual’s unique DNA. All humans, with the exception of
identical twins, have unique DNA. DNA fingerprinting produces a
visual representation of a person’s unique patterns of noncoding DNA. It is used in forensics, ancestry
analysis, and in medical research.