Establishing Identities: DNA
If identification officers find biological evidence (e.g., bodily fluids, skin, or hair) at a crime scene, they send it to a forensic biologist for DNA analysis. In this lesson you will learn what DNA is and how it can help determine who left that biological evidence at the crime scene.
Identification officers find biological evidence (e.g., bodily fluids, skin, or hair) at a crime scene, they send it to a forensic biologist for DNA analysis.
What is DNA?
- The human body is made up of millions and millions of cells.
- Inside the nucleus of each of these cells are structures called chromosomes.
- And within each of these chromosomes is genetic material called deoxyribonucleic acid, or DNA.
- DNA contains the "blueprint" for what you look like.
- For example, if you have brown hair it is because your DNA has the code for brown hair.
- The same goes for things like your eye colour or the shape of your nose.
- DNA determines almost all of your physical characteristics.
DNA Profiles
- Most of the genetic information in your DNA is identical to every other person on Earth.
- However, a small amount of it is unique to you (unless you have an identical twin, triplet, etc.).
- Using modern technology, scientists can locate that unique genetic information and create what is known as a DNA profile.
- The first step in making a DNA profile is to separate the DNA from all other cellular components.
- Next, the amount of DNA in the sample is measured to determine how much more of it needs to be replicated.
- Using a process called polymerase chain reaction (PCR), the scientist then replicates millions of times over the sequence of DNA information that is unique to that individual.
- This unique information is known as a short tandem repeat.
- Next, the scientist sorts the DNA fragments by size using a technique called electrophoresis.
- In this technique, the DNA fragments are put into gel-filled tubes.
- The DNA/gel mixture is then zapped with a strong electrical current.
- This causes the DNA fragments to move towards the opposite end of the gel.
- The shorter fragments move faster than the longer ones, so when the current is shut off, the fragments are sorted according to length.
- A laser then scans this DNA pattern into a computer.
- This pattern is the DNA profile.
- A DNA profile looks something like this:
Uses of DNA
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Did you know?
In 1970, David Milgaard was found guilty of the rape and murder of a Saskatoon nursing assistant named Gail Miller. Milgaard spent the next 22 years in jail - all the while claiming he was innocent. Finally, in 1992, Milgaard was released. DNA testing later proved that he was telling the truth: Milgaard's DNA did not match the DNA found at the crime scene. More DNA testing proved that the real killer was Larry Fisher, a man who was living in the basement of the house Milgaard was visiting. |
DNA Databases
- Whenever a DNA profile is created from crime scene evidence, it is compared against other DNA profiles stored in a database.
- CODIS (Combined DNA Index System), which is funded by the United States Federal Bureau of Investigation (FBI).
- CODIS contains the DNA profiles of convicted criminals as well as all unknown DNA profiles from crime scenes.
- If there is a match, it proves the person recorded in CODIS was at the crime scene.
- Canada has a similar DNA database, known as the National DNA Bank includes CODIS which is provided at no cost to the RCMP by the FBI and U.S. Department of Justice.
Important Terminology
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Terminology Defined
Chromosomes: Structures containing genes that are found in the nucleus of a cell. Each cell contains 46 chromosomes, 23 from the mother and 23 from the father.
DNA (Deoxyribonucleic Acid): A chemical inside our cells that determines what we look like. For example, our DNA determines our hair, eye, and skin colour. DNA is unique to each individual.
DNA Profile: A person's unique sequence of DNA segments, which can be isolated in a lab procedure. A DNA profile is the same for every cell, tissue, and organ of a person, and cannot be altered.
DNA (Deoxyribonucleic Acid): A chemical inside our cells that determines what we look like. For example, our DNA determines our hair, eye, and skin colour. DNA is unique to each individual.
DNA Profile: A person's unique sequence of DNA segments, which can be isolated in a lab procedure. A DNA profile is the same for every cell, tissue, and organ of a person, and cannot be altered.