DNA Power Point

DNA Evidence Notes

What is DNA?

• DNA is a very small but long molecule found in the nucleus of every cell.

• It is a code (set of instructions) that makes each person unique.

• Every person shares 99.9% of their DNA; the 0.1% difference makes individuals unique.

• Each cell in the human body contains 6 feet of DNA with 3 billion "letters."

• Identical twins are the only people who share the same DNA.

History of DNA Evidence

• DNA Fingerprinting (Profiling) was developed by Alec Jeffreys in the 1980s.

• The first court case using DNA evidence was in 1986 to solve a double rape-murder case.

• DNA evidence proved a 17-year-old suspect innocent and led to the real killer’s arrest.

• DNA Fingerprinting became a crucial tool in forensic investigations.

DNA at the Crime Scene

• DNA is in every cell of the body, so it can be found in:

  • Body fluids (saliva, blood)

  • Hair roots

  • Bone marrow

  • Even from fingerprints in some cases

Collection of DNA Evidence

• Investigators wear protective gear to prevent contamination.

• Precautions include:

  • Collecting blood pools/spatter with sterile cotton swabs

  • Storing body fluids in porous containers (paper bags, cardboard boxes)

  • Drying samples to prevent decomposition

  • Storing DNA evidence in freezers to preserve it

How to See DNA Evidence

1. Cut the DNA with a restriction enzyme (works like scissors).

2. Copy the DNA using a PCR machine to make millions of copies.

3. Separate DNA fragments by size using a Gel Electrophoresis Machine.

4. Compare DNA Fingerprints to find matches between crime scene evidence and suspects.

DNA Fingerprints

• Larger DNA fragments move slower; smaller ones move faster in the gel.

• Each band represents a clump of DNA fragments of the same size.

• DNA Fingerprints match only if they come from the same person.

Limitations of DNA Evidence

• Presence of DNA does not prove guilt—it only shows that a person was in a location or touched an object.

• DNA can degrade due to heat, bacteria, moisture, or sunlight.

• Not all samples provide enough DNA for testing.

Example: Jewelry Store Robbery

• Multiple DNA samples found:

  • G. Garrett’s DNA on the cash drawer (he works there).

  • J. Jones, E. Edwards, and S. Smith’s DNA found in the store (they were customers the day before).

• Defense attorneys could argue that DNA presence does not prove involvement in the crime.

CODIS Database

• CODIS (Combined DNA Index System) stores over 3 million DNA profiles.

• Includes:

  • Arrestees (varies by state laws)

  • DNA from unsolved cases

• Investigators compare crime scene DNA with CODIS records to find matches.

• If no match is found, the suspect may not have a criminal record.

John Doe Warrants

• If DNA evidence is found but no suspect is identified, a "John Doe Warrant" is issued.

• This prevents the statute of limitations from expiring.

• If the suspect is arrested later and their DNA is entered into CODIS, they can be linked to the crime.

When There is No Body

• If only blood or body parts are found, the victim’s identity can be determined by comparing DNA to relatives.

• This method is also used for paternity and maternity tests.

Post-Conviction DNA Testing

• Over 300 people have been exonerated by post-conviction DNA testing.

• Some were on death row before being proven innocent.

• The Innocence Project has been leading efforts to use DNA to overturn wrongful convictions since 1992.