1. DNA Evidence and Forensics
Ms MacCormack
Spring 2017

2. Review of DNA
A nucleic acid that contains the code for making proteins.
Makes up the chromosomes found in the nucleus of our cells.
Also found in mitochondria Mitochondrial DNA
Made up of nucleotides
Sugar = deoxyribose
Phosphate
Nitrogenous Bases = Adenine, Cytosine, Guanine and Thymine

3. DNA Strucutre

4. Why is DNA important in Crime Scenes?
The order of the bases determines the genetic code.
Each person’s DNA is different from other people (except identical twins).
DNA collected from a crime scene can do one of two things:
Link a suspect to the evidence
Eliminate a suspect
DNA can also be used to identify a victim through DNA from relatives, even when a body can not be found.

5. Why is DNA important in Crime Scenes?
DNA can also be used to identify a victim through DNA from relatives, even when a body can not be found.
DNA can link crime scenes together by linking the same perpetrator to different scenes locally, statewide and across the nation
DNA can place an individual at a crime scene, in a home or in a room where the suspect claimed not to have been.
DNA can refute a claim of self-defense and put a weapon in the suspect’s hand.
It can change a story from an alibi to one of consent.

6. DNA Collection and comparison
Investigators gather samples from the crime scene and from suspects and then analyze it for a set of specific DNA regions or markers.
A match of one marker is not usually unique, but if a sample matches four or five markers, there is a very good chance it is a match.

7. DNA COllection
DNA is collected at crime scenes in a variety of ways using tools such as:
Smear slides, scalpels, tweezers, scissors, sterile cloth squares, UV light, luminol and/or blood collection kits (for sample collection of suspects or living victims)

8. DNA samples can be from:
DNA Collection
DNA samples can be from:
Saliva, blood, hair strands, skin, finger or toe nails, and/or a tooth with root material

9. DNA Collection and Comparison
How is blood collected?
Blood on Clothing?
Investigators submit whole pieces of clothing or they may use a sterile cloth square and a small amount of distilled water
Dried blood on furniture?
Investigators send the whole object to the lab
Dried blood on a wall, tub or some other object too big or difficult to move to the lab?
Investigators scrape the blood sample into a sterile container for further analysis

10. Analysis of DNA Uses various DNA Technologies RFLP PCR STR
Mitochondrial DNA Analysis

11. RFLP: Restriction Fragment Length Polymorphism
Analyzes variable lengths of DNA fragments
One of the original applications of DNA analysis
Not used as much anymore because it requires a large quantity of DNA sample and samples degraded by the environment do not work well with RFLP

12. PCR: Polymerase Chain Reaction
Used to make millions of exact copies of DNA from a biological sample
Allows very small samples to be analyzed, such as a sample of a few skin cells
Must be very careful about contamination in this process

13. STR: Short Tandem Repeat
Evaluates specific regions (loci) within nuclear DNA
FBI uses 13 standard specific STR regions for CODIS

14. Mitochondrial DNA Analysis
Used for samples that cannot be analyzed using RFLP or STR
Uses DNA extracted from mitochondrion rather than nuclear DNA
Especially useful in old cases and old samples

15. What happens next? What happens after the samples are collected?
A DNA profile is created….how??
Markers are found by designing small pieces of DNA (probes) that will seek out and bind to complementary DNA sequences. This creates a distinct pattern. Again, one marker is not usually unique, but with four or five regions the match is likely
The DNA profiles are compared with samples from suspects to find possible matches.
If there are no suspects, a national database called CODIS may be used to find potential suspects.

16. More about CODIS Stands for Combined DNA Index System
National Network that helps identify leads for crimes with no suspects
Three tiers: Local (LDIS), State (SDIS), National (NDIS)
Uses 13 DNA regions that vary from person to person
Looks for matches at more than one location on a genome for more accurate results

17. Sources of DNA at Crime Scenes
Examples of sources from real cases:
Saliva on the stamp of a stalker’s threatening letter
Skin cells shed on a ligature of a strangled victim
Perspiration on a baseball cap discarded by a rapist was compared with the DNA in the saliva swabbed from a bite mark on a different rape victim
DNA analysis of a single hair (without the root) found deep in a victim’s throat
Maggots can contain DNA of a perpetrator

18. How can DNA evidence be planted?
Sneezing or coughing over evidence
Person touches their mouth, nose or other part of the face and then touches the area that may contain the DNA to be tested.
Scene personnel can deposit hairs, fibers, or trace material from their clothing
Wind can carry in contaminants

19. Paternity Testing and Proving Family Relations
Other Uses of DNA
Paternity Testing and Proving Family Relations
Identification of John or Jane Does
Study of evolution and ancestry
Studying Inherited Disorders