1. What do these items have to do with one another?

2. Deoxyribonucleic Acid (DNA)
Unit 8A
Forensic Science

3. Unit 8A: DNA – Contents Structure of DNA DNA Typing
Collecting and Packaging Biological Evidence
Isolating DNA
DNA Sequencing: PCR
DNA Sequencing: Gel Electrophoresis
CODIS

4. 1. Structure of DNA
Like fingerprints, DNA is unique to each individual  individual evidence!
DNA can definitively link a suspect to a victim or crime scene.
The primary unit of DNA is called a gene
Each gene contains instructions that control our genetic traits

5. 1. Structure of DNA
double helix
DNA (deoxyribonucleic acid) is a molecule comprised of repeating units called nucleotides
A nucleotide consists of
Sugar-phosphate backbone
Deoxyribose sugar
Phosphate
Nitrogenous base
adenine, guanine, cytosine, thymine
Adenine bonds only to thymine, and guanine bonds only to cytosine
The order of bases is our genetic code.

6. 1. Structure of DNA
Four types of bases are associated with the DNA structure: adenine (A), guanine (G), cytosine (C), thymine (T).

7. 1. Structure of DNA
The long strands of DNA are coiled upon themselves into shapes called chromosomes
When paired, chromosomes resemble the letter X
Humans have 22 pairs of autosomal (standard) chromosomes, and 1 pair of 2 sex chromosomes
Females are XX
Males are XY

8. Unit 8A: DNA – Contents Structure of DNA DNA Typing
Collecting and Packaging Biological Evidence
Isolating DNA
DNA Sequencing: PCR
DNA Sequencing: Gel Electrophoresis
CODIS

9. 2. DNA Typing
DNA typing (a.k.a. DNA Fingerprinting) was developed by British geneticist Sir Alec Jeffreys in 1984.
This technique converts DNA into readable bands on a gel
With these bands, we can compare suspect and crime scene DNA, or child and possible father, etc.

10. 2. DNA Typing: Short Tandem Repeats (STRs)
A common method of DNA typing
There are locations (loci) on a chromosome that contain short segments of 3 – 7 bases that repeat themselves
STR’s are less susceptible to degradation (breaking down) and can be recovered from bodies or stains that have been subject to extreme decomposition
With the technology of PCR one can extract and amplify a combination of different STR’s.

11. 2. DNA Typing: Short Tandem Repeats (STRs)
Serve as useful markers for identification because they are found in great abundance throughout the human genome.
What is important to understand is that all humans have the same type of repeats, but there is tremendous variation in the number of repeats each of us have.

13. Unit 8A: DNA – Contents Structure of DNA DNA Typing
Collecting and Packaging Biological Evidence
Isolating DNA
DNA Sequencing: PCR
DNA Sequencing: Gel Electrophoresis
CODIS

14. 3. Collecting and Packaging Biological Evidence
At the crime scene, the following are to be collected and sent to the forensic laboratory:
Victim’s clothing
Fingernail scrapings
Head and pubic hairs
Blood
Other sources of DNA

15. 3. Collecting and Packaging Biological Evidence
Other possible sources of DNA
Skin
Semen
Sweat
Urine
Blood
Hair (root)
Mucus
Ear Wax
Saliva
Vaginal or rectal cells
Tissue

16. 3. Collecting and Packaging Biological Evidence
Photograph evidence first
Wear gloves at all times
Package each stained article separately in paper or a well-ventilated box (to avoid bacterial or fungal growth)
Remove dried blood using a sterile swab moistened with distilled water
Store biological evidence in the refrigerator or a cool location until it is delivered to the lab

17. Unit 8A: DNA – Contents Structure of DNA DNA Typing
Collecting and Packaging Biological Evidence
Isolating DNA
DNA Sequencing: PCR
DNA Sequencing: Gel Electrophoresis
CODIS

18. 4. Isolating DNA Most DNA extraction procedures consist of two parts:
A technique to lyse (cut open) the cells and expose the DNA
An enzyme or chemical wash to remove contaminates such as: proteins, RNA, or macromolecules

19. Unit 8A: DNA – Contents Structure of DNA DNA Typing
Collecting and Packaging Biological Evidence
Isolating DNA
DNA Sequencing: PCR
DNA Sequencing: Gel Electrophoresis
CODIS

20. 5. DNA Sequencing: PCR

21. 5. DNA Sequencing: Polymerase Chain Reaction (PCR)
A technique for making many copies of a specific piece of DNA to be analyzed forensically
Can amplify very minute quantities of DNA millions of times!
This method works by cycling through different temperatures
for each part of the process
A device called a thermocycler controls the temperatures, allowing for fast and accurate copying of DNA

22. 5. Polymerase Chain Reaction (PCR)
d. The steps of PCR
Step 1: Denaturation.
Extracted and purified DNA is heated to “unzip” (separate) the double helix
This is done at high temperature, about 94°C

23. 5. Polymerase Chain Reaction (PCR)
d. The steps of PCR
Step 2: Annealing.
Short template pieces called “primers” bind with the separated strands for new DNA to build upon.
This occurs at ~65°C

24. 5. Polymerase Chain Reaction (PCR)
d. The steps of PCR
Step 3: Extension/Elongation.
Taq Polymerase, a DNA building enzyme, adds free nucleotides from the surrounding solution onto the template primers
In this way, new strands are built out of the original 2 separated stands
This happens at 72°C
new DNA strands

25. 5. Polymerase Chain Reaction (PCR)
Each step only requires a few minutes
The thermocycler machine cycles through these temperatures for several hours
Each cycle doubles the number of copied DNA strands
PCR is specific to your “region of interest.” Different primers will selectively amplify different genes

27. Unit 8A: DNA – Contents Structure of DNA DNA Typing
Collecting and Packaging Biological Evidence
Isolating DNA
DNA Sequencing: PCR
DNA Sequencing: Gel Electrophoresis
CODIS

28. 6. Gel Electrophoresis

29. 6. Gel Electrophoresis
DNA can be visualized through the process of electrophoresis
In the lab, DNA molecules are cut by restriction enzymes into fragments of various sizes. Restriction enzymes cut at specific sequences throughout the DNA.
The resulting fragments are forced to move along a gel-coated plate under the influence of an electrical field.

30. 6. Gel Electrophoresis
The DNA molecules have a slight negative charge, so they are attracted to the positive end of an induced electric field.
DNA fragments are separated by size
Larger fragments move more slowly
Smaller fragments move farther along the gel

32. 6. Gel Electrophoresis
After the fragments have “migrated” across the gel, the gel can be stained to show the “bands” or fragments easily
Then comparisons can be made
Example: crime scene sample to suspect

34. Unit 8A: DNA – Contents Structure of DNA DNA Typing
Collecting and Packaging Biological Evidence
Isolating DNA
DNA Sequencing: PCR
DNA Sequencing: Gel Electrophoresis
CODIS

35. 7. Combined DNA Information System (CODIS)
CODIS maintains a database of DNA profiles from
convicted offenders
unsolved crime scene evidence
profiles of missing persons

36. Resources
Saferstein, Richard. Forensic Science: An Introduction. New Jersey: Pearson Prentice Hall, 2008
Saferstein, Richard. Forensic Science: An Introduction. 2nd ed. New Jersey: Pearson Prentice Hall, 2011
Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 8th ed. Upper Saddle River, NJ; Pearson Prentice Hall, 2004