1. DNA Evidence

2. Scientific Breakthrough
The discovery of the structure of DNA, coupled with a new understanding of how it is replicated and transcribed from gene to protein may be the single most amazing discovery of the 20th century!
Applying this new knowledge to forensics wasn’t apparent right away…

3. History
1985 – Alec Jeffreys Leicester University While investigating a human gene, they discovered that portions of the DNA structure of certain genes are unique to each individual. DNA Fingerprinting/Typing was developed

4. Forensic Scientists jumped at the opportunity to turn this biological evidence from class to Individual
Courts now accept DNA evidence as reliable

5. DNA Refresher There are 60 trillion cells in the human body
Chromosomes – rod-like structures made up of DNA that is coiled around proteins
Human DNA contains
3 billion base pairs
approximately 25,000 genes
Genes instruct the body cells to make proteins that do anything including structural proteins to make new cells, enzymes, determine hair and eye color, etc.

6. Complementary Base-pairing
DNA Refresher
DNA is a polymer (a string of many molecules put together)
The pieces of the string are nucleotides
Structure:
Double helix
Sugar phosphate backbone
Joined together by A T C G
Complementary Base-pairing
A-T
G-C

7. DNA Refresher - Replication
DNA Helicase binds to the DNA strand and causes the helix to unwind and the 2 strands break apart
With the 2 strands separated there is now a template strand for free nucleotides in the nucleus to attach
DNA Polymerase is an enzyme that matches up the free nucleotides with their complementary base on the template strand of DNA
DNA Polymerase is also the proofreader – checking for errors in base pairing and repairing as needed

8. DNA Fingerprint
Bozeman Science DNA Fingerprinting Video:

9. Human DNA is approximately 3 billion base pairs long, and we have about 30,000 different genes.
In order to study a specific gene we need to cut down the long strand of DNA and select only the specific gene or region that we are working on Restriction Enzymes = “molecular scissors”
Image source: Microsoft Clipart Gallery

10. Cuts can be ‘blunt’ or ‘sticky’
Restriction Enzyme – Eco R1
Recognition Site
ACTGGTACGAATTCCGTA
TGACCATCCTTAAGGCAT
AATTCCGTA
GGCAT
ACTGGTACG
TGACCATCCTTAA
Sticky Ends!
Sticky ends will bind to complementary bases that come along, making them useful

11. Once DNA has been cut into fragments by restriction enzymes, the fragments can be sorted or separated by size
How?

12. Gel Electrophoresis
An electrical current is used to separate a mixture of different fragments of DNA
DNA Sample
Negative electrode
Positive electrode

13. Gel Electrophoresis
Because DNA has a negative charge, the fragments move toward the positive electrode
Pores in the gel allow small molecules to move through the gel quickly, while large molecules get “stuck” and move slowly

14. Gel Electrophoresis
So, the length of a DNA fragment can be estimated compared to other fragments by measuring how far each fragment migrated through the gel
DNA fragments appear as different bands or lines on the gel (after staining)

15. Restriction Maps
The band pattern can be thought of as a map of the original strand of DNA
The map shows the lengths of DNA fragments between restriction sites on a DNA strand

16. How can we get a large enough sample of DNA to work with?
Polymerase Chain Reaction (PCR)
1 Cycle
2 Cycles
3 Cycles

17. Polymerase Chain Reaction (PCR)
A technique that produces millions (or billions!) of copies of a specific DNA sequence in hours
Invented by Kary Mullis in 1983
He patented the process and sold the rights for $300 million
Won the Nobel Prize in Chemistry in 1993
Under the right set of conditions, DNA polymerase enzymes will make new copies of DNA in a test tube, just like they do in your cells!
Sample size is no longer an issue in investigations!

18. Polymerase Chain Reaction (PCR)
4 Starting Materials:
DNA strand
DNA polymerase
Nucleotides
Primers
Separating
Tube with all reactants is heated to over 90C to separate the strands of DNA
Binding
Tube is cooled to 55C, primers bind to DNA
Copying
Tube heated to 72C, polymerase binds new nucleotides until segment is copied

19. Polymerase Chain Reaction (PCR)
Each PCR cycle doubles the number of DNA copies
1 > 2 > 4 > 8 > 16 > 32 > 64 > 128 > 256 > 512…
After 30 cycles, more than 1 billion copies!
What is a primer? A short segment of DNA that acts as a starting point for the new strand

20. DNA Fingerprinting – a restriction map!
Our DNA is 99.9% identical, how do we find the differences?
The greatest differences in DNA sequence between people are in the non-coding sections of our DNA (introns)
So….DNA fingerprinting focused on the introns, which often include stretches of nucleotide sequences that repeat several times
Each person’s DNA differs in how many repeats they have

21. DNA Fingerprinting – a restriction map!
DNA fingerprinting has become reliable and widely used in US courts since the 1990s
In most forensic cases, at least 5 regions of DNA are analyzed
This technique can be used to:
identify perpetrators of a crime
identify fathers in paternity cases
identify unknown remains
clear persons wrongly accused
identify crime and catastrophe victims
match organ donor with recipients in transplant programs