1. DNA Fingerprinting
Ch 7 – Unit 5

2. Introduction
No 2 people on Earth have the exact same DNA- except identical twins! (Why?)
There are some people that have 2 sets of DNA in one body. (How can that happen?)
Since the 1980s, DNA evidence has been used to investigate crimes, establish paternity, & identify victims of war & large-scale disasters.
DNA is considered to be individual evidence not class evidence!

3. History of Biological evidence in forensics
Several types of biological evidence (skin, blood, saliva, urine, semen, & hair) are used in forensics for identification purposes.
Is examined for the presence of inherited traits, such as blood type of enzyme variants.
When human cells are present in evidence- their chromosomes can be examined to determine if the evidence comes from a male or a female, called karyotyping.

4. History of Biological evidence in forensics
Blood-typing techniques- 1st used to make blood transfusions safer, can now be used to exclude suspects or to determine if blood found at the scene is from one person or multiple people. It can also be used in paternity testing.

5. History of Biological evidence in forensics
DNA FINGERPRINTING – (DNA profiling)
Used in legal cases to determine identity or parentage.
DNA can be extracted from small amounts of biological evidence, like a drop of blood or a single hair follicle.

6. The function & structure of Dna
DNA- contains all the genetic material for a cell.
Genetic information is stored in molecules of DNA making up structures called chromosomes.
Tease apart a chromosome, you can see each DNA strand is tightly coiled around protein molecules & itself, called a double helix.
The outer part, the backbone is made of alternating sugar and phosphate molecules
The rungs are made of pairs of molecules called nitrogenous bases.

7. The function & structure of Dna
The Different DNA Bases (4 bases)
A (adenine)
C (cytosine)
G (guanine)
T (thymine)
Each base is attached, on 1 end to the sugar molecule on the backbone and can hydrogen bond with 1 specific base on the other strand of DNA.
A only binds with T
C only binds with G
Human cells hold 23 pairs of chromosomes (46 total).
Exception – egg and sperm cells have 1 if each of the 23 chromosomes)
You inherit half your chromosomes from your mother and half from your father.
The human genome- total amount of DNA in a cell
Each chromosome contains 23,688 encoded genes, with each gene having about 3,000 base pairs. This is LESS than 1.5% of the DNA in the genome.
The rest is noncoding DNA- called junk DNA, has no known function!

8. The function & structure of Dna
Genes and Alleles
Genes – DNA sequences that have instructions that determine our inherited characteristics or traits, such as blood types
Alleles – 1 or 3 more alternative forms of a gene.
For example, one allele might code for normal hemoglobin, while another codes for abnormal hemoglobin. One comes from your mother, the other from your father.

9. DNA Identification
Junk DNA contains many variations that can be used to identify individual people based on those base pair patterns. –called polymorphisms in humans.
In 1984, Dr. Alec Jeffreys developed the 1st technique of isolating & analyzing these polymorphisms, called DNA fingerprinting.
In DNA fingerprinting, the variable regions appear as a pattern of bands (the base pairs)on X-ray film, that can be used to identify individual people.

10. Dna indentification
Forensic scientists focus on 2 types of repeating DNA sequences in the noncoding sections of DNA known as
VNTRs (variable numbers of tandem repeats) &
STRs (short tandem repeats)

11. VNTR
In the Junk DNA, certain short sequences of DNA are repeated multiple times & the number of copies of repeated base sequences can vary among individuals.
The length of a VNTR cane be from 9 – 80 bases in length.
For example –
Person 1 may have 3 copies of a repeated base sequence, CATACAGAC, while
Person 2 may have 7 copies of the same base sequence.

12. STR
DNA sequences with a high degree of polymorphism are most useful for DNA profiling.
A short tandem repeat (STR) is a short sequence of DNA, usually 2-5 base pairs in length within the Junk DNA.
For example, an STR, GATA may repeat 4 times: GATAGATAGATAGATA.
The use of STRs is becoming the preferred method of analysis because of its accuracy & because small & partially degraded DNA samples may be used to identify people.

13. DNA profile
VNTR and STR data from DNA fingerprints are analyzed for 2 main purposes
Tissue matching
2 samples that have the same band pattern are from the same person. Used in matching DNA evidence from a crime scene to a suspect or victim.
Inheritance matching
The matching bands in the DNA fingerprints must follow the rules of inheritance. Each band in a child’s DNA fingerprint MUST be present in at least one parent.

14. Population genetics & dna databases
Population genetics- the study of variation in genes among a group of individuals
For example – varying allele frequencies among populations concerns ABO blood types. Among the European populations of Great Britain & the U.S., 46% have type O blood. Among native South Americans, almost 100% have type O blood.
To calculate these percentages, population studies are done and the information is entered into a database.

15. Sources of DNA DNA is found in the nucleus of cells in the human body
A perpetrator may leave biological evidence, such as saliva, blood, seminal fluid, skin, or hair, at a crime scene.
This is individual evidence, not class evidence.
When a small amount of evidence is left at a crime scene, it is called trace evidence.
One problem with this is that all the trace evidence can be consumed during forensic testing.
The use of polymerase chain reaction (PCR) technique was developed in 1993 to create multiple copies of DNA evidence so it doesn’t all get used up.

16. Collection & preservation of DNA evidence
To avoid contamination, the following precautions are used in collecting evidence:
Wear disposable gloves & change them often.
Use disposable instruments for handling each sample.
Avoid touching the area where you believe DNA evidence may exist.
Avoid talking, sneezing, & coughing when collecting & packaging evidence.
Avoid touching your face, nose, & mouth when collecting & packaging evidence.
Air-dry evidence thoroughly before packaging.
Put evidence into new paper bags or envelopes.
If wet evidence cannot be dried, it may be frozen.
Keep evidence cool & dry during transportation and storage.

17. Steps of DNA fingerprinting
Extraction
Cells are isolated from the biological evidence.
The cells are disrupted to release DNA from the proteins and other cell components.
Then the DNA can be extracted from the cell nucleus.
Restriction Fragments
The DNA sample is mixed with a restriction enzyme to cut the long DNA strands into pieces called DNA restriction fragments (not needed in STR, just VNTR)

18. Steps of dna fingerprinting
Amplification
With some DNA analyses, PCR (polymerase chain reaction) can be used to amplify certain pieces of DNA that contain the VNTRs.
Electrophoresis
The DNA fragments are separated on an agarose gel.
An electric current is passed thru the gel separating the fragments by size.

19. DNA & Probes
A DNA probe is made up of a synthetic sequence of DNA bases that is complementary to a small portion of the DNA strand.
Anywhere the complimentary sequence is found on the DNA blot, the probe binds to it.
In most cases, 6-8 probes are used.

20. Analysis of DNA fingerprinting
Applications
Bands & widths are significant in S
Several countries have DNA databases
U.S. – CODIS: Combined DNA Index System
Every state maintains a DNA index of individuals who have been convicted of certain crimes, such as rape, murder, or child abuse.
DNA fingerprinting can
Match crime scene DNA with a suspect
Identify paternity and maternity
Eliminate a suspect
Free a falsely imprisoned person
Identify human remains

21. Preparing DNA Samples for Fingerprinting
DNA is mixed with special enzymes that cut the DNA in specific places, forming different sized fragments.
Using the process of gel electrophoresis