1. The Indispensable Forensic Tool
DNA
The Indispensable Forensic Tool

2. DNA Review
DNA is the keeper of the all the information needed to recreate an organism. All DNA is made up of a base consisting of sugar, phosphate and one nitrogen base. There are four nitrogen bases, adenine (A), thymine (T), guanine (G), and cytosine (C). The nitrogen bases are found in pairs, with A & T and G & C paired together.

3. More DNA Review
The sequence of the nitrogen bases can be arranged in an infinite ways, and their structure is known as the famous “double helix”. The sugar used in DNA is deoxyribose. The four nitrogen bases are the same for all organisms. The sequence and number of bases is what creates diversity.

4. Even More DNA Review
DNA does not actually make the organism, it only makes proteins. The DNA is transcribed into mRNA and mRNA is translated into protein, and the protein then forms the organism. By changing the DNA sequence, the way in which the protein is formed changes. This leads to a different protein.

6. DNA Replication
DNA replication begins with the unwinding of the double helix. Each strand is then open to collect free nucleotides. The result is two identical copies of DNA.
The cell passes on this DNA when it divides.

7. DNA Replication

8. DNA Polymerases
There are enzymes and proteins that are responsible for unwinding and assembling a new DNA strand in the proper sequence.
DNA polymerases are enzymes that assemble a new DNA strand.
The polymerases also “proofread” the growing DNA strand to ensure there are no mismatches (mutations) in base pairs.

10. PCR
Polymerase Chain Reaction (PCR), is the technique of replicating a portion of a DNA strand outside a living cell. PCR allows the forensic scientist to replicate minute samples of evidence for analysis. The forensic scientist is no longer limited by sample size.

11. Polymerase Chain Reaction
Amplify the amount of DNA by PCR:
Separate strands of DNA - Heat
Add primers to the single strands
Add polymerase and free nucleotides
Two new DNA molecules are formed

13. Recombinant DNA
Recombinant DNA is the general name for taking a piece of one DNA, and and combining it with another strand of DNA. Thus, the name recombinant!
By combining two or more different strands of DNA, scientists are able to create a new strand of DNA.
Scientists are no longer limited by sample size.

15. Recombinant DNA
Once a portion of the DNA strand has been cut out with the aid of a restrictive enzyme, the next step in the recombinant DNA process is to insert the isolated DNA segment into a foreign DNA strand (usually bacterium).

16. Recombinant DNA
As the bacteria multiplies, the new recombinant DNA is passed on.
Recombinant DNA can be used to make large samples of DNA evidence for identification of suspects.

17. Restrictive Enzymes
Recombinant DNA relies on the ability of certain chemicals, known as Restrictive Enzymes to cut DNA into fragments that can later be incorporated into another DNA strand (Recombiant DNA).

18. Restriction Enzymes
In other words, Restrictive Enzymes are chemicals that act as scissors to cute DNA molecules at specific locations.

21. DNA Fingerprinting
DNA fingerprint (genetic fingerprint)- a technique for comparing the nucleotide sequences of fragments of DNA from different sources. The fragments are obtained by treating the DNA with enzymes that break DNA strands at specific sites.

22. DNA Fingerprinting
A method used to identify multiple DNA banding patterns that are specific to an individual by exposing a sample of the person's DNA to molecular probes and various techniques. DNA fingerprinting is often used to provide evidence in criminal law cases.

23. DNA Fingerprinting DNA fingerprinting can be used to:
Trace inheritance of genetic disorders
Identify the origins of blood, semen, hair or saliva sample in a criminal investigation
Establish paternity or maternity

25. DNA Fingerprinting
Accuracy of DNA fingerprinting can be within 1 in 10,000,000,000 people
Limitations in the techniques prevents two samples from being identified as “Perfect matches” (results are in statistical probabilities i.e. percentages)

33. Polymorphism A polymorphism is a variation
For every 1,000 nucleotides inherited there is one variation
Change in length of DNA fragments is produced by the digestion of restriction enzymes
Exact numbers digested differs from person to person

35. RFLP
Fragments resulting from polymorphisms are called Restriction Fragment Length Polymorphism (RFLP)
These RFLP’s can be separated by electrophoresis to create a “DNA Fingerprint”

36. RFLP
Scientists have identified 3,000 RFLP’s in the human genetic code- many of which are highly variable among individuals.
It is this variability that allows scientists to identify unique individuals.

37. RFLP Steps of making an RFLP:
Add a restriction enzyme to cut the DNA into fragments - Exact number and size of fragments produced varies from person to person
Fragments are separated by electrophoresis
The smaller fragments travel further than the large fragments
This creates a DNA Fingerprint

42. VNTR’s Variable Numbers of Tandem Repeats (VNTR)
Most of the DNA in a chromosome is not used for the genetic code
These regions contain nucleotide sequences that repeat from times, these are Short Tandem Repeats (STR)

43. VNTR
There are several variations in these STR’s in the human population
Scientists can look at 4-6 different STR’s in the same person –the more STR’s identified the higher probability of a match

44. VNTR VNTR is more commonly used than RFLPs Advantages of VNTR to RFLP
Less complex patters
STR’s are less susceptible to degradation because they are usually less than 400 base pairs
Can be used on stains or materials that are older or exposed to extreme decomposition.

48. Mitochondrial DNA DNA in the mitochondria
Inherited maternally (from mother)
Forensic Casework
Analyze sample materials that are not suitable for nuclear DNA
Test hair strands without the root
Test highly degraded tissue
Test skeletal remains

51. Problems with mDNA Used for determining maternal relationships only
Results take up to two months to complete
The specific tested alleles cannot be compared against standard tested alleles
Exclusions are 100%, Inclusions are 95%.

52. CODIS Combined DNA Index System (CODIS) The FBI’s DNA database
Built from the databases of 40 states that mandated DNA samples from convicted offenders (esp. sexual offenders)

54. CODIS
1991- Minnesota became the first state to match a DNA sample left at the crime scene of a crime with a sample on file in the DNA database. At the crime there was no other investigative evidence.