The Indispensable Forensic Tool DNA The Indispensable Forensic Tool

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.

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.

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.

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.

DNA Replication

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.

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.

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

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.

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).

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.

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).

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

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.

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.

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

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)

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

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”

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.

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

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 20-100 times, these are Short Tandem Repeats (STR)

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

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.

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

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%.

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)

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.