FORENSICS & DNA

The Cell

• Smallest unit of life

• Compose all living things

• The “nucleus” (one of many organelles) contains genetic information the cell needs to exist and reproduce

- most cells organize genetic information into chromosomes

DNA

• Each chromosome and every gene is made of deoxyribonucleic acid (DNA)

• DNA is a polymer of repeating units called nucleotides

• Each nucleotide contains three parts

- phosphate group

- sugar (deoxyribose)

- nitrogenous base

The DNA Backbone

• Nucleotides are linked together with alternating P-S-P-S-P……..

The DNA Double Helix

• DNA is normally double stranded

• The two nucleotide chains are held together by hydrogen bonds

•A always pairs with a T on the other strand; C always pairs with G

The DNA Double HelixThe two strands wrap around each other to form

helical structure shown (double helix)

DNA and EVIDENCEEach person’s DNA is different than other people◦Exception: identical twins!

DNA collected from a crime scene can either link a suspect to the evidence or eliminate a suspect, similar to the use of fingerprints.

DNA and EVIDENCEDNA can identify a victim through DNA from relatives, even when no body can be found.

DNA can link crime scenes together by linking the same perpetrator to different scenes locally, statewide, and across the nation.

Factors that Affect DNA EvidenceEnvironmental factors

◦Heat/sunlight◦Moisture◦Bacteria◦Mold

DNA testing cannot identify when the suspect was at the crime scene or for how long

Polymerase Chain Reaction (PCR)

• Technique devised in 1983 to amplify small amounts of DNA

• Can be performed on DNA from a single cell

- cigarette butt, a licked stamp, root of a single hair, 1/50,000 a drop of blood (0.1 microliters)

• The amplified DNA can then be used to:

- identify a suspect or victim

-determine an individual’s sex

-determine species (if not human)

PCR to Amplify a Person’s DNA

Steps Involved:

1. Isolate repeating loci from a person’s DNA using restriction enzymes

2. Design primers – short segments of synthetic DNA that are complementary to DNA on either side of the VNTR regions

3. Add vast excess of the primers and heat mixture to 75 oC

This causes DNA strands to separate by breaking hydrogen bonds between bases

4. Cool to 15 oC. Primers hydrogen bond (anneal) to complementary strands

5. Add DNA polymerase and all four types of nucleotides. The polymerase (enzyme used in DNA replication) will fill in the rest of the two strands.

You now have two identical copies of the DNA you started with.

6. Repeat steps. Heat to break hydrogen bonds. Cool to anneal more primers (still there in vast excess). Allow DNA polymerase to fill in the remaining strands. Two strands of DNA become four. Etc…Etc…Etc…..

PCROriginally, the DNA polymerase would have to be added between each heating step because it would fall apart at 75 degrees. Now, an enzyme called Taq DNA polymerase is added. This is a very stable enzyme isolated from bacteria living at thermal vents in the ocean (up to 95 oC)

In just 32 rounds of PCR, 1 copy of DNA becomes 4.2 billion copies. This would take about 3 hours to perform in lab.

PCR Animation

DNA ProfilingTechnique used by scientists to distinguish between individuals of the same species using only their DNA

InventorAlec JeffreysUniversity of Leicester in 1985

He was knighted in 1994

ElectrophoresisStep 1:

◦Cells are broken down to release DNA

◦If only a small amount of DNA is available it can be amplified using PCR

Stages of DNA ProfilingStep 2:

◦The DNA is cut into fragments using restriction enzymes

◦Each restriction enzyme cuts DNA at a specific base sequence

Stages of DNA ProfilingThe sections of DNA that are cut

out are called restriction fragments.

This yields thousands of restriction fragments of all different sizes because the base sequences being cut may be far apart (long fragment) or close together (short fragment).

source: www.worldofteaching.com

Stages of DNA Profiling

Step 3:Fragments are

separated on the basis of size using a process called gel electrophoresis.

DNA fragments are injected into wells and an electric current is applied along the gel.

source: www.worldofteaching.com

Stages of DNA Profiling

DNA is negatively charged so it is attracted to the positive end of the gel.

The shorter DNA fragments move faster than the longer fragments.

DNA is separated on basis of size.

source: www.worldofteaching.com

Stages of DNA Profiling

A radioactive material is added which combines with the DNA fragments to produce a fluorescent image.

A photographic copy of the DNA bands is obtained.

source: www.worldofteaching.com

Stages of DNA Profiling

Step 4:The pattern of fragment

distribution is then analysed

source: www.worldofteaching.com

DNA FingerprintingUsed to identify individuals by their repeat regions (usually STR)

regions:

Steps involved:

1. Isolate and amplify DNA if needed

2. DNA is cleaved into smaller pieces with restriction enzymes

3. DNA is separated with gel electrophoresis

4. DNA is transferred to a nylon membrane (Southern blotting)

5. A radioactive primer is designed that will be complementary to unique regions (STR, etc, regions). Add this to nylon membrane containing DNA.

6. Wash off excess primer and hold nylon up to a photographic plate to expose. The pattern will be unique to the individual.

Clearly, suspect one is the match…..

If all STR regions are considered, there is a one in 3.4 billion chance of error. This means there may be one other person on the planet that would be too similar to tell the difference.

If all other satellite regions are also considered, the chances of error go way, way down…

1 in 53,581,500,000,000,000,000

Uses of DNA ProfilingDNA profiling is used to solve crimes and medical problems

CrimeForensic science is the use of

scientific knowledge in legal situations.

The DNA profile of each individual is highly specific.

The chances of two people having exactly the same DNA profile is 30,000 million to 1 (except for identical twins).

source: www.worldofteaching.com

Biological materials used for DNA profiling

BloodHairSalivaSemenBody tissue cellsDNA samples have

been obtained from vaginal cells transferred to the outside of a condom during sexual intercourse.

source: www.worldofteaching.com

DNA Profiling can solve crimes

The pattern of the DNA profile is then compared with those of the victim and the suspect.

If the profile matches the suspect it provides strong evidence that the suspect was present at the crime scene (NB:it does not prove they committed the crime).

If the profile doesn’t match the suspect then that suspect may be eliminated from the enquiry.

source: www.worldofteaching.com

ExampleA violent murder occurred. The forensics team retrieved a

blood sample from the crime scene.

They prepared DNA profiles of the blood sample, the victim and a suspect as follows:

source: www.worldofteaching.com

Was the suspect at the crime scene?

Suspects Profile

Blood sample from crime scene

Victims profile

source: www.worldofteaching.com

Solving Medical ProblemsDNA profiles can be used to determine

whether a particular person is the parent of a child.

A childs paternity (father) and maternity(mother) can be determined.

This information can be used in• Paternity suits• Inheritance cases• Immigration cases

source: www.worldofteaching.com

Example: A Paternity TestBy comparing the DNA profile of a mother and her child it is possible to identify DNA fragments in the child which are absent from the mother and must therefore have been inherited from the biological father.

source: www.worldofteaching.com

Is this man the father of the child?

Mother Child Man

source: www.worldofteaching.com

A. Who done it?

Which suspect matches the bloodstain?

B. Whose your daddy?Which sample is most likely to be the father?

F1 or F2

C. Identical or not?

Which sets of twins are identical twins?

The Killer’s Trail

Video QuizChoose the best answer for each.

1. Who was the victim?

A. Marilyn Sheppard B. Sam Sheppard C. Sam Sheppard, Jr.

2. What are the keys to DNA fingerprinting?

A. Chromosomes B. Alleles C. Nitrogen bases

3. Where did the scientist get the sample of DNA for Marilyn Sheppard?

A. Hair B. Skin C. Fingernail

4. Whose blood was found in the blood trail?

A. Marilyn Sheppard B. Sam Sheppard C. Neither

Famous Cases

Colin Pitchfork was the first criminal caught based on DNA fingerprinting evidence.

He was arrested in 1986 for the rape and murder of two girls and was sentenced in 1988.

source: www.worldofteaching.com

Famous Cases

O.J. Simpson was cleared of a double murder charge in 1994 which relied heavily on DNA evidence.

This case highlighted lab difficulties.

source: www.worldofteaching.com

CODIS – Combined DNA Index System

• National software developed by the FBI

• Distributed to local, state, and national crime labs

• All 50 states mandate inclusion of DNA fingerprint (if available) from violent and sexually motivated crimes

• Mostly a database of STR regions

• Thousands of matches have led to the capture of criminals that otherwise would not have been caught

This has led numerous people to suggest a national DNA database that would include only polymorphism information…

The 23 human chromosomes and 13 chromosomal locations at which the FBI looks for repeating DNA sequences.

Who Robbed the Bank?A robbery takes place at a bank. As the

thief escapes the building, a security guard grabs one of the bank robber's gloves.

The bank robber leaves the scene in a phone service van.

The phone company identifies three employees who may have been in the vicinity of the bank at the time of the robbery. 

Police found hair with DNA in the glove