Forensic SciencePresents

DNA

B. History

1. James Watson and Francis Crick—in 1953 discovered the configuration of the DNA molecule.

2. Alec Jeffreys– the first to recognize DNA is unique to everyone. He isolated DNA markers and called them DNA “fingerprints.”

A. Terminology

1. Chromosomes –are strands of genetic material (DNA) The avg human chromosome contains 100 million base pairs

Genes – fundamental unit of heredity; they instruct cells to make proteins

Structure of DNA

Nucleotides - the “monomers” of DNA; consist of a 5 carbon sugar, a phosphate group, and a nitrogenous base. ( 4 bases are A,T,G,C ) Double Helix

A-T G-C

Protein Synthesis

. DNA codes for the assembly of Amino acids into proteins,

Each sequence of 3 bases codes for a particular Amino Acid (building blocks of proteins)

Human genome Project

A 13 year project designed to determine the order of bases on all 23 pairs of human chromosomes.

• The project is now complete.

• Knowing where on a specific chromosome DNA codes for a particular protein is useful for diagnosing and treating genetic diseases.

DNA Replication

• 1. DNA open up – helicase breaks H bonds.

• 2. DNA polymerase assembles New DNA strand in the proper sequence A-T, G-C

• (polymerase also makes needed corrections to new DNA stand)

• 3. Strands rewind and 2 identical strands are formed

Recombinant DNAopening up base pairs of the helix and

recombining it with another strand

Restriction enzymes

Chemicals that cut DNA into fragments that can later be incorporated into another DNA strand; about 150 are commercially available. Used in recombinant DNA and DNA Typing

At a crime scene

• There is usually a small sample of genetic material……if it can be isolated and not contaminated….

• The DNA found must be copied for multiple tests

DNA Typing (DNA fingerprinting DNA

Profiling)Tandem repeats – portions of DNA that contain sequences repeated many times, not used to code for proteins (30% of human genome contains the repeated segments of DNA. These can be used to ID humans

These are called RFLP (restriction fragment length polymorphisms) restriction enzymes can be used to cut out these tandem repeatsCan also use Short Tandem Repeats (STR) – requires less DNA, newest method, ideal for PCR, less susceptible to degradation. CODIS requires 13 STR

What if there isn’t enough DNA in the sample?

• Tiny amounts of DNA can be amplified (copied) by a technique called PCR (polymerase chain reaction)

Steps for DNA Typing1. Collect DNA2. Use polymerase chain reaction to create enough

DNA to test

3. Use restriction enzyme to cut the DNA into segments

4. Gel electrophoresis is then used to sort out the fragments

5. DNA band pattern is transferred to a nylon membrane (called southern blotting)

6. DNA probe is added to bind to specific fragments

7. X-ray film detects the probes, and is developed revealing the DNA fingerprint pattern

Genetic technology terms

• Restriction enzymes- enzyme that cuts DNA at a specific sequence of nucleotides

• Restriction enzymes are specific- They will cut a particular sequence at only a particular site

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Electrophoresis

An electrical current moves through a substance causing molecules to sort by size.

Smaller, lighter molecules will move the furthest on the gel.

Electrophoresis

Pipette the DNA.

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Electrophoresis (cont.)

Load DNA into the gel wells.

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Electrophoresis (cont.)

• Run the gel.

• Observe and compare bands of DNA.

DNA plus restriction enzyme

Mixture of DNA fragments

Gel

Power source

Longer fragments

Shorter fragments

Section 13-2

Figure 13-6 Gel Electrophoresis

Go to Section:

Process of Gel Electrophoresis – sorts out the fragments into sections

DNA fingerprintDNA is negative, so will move away from

the wells toward the other end of the gel…smallest pieces travel the furthest!!

Figure 20.17 DNA fingerprints from a murder case

I. Crime Scene DNA:If you are the DNA analyst, you should

conclude that: A. Both Suspects A and B are excluded as

the source of the evidence. B. Suspect A is excluded as the source of

the evidence, but Suspect B cannot be excluded.

C. Suspect B is excluded as the source of the evidence, but Suspect A cannot be excluded.

D. Neither Suspect A or B can be excluded as a source of the evidence.

E. Suspect B cannot be excluded as a source of the evidence. The results with Suspect A are inconclusive.

II. Crime Scene DNA:If you are the DNA analyst, you should

conclude that: A. Both Suspects A and B are excluded as

the source of the evidence. B. Suspect A is excluded as the source of

the evidence, but Suspect B cannot be excluded.

C. Suspect B is excluded as the source of the evidence, but Suspect A cannot be excluded.

D. Neither Suspect A or B can be excluded as a source of the evidence.

E. Suspect B cannot be excluded as a source of the evidence. The results with Suspect A are inconclusive.

Gel Electrophoresis Video clip

• http://www.youtube.com/watch?v=6_4AY3lYRgo

To reveal the DNA fingerprint

Radioactive probe is added to bind to specific fragments

autoradiography-a nylon sheet is placed against x-ray film and exposed for several days.

Mitochondrial DNA

location—in the cytoplasm (in the mitochondria)Inherited solely from mom to offspringImportance – more mitochondria can be found

in one cell so you don’t need as much

Difference between mtDNA and nuclear DNA testing – MtDNA is more rigorous, time consuming and more costly$$$

Collection and System ID of DNA

1. CODIS – Combined DNA Index System; a national system of DNA identification. Forensic labs can store and match DNA records from convicted offenders and crime-scene evidence.

2. Steps taken before collection – notes, sketches, close-up photography

Biological materials used for DNA profiling

• Blood• Hair• Saliva (secreter)• Semen• Body tissue cells• DNA samples have been

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

Collection of DNA

Precautions - disposable latex gloves; glasses or goggles; face masks; shoe covers; coveralls; very little personal contact!

Packaging – separately in a paper bag or well-ventilated box. Allow swabs to dry 5-10 minutes and place in a manila envelope. Place in refrigerator or cool place.

Example

• A 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:

Was the suspect at the crime scene?

Suspects Profile

Blood sample from crime scene

Victims profile

Solving Medical Problems

DNA 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

Example: A Paternity Test

• By 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.

Is this man the father of the child?

Mother Child Man