1. Unit 4: DNA Evidence
Structure of DNA

2. What is DNA? Stands for Deoxyribonucleic Acid
Carries our genetic material from one generation to the next in all living things
DNA is arranged in a double helix-two strands are twisted around each other like a winding staircase.
If you unwind a single human chromosome of DNA, it will be about 5 feet long!

3. What does our DNA do?
Only about 8% of our genetic sequence is made up of our genes
Up until recently, scientists believed that the rest of our DNA was noncoding-they thought these sequences were “junk”!

4. However, a research project called Encode studied these sections and found:
Parts of the DNA that don’t code for genes contain about 4 million gene switches that have important roles:
Determine when the gene turns on and off
Determine how much protein to make
Switches for a variety of diseases
Affects every cell, but can do so at different times in our lifetime
Can be sequenced and used for identification purposes in crime cases
Examples:
Instructions for the cells to be different types of cells (brain cell vs liver cell)
Instructions for pancreas cell to make insulin after a meal
Instructions for cells to reproduce to replace dead or damaged cells (like skin cells replacing those that sloughed off)

5. Structure
Nucleic Acids: Macromolecules, made up of nucleotides linked together
Nucleotides are made up of 3 things:
A phosphate group (same on all nucleotides)
5 Carbon sugar molecule (same on all nucleotides)
Nitrogen containing base (Different on each nucleotide)
Adenine (Purines: 2 rings of
Guanine Carbon and Nitrogen)
Thymine (Pyrimidines: single ring of
Cytosine Carbon and Nitrogen)

6. Nucleotides

7. Structure
The phosphate of one nucleotide is attached to the sugar of the next nucleotide with a covalent bond.
The base pairs on each strand of DNA are bound together with hydrogen bonds.

8. Base Pairing Purines are always paired with Pyrimidines
Adenine always pairs with Thymine (forms 2 hydrogen bonds)
Guanine always pairs with Cytosine (forms 3 hydrogen bonds)
This is called complementary base pairing-the sequence on one strand determines the sequence on the following pair.
DNA has a direction-it is polar!
It goes from the 5’ end to the 3’ end.
The opposite strand also goes from the 5’ to 3’ end, but in the opposite direction!

9. Mitochondrial DNA
The mitochondria in our cells also contains its own DNA (mtDNA) that tells the mitochondria what to do and how to replicate.
Mitochondrial DNA comes from our mothers
This is because the portion of sperm that is able to penetrate the egg does not contain mitochondria.
Mitochondrial DNA is much more prevalent than nuclear DNA and therefore might be easier to find and analyze at a crime scene.
However, since the DNA comes from our mother, it is identical in all children produced by the same woman.

10. Collection of Biological Materials

11. Arriving At a Crime Scene
When biological materials are found at a crime scene, the first step is to protect the investigator and the sample from contamination by taking precautionary measures.
A scientist should always assume that biological materials are contagious and should wear the following protective gear:
Latex or Nitrile Gloves
Face mask
Boot covers
Eye protection
Lab coat/coveralls
Once the investigator has on the proper protective gear, photographs and sketches of the blood should be taken at the scene.
A substrate control should also be collected at the scene.
A substrate control is a piece of the material that is close to but not part of the biological material being collected.

12. Collecting Evidence Possible Sources of DNA: Blood Semen Sweat Skin
Saliva
Buccal swabs: a swab is used
to collect cheek cells out of
your mouth

13. Collecting Evidence (cont.)
Collecting DNA samples:
Biological materials should be placed in a paper bag or well ventilated box to discourage the growth of bacteria
Dried blood is swabbed with a Qtip that has been dipped in distilled water. The Qtip then goes into a well ventilated box for transport.
Soil that contains blood is packaged in an airtight container and immediately frozen. This is to discourage the growth of bacteria that is naturally in the soil.

14. Types of DNA Analysis

15. DNA Analysis Techniques
Before any analysis can be done, the DNA sample has to be amplified, or copied, through a process called Polymerase Chain Reaction (PCR).
After enough of the sample is created, we can use one of two techniques to analyze the sample:
Restriction Fragment Length Polymorphism (RFLP): a sample of DNA is separated using gel electrophoresis based on the size of the DNA fragments.
Short Tandem Repeats (STR): looks at the non-coding part of our genome and counts how many times a specific sequence of nucleotides repeats.