DNA

General DNA Information Double helix—two coiled DNA strands Composed of nucleotides—units containing sugar molecule (deoxyribose), phosphate group nitrogen-containing base adenine deoxyribose PO4

General DNA Info In humans, the order of these bases is 99.9% the same. Four bases Adenine Cytosine Guanine Thymine Bases always pair A to T and G to C

Where Is DNA Found? Genes are portions of DNA that code for specific proteins DNA is found in all nucleated body cells—white blood cells, semen, saliva, urine, hair root, teeth, bone, tissue Most abundant in buccal (cheek) cells Red blood cells have no nuclei; and therefore, no nuclear DNA

Types of DNA Nuclear Mitochondrial found in the nucleus constitutes 23 pairs of chromosomes inherited from both parents each cell contains only one nuclei Mitochondrial found in the cytoplasm is inherited only from mother each cell contains hundreds to thousands of mitochondria can be found in skeletal remains

Mitochondrial DNA Analysis of mDNA is more: rigorous time consuming costly than nucleic testing of DNA mDNA is constructed in a circular or loop

Polymerase Chain Reaction Technique used for making copies of a particular part of DNA This can be valuable when the amount of evidence is minimal. Millions of copies of DNA can be made from a single speck of blood.

PCR The outcome is a doubling of the number of DNA strands. The cycle is typically 25 to 30 times, yielding more than one million copies of the original DNA molecule. Each cycle takes less than 2 min from start to finish.

PCR Procedure 1) Heat the DNA strands, causing the strands to separate (unzip). 2) Cool the mixture and add a primer, a short sequence of base pairs that will add to its complementary sequence on the DNA strand. 3) Finally, add a DNA polymerase and a mixture of free nucleotides to the separated strands.

PCR Animation PCR Animation

Electrophoresis Separates DNA fragments by length Longer strands move through the gel more slowly The smaller, lighter molecules will move the furthest on the gel.

Electrophoresis Your restriction fragments will be different lengths than someone else’s restriction fragments Every individual’s electrophoresis results look different because everyone has unique DNA sequences

Electrophoresis

Short Tandem Repeats (STRs) Shorter than restriction fragments from PCR More easily recovered from the often degraded and limited quantities of DNA typically found in evidence. 13

Short Tandem Repeats (STRs) The FBI uses 13 core STRs for identification of Americans. 14

Short Tandem Repeats (STRs) For all 23 PAIRS of chromosomes, 1 is from mom, and 1 is from dad. 15

Short Tandem Repeats (STRs) 16

Combined DNA Index System FBI’s CODIS DNA Database Combined DNA Index System Launched October 1998 Links all 50 states Requires >4 RFLP markers and/or 13 core STR markers Used for linking serial crimes and unsolved cases with repeat offenders

How to Collect DNA Evidence 1. Wear disposable gloves and change them often. 2. Use disposable instruments for handling each sample. 3. Avoid touching the area where you believe DNA may exist. 4. Avoid talking, sneezing, and coughing over evidence. 5. Avoid touching your face, nose, and mouth when collecting and packaging evidence. 6. Air-dry evidence thoroughly before packaging. 7. Put evidence into new paper bags or envelopes. 8. If wet evidence cannot be dried, it may be frozen.

Chimeras Rarely, people can have 2 different sets of DNA Can be caused by a twin dying early in utero Resultantly, can cause 2 blood types and different DNA in sex cells and other cells Lydia Fairchild tested as not the biological mother of her children

Chimeras & Bone Marrow Bone marrow transplants can cause chimerism New bone makes blood cells of original donor in new person’s body Alaskan case where DNA taken matched a person in jail. He had donated bone marrow to his brother, who now had his brother’s DNA in his body.