Forensic DNA analysis

Deoxyribonucleic acid (DNA) is the genetic material that is unique to each individual. Probability of a match is about one in 6 billion (approximately the same as the world population)

Two discoveries in the 80’s led to its use in forensics

Discovery of Polymerase Chain reaction (PCR) by Kary Mullis at Cetus to amplify copies of DNA molecules
Alec Jeffries used restriction enzymes to cut DNA of repeated sequences (short tandem repeats) to restriction fragment length polymorphisms (RFLP) for DNA fingerprinting

To save DNA samples store dry samples (hair) in paper bags and freeze wet samples (blood) immediately to prevent degradation. Collect all samples with sterile gloves and sterile tools and make sure they do not touch one another or mix.

Structure: DNA is made of individual subunits called nucleotides each having a 5 carbon sugar, deoxyribose, a nitrogen base (adenine, thymine, cytosine, and guanine), and a phosphate group.

Nuclear DNA is inherited from mom and dad but mitochondrial DNA is inherited only from the maternal side.

Chargaff’s rules state that A pairs with T and C pairs with G to make a double-stranded helix, whose structure was described by Watson and Crick in 1953.

Short tandem repeats are repeated different numbers of times for different people. Since they are short, they decompose more slowly than larger molecules. Mom and Dad will be either homozygous or heterozygous for each section of DNA (have the same or different numbers of a particular repeat, for instance Mom may have a gene for 5 and a gene for 8 and Dad may have two genes for 6..). During meiosis, formation of an egg or sperm cell, the two versions separate so each child will get one version (called an allele) from Mom and one version from Dad. During fertilization the sperm and egg unite to make a new individual. Genes sort randomly, but a child will have one allele from Mom and one version from Dad.

Gel electrophoresis: Cut DNA into fragments with enzymes and place on a gel. Run in an electric field. Because of the negatively-charged phosphate groups, fragments move toward the positive electrode. Smaller pieces move more quickly.

In the US, there are a standard 13 different STRs that are stored in the Combined DNA Index System (CODIS). The probability of sharing all of the same genes is very small.