Chapter 13 DNA: The Indispensable Forensic Science Tool

DNA Deoxyribonucleic acid A polymer In 1985 it was discovered that certain genes are as unique as fingerprints DNA markers Discovered in 1868 In the 1950s, James Watson and Francis Crick figured out the structure of DNA

DNA base pairing There are four DNA bases Adenine, Cytosine, Guanine, and Thymine They link together to form DNA, with pairs spanning the double-helix A and T always pair C and G always pair The sequence is different for each individual

Proteins Play basic roles in the structures and functions of living things Made of amino acids There are 20 They link to form proteins Different sequences make different proteins

Abnormalities A difference of one amino acid in a protein can cause big differences

Human Genome Total DNA content About 3 billion base pairs Has been mapped

DNA replication Making new DNA from existing DNA Two strands unwind Each recreates the double helix

PCR Polymerase chain reaction Used to copy and fill in small pieces of DNA from crime scenes Sample size is no longer a limit

Recombinant DNA Cutting DNA into fragments that can later be incorporated into another DNA strand Uses restriction enzymes Chemicals that act as scissors to cut DNA at specific locations Used for genetic engineering Making fish grow faster Growing insulin through bacteria Making plants grow faster and/or produce higher yields

Tandem Repeats A region of a chromosome that contains a repeating DNA sequence About 30% of human genome Origin and significance not known Possibly fillers Helpful in DNA typing Distinguish individuals

RFLP Restriction Fragment Length Polymorphisms Repeat segments that are cut from DNA molecule with restriction enzymes Used for DNA typing

Hybridization Joining two complementary strands of DNA to form a double-stranded molecule

DNA technology over time 1985 to mid 1990s RFLP (Restriction Fragment Length Polymorphisms) RFLP Portions of DNA are repeated numerous times (over 30%) Act as fillers or spacers Restriction enzymes are used to cut the fillers Everyone has different numbers and order of these fillers Mid 1990s on PCR (Polymerase Chain Reaction) Copies or multiplies DNA strands

Short Tandem Repeat (STR) Latest method of DNA typing Locations on DNA that contain short segments of 3 – 7 repeating base pairs Abundant in human genome Easier to multiply using PCR 13 STRs used by CODIS

Multiplexing Simultaneously detecting more than one DNA marker in a single analysis

Amelogenin Gene Used to determine gender Not one of the 13 CODIS STRS, but can also be useful

Y-STRs STRs on the Y chromosome Useful for separating DNA when multiple males are involved in a sexual assault

Sequencing A procedure used to determine the order of the base pairs that make up DNA

Mitochondria Small structures in the cell Supply energy to the cell Contain maternal DNA Mitochondrial DNA or mtDNA More difficult, time-consuming, and costly to analyze than nuclear DNA

mtDNA Circular or loop configuration Contains 37 genes Found in hair that lacks nuclear DNA Found in skeletons Reference samples can be taken from maternal relatives

mtDNA First used in State of Tennessee v. Paul Ware in 1996 Used to identify the individual buried in the tomb of the Vietnam War’s unknown soldier

CODIS Combined DNA Index System Local, Regional, and National DNA databases DNA is collected from convicted offenders of particular crimes (especially sexual assault) Also contains DNA from unknown suspects and missing persons

DNA evidence Only about 36 cells are needed to get an STR profile Low copy number – fewer than 36 cells Epithelial cells Outer layer of skin cells Easily rubbed off Can yield DNA

Collecting evidence Must be photographed first Don’t disturb blood pattern until after it has been analyzed for shape and location Minimal personal contact Assume all fluids are infectious Each stained article should be packaged separately in a paper bag or a well-ventilated box

Substrate control Unstained material from near the bloodstain Confirm test results Swabs must be dried before being packaged Takes 5 – 10 minutes Biologicals should be stored in a cool place, refrigerated, or frozen until testing

Collecting reference samples Blood samples Buccal swabs Inner cheek lining Hairbrushes, toothbrushes, etc.

Contamination Coughing or sneezing on sample Incorrect packaging Avoiding Change gloves for each piece of evidence Substrate controls Use disposable forceps (tweezers) to pick up evidence Separate containers