The Indispensable Forensic Science Tool Tanya Ricketts DNA The Indispensable Forensic Science Tool Tanya Ricketts

Deoxyribonucleic acid – DNA 1985 – discovery that portions of the DNA structure of certain genes are as unique to each individual as fingerprints Alec Jeffreys and colleagues at Leicester University named process for isolating and reading DNA markers DNA fingerprinting

Later called DNA profiling and DNA typing READ page 384 – 387 OJ Simpson

WHAT IS DNA? Inside each of 60 trillion cells in the human body are strands of genetic material called chromosomes Along the ch. Are nearly 30,000 genes The gene is the fundamental unit of heredity. It instructs the body cells to make proteins that determine everything from hair color to susceptibility to disease

James Watson and Francis Crick – 1950s Deduced the structure of DNA Controls the genetic traits of all living cells, plants and animals Four types of bases associated with DNA Adenine, cytosine, guanine, and thymine

DNA is composed of two DNA stands coiled into a double helix A opposite T G opposite C Known as complementary base pairing

DNA is a polymer Nucleotide – unit of DNA consisting of one of four bases attached to a phosphate-sugar group Average human chromosome has DNA containing 100 million base pairs.

All of the human chromosomes taken together contain about 3 billion base pairs. From these numbers, we can begin to appreciate the diversity of DNA and hence the diversity of living organism consisting of the alphabet A,T,G,C

DNA at work Inheritable traits are controlled by DNA arise out of its ability to direct the production of complex molecules called proteins. Proteins are made by linking a combination of amino acids 20 known amino acids

DNA REPLICATION DNA replication begins with the unwinding of the DNA strands in the double helix. Each strand is then exposed to a collection of free nucleotides. Letter by letter , the double helix is recreated as the nucleotides are assembled in the proper order, base pairing

DNA polymerases are enzymes that assemble a new DNA strand in the proper base sequence determined by the original or parent DNA strand. PCR polymerase chain reaction – using DNA polymerases to copy a DNA strand located outside a living cell – new lab technology

Small quantities of DNA or broken pieces of DNA found in crime-scene evidence can be copied with the aid of a DNA polymerase A DNA Thermal Cycler is used for the copying process Sample size no longer a limitation in characterizing DNA recovered from csevi

R/ship b/t base letters on a DNA strand and the type of protein specified for manufacture by the sequence of these letters is called the genetic code

Key to understanding DNA typing lies in the knowledge that within the world’s population numerous possibilities exists for the number of times a particular sequence of base letters can repeat itself on a DNA strand. Possibilities become greater when one deals with two ch. Each containing different lengths of repeat sequ.

It’s the length differences associated with DNA strands or RFLPs(Restriction Fragment Length Polymorphisms) that allow forensic scientists to distinguish one person from another

Once DNA molecules have been cut up by the restriction enzyme, the resulting fragments must be sorted out – by electrophoresis – DNA from various sources, cut up by restriction enzymes, is placed in an separate lanes on an electrophoretic gel and subjected to an electric field

During this process, DNA fragments will migrate across a gelcoated plate – fragments are chemically treated so that the strands separate from each other RFLP DNA typing- used in impeachment trial of President Bill Clinton – semen stained dress of Ms. Lewinsky pg 372

SIGNIFICANCE About 25% of the DNA exams conducted by the FBI lab since 1989 have excluded suspects identified by police as the source of DNA evidence collected from the crime scene

Mitochondrial DNA Costly In circular or loop rather than linear strands Time consuming, rigorous Used where charred remains, small quantity such as a hair shaft Can use for maternal lineage

Individuals of same maternal lineage will be indistinguishable by mtDNA analysis READ STORY page 383 Criminalistics One of the most publicized cases performed on human remains was the id of the individual buried in the tomb of the Vietnam War’s unknown soldier

Were believed to belong to First Lt. Michael J Were believed to belong to First Lt. Michael J. Blassie, whose A-37 warplane was shot down near An Loc, South Vietnam, in 1977 1984 – US Army Central Identification Lab – failed to identify remains by physical characteristics

1998 – Blassie family, mtDNA analysis analysis and was confirmed to be consistent with Lt. Blassie’s family

Combined DNA Index System CODIS All 50 states have legislatively mandated the collection of DNA samples from convicted offenders of particular crimes and the establishment of DNA databases for law enforcement purposes

CODIS – computer software program developed by the FBI that maintains local, state, and national databases of DNA profiles from convicted offenders, unsolved crime scene evidence, and profiles of missing persons READ STORY page 390

Luminol – can be used to locate traces of blood and areas that have been washed nearly free of blood without compromising the potential for DNA typing READ CASE page 395

Criminalistics Chapter 13