DNA

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DNA

• The Indispensable Forensic Science Tool
• Tanya Ricketts

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• 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

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

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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

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• 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

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

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• 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.

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• 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

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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

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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

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• 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

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• 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

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• 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

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• Key to understanding DNA typing lies in the
  knowledge that within the worlds 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.

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

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• 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

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• 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

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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

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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

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• 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 Wars unknown soldier

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• 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

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• 1998 Blassie family, mtDNA analysis analysis
  and was confirmed to be consistent with Lt.
  Blassies family

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Combined DNA Index SystemCODIS

• 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

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• 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

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• 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

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• Criminalistics Chapter 13