Fs Ch 14

Chapter 14

Biological Stain Analysis: DNA

Forensic ScienceRichard Saferstein

© 2009 Pearson Education, Upper Saddle River, NJ 07458.

All Rights Reserved.2

Nature of Blood Blood:-cells/enzymes/proteins/inorganic

substances Plasma:-fluid portion/water Red blood cells (erythrocytes) White blood cells (leukocytes) Platelets Antigens:-proteins-surface of red blood cells/blood-type




Blood Typing A-B-O and Rh systems Type A Type B Type AB Type O Rh factor:

-D antigen.

-Rh positive

-Rh negative. agglutination




Forensics of Blood1. Blood?

2. Species

3.Human-particular individual?

Preliminary color test.




Blood Tests Kastle-Meyer-color test-deep pink Luminol:-luminescent-old blood Microcrystalline tests:-Takayama -Teichmann




Blood Origin Testing Blood? Yes

Origin?

-precipitin test

-antisera




Individualizing Bloodstains Human? Yes:

-particular individual

-Deoxyribonucleic Acid (DNA)




Testing for Seminal Stains Sex offenses:

-seminal stains-acid phosphatase color test-purple color = acid phosphatase enzyme-spermatozoa-p30-DNA typing




Sexual Assault Evidence Victim

-medical examination-ASAP -clothing/hairs/vaginal and rectal-clothing removed/packaged separately -bedding/object




SEXUAL ASSAULT EVIDENCE Suspect:-24 hours-victim’s DNA -penile swab-clothing/pubic hair/head hair/penile swab-blood sample/buccal swab-physical contact between victim and

assailant-transfer:-blood/semen/saliva/hairs/fibers




Introduction DNA structure:-unique as fingerprints

Gene:-unit of heredity. -composed of DNA-positioned on chromosomes-threadlike bodies-nucleus of cell




Sex Determination X chromosome-mom Y chromosome-dad

-XX=female-YY=male




Alleles Locus:-gene position Alleles:-alternate form of genes Homozygous:-2 similar alleles Heterozygous:-2 different alleles




The Structure of DNA DNA:-large molecule/-nucleotides-sugar/phosphorous-containing group/nitrogen-

containing molecule called a base Four types:-adenine (A)-guanine (G)-cytosine (C) -thymine (T). double-helix configuration Base pairing-A w/TG w/ C




DNA Replication Prior to cell division/unwinding of double helix Strands exposed to collection of nucleotides:-recreate the double helix-letter by letter-base pairing

Polymerase chain reaction (PCR):-replicates small quantities-sample size no longer limiting




PCR Testing Advantage-amplify minute quantities-millions of times

Heated to separation Primers:-target specific regions-hybridize with the strands DNA polymerase/free nucleotides added:-rebuild each strands.-process repeated 25 to 30 times




DNA Typing DNA molecule

-sequences of bases repeated numerous times (tandem repeats)

-means of distinguishing one individual from another DNA typing.

-filler/spacers between coding regions of DNA

-all humans have same type of repeats-tremendous variation in number of repeats indivually




Short Tandem Repeats (STRs) STR analysis;-most successful/widely used DNA profiling

procedure-STRs: locations on chromosome that contain

short sequences that repeat themselves within the DNA molecule

-useful markers for identification -great abundance throughout the human

genome




STR Advantages STRs:

-repeating sequences of 3-7 bases in length-entire strand very short-less susceptible to degradation-recovered from bodies/stains subjected to

decomposition-ideal candidates for multiplication by PCR-overcoming limited sample size associated

with crime-scene evidence




The Power of STR Hundreds of different types of STRs are

found in human genes:

-more STRs characterized-smaller the percentage of the population a particular combination of STRs came from

-multiplexing-simultaneously extract/amplify combination

of different STRs




Standardizing STR Testing U.S. crime laboratories-standardized on 13

STRs for entry into CODIS

-high degree of discrimination/individualization attained by multiplexing/determining product of their frequencies

-125 picograms of DNA for analysis




Capillary Electrophoresis Capillary electrophoresis:- separates DNA fragments on a thin glass

column-end immersed in reservoir of buffer liquid

w/ electrodes-sample solution is injected into one end of

column-STR fragments move through the electrified

column at a speed that is related to the length of the STR fragments

-DNA peaks pass through the detector (electropherogram)




Sex Determination by STRs Amelogenin gene:

-tooth pulp-shorter by six bases in the X

chromosome-amelogenin gene amplified by

PCR/separated by electrophoresis-males-two bands-females-one band




Mitochondrial DNA Located outside nucleus:

-mother-structures found in all energy cells-single mitochondria contains

several loops of DNA




Mitochondrial DNA Testing Reserved for samples (hair) when STR not

possible-analysis more rigorous-time consuming-costly -compared to nuclear DNA analysis-same maternal lineage will be indistinguishable

Two regions highly variable-sequencing-determines the order of base pairs




CODIS DNA typing:-compare DNA types from crime scene

convicted sex offenders/felons CODIS (Combined DNA Index System):-computer software program -maintains DNA profiles from convicted

offenders/ unsolved crimes/profiles of missing persons




Packaging Biological Evidence Photograph/sketches

Latex gloves

Clothing from victim/suspect with blood evidence must be collected

Paper versus plastic




Packaging Biological Evidence

Package separately in a paper bag/well-ventilated box

Dried blood:

-sterile cotton swab w/distilled water

-air dry

-swab box

-paper/manila envelope

-refrigerated

-Standard/reference specimens collected

Technology

Fs Ch 14