Chapter 2: Sources of DNA Evidence

 DNA is located in cells which are the building blocks of the human body  Two Types of cells:  Sex Cells  Somatic Cells  Where are these cell types located in a human being?  How are they different from one another? 2

 DNA can be found in any nucleated cell  All nucleated cells in a body contain the same DNA  Rare mutations can lead to slight differences  Many types of crime scene evidence can contain nucleated cells and thus be a source of DNA for typing 3

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 Blood  Very common at violent crime scenes  Consists of erythrocytes (red blood cells), leucocytes (white blood cells), and platelets  Nuclear DNA obtained from leucocytes; erythrocytes and platelets lack nuclei and DNA 7

8 Nucleated leukocyte surrounded by enucleated erythrocytes

 Hair  Consists of root and shaft  Medulla  Cortex  Cuticle  Grows at approximately 0.3mm per day  Anagen phase- growing phase  Catagen phase- follicle becomes inactive  Telogen phase- growth cycle ends Forensic Biology by Richard Li9

10 A hair follicle

11 Cross section of a hair shaft

 Hair  The shaft contains very little nuclear DNA but lots of mitochondrial DNA  More mtDNA genomes per cell than nuclear genomes  The root contains enough nuclear DNA for standard DNA profiling  Rooted hairs are less common at crime scenes than unrooted (shed) ones  People shed more than 50 hair shafts per day  Rooted hairs must be pulled out by force Forensic Biology by Richard Li12

 Hair as a source of DNA evidence  PCR allowed more productive analysis of hair  Formerly just analyzed morphology and comparisons  mtDNA can be isolated from roots and shafts  Nuclear DNA analysis or hair is preferred ▪ Plucked hair with roots ▪ Naturally shed hairs contain little nuclear DNA ▪ Most hairs at crime scenes are naturally shed Forensic Biology by Richard Li13

14 Hair morphology: Differences in human hair color

15 Hair morphology: Absence (above) or presence (below) or medulla

16 Hair morphology: Distinct cuticle layering patterns

17 Pulled dreadlocks recovered from a crime scene

 Bone  Compact vs. Spongy ▪ Spongy degrades more quickly after death than compact  Osteocytes ▪ Most abundant cells in bone but cannot divide ▪ Contains most of the DNA in compact bone  Osteoblasts ▪ Produce new bone matrix  Osteoclasts ▪ Dissolve and recycle the bone matrix 18

19 Anatomy of a bone

 Bone  Rate of degradation of human remains varies greatly with environmental conditions  Soft tissue is lost  Bones are more stable  Mass Fatalities-identify victims  Sanding- used to reduce potential of comingled remains, adhering inhibitors, and bacterial contamination Forensic Biology by Richard Li20

 Teeth  Deciduous /Primary teeth (20) vs. Secondary (32)  Dentin- Does not contain cells  Pulp Cavity ▪ Contains lots of cells and DNA  Root ▪ Contains root canal with cells and DNA Forensic Biology by Richard Li21

22 An adult human tooth

 In addition to DNA analysis, teeth can be used to:  Identity a person ▪ Characteristics of teeth ▪ Alignment ▪ Overall structure ▪ X-rays  Compare suspect’s with bite marks on victim ▪ What else might be present on a bite mark that could help identify a criminal? 23