Forensic Biology by Richard Li

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Forensic Biologyby Richard Li

• Chapter 2 Sources of DNA Evidence

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Introduction to Cells

• 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?

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Introduction to Cells

• 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

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Examples of Biological Evidence

• 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

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Nucleated leukocyte surrounded by enucleated
erythrocytes
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Examples of Biological Evidence

• 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

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A hair follicle
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Cross section of a hair shaft
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Examples of Biological Evidence

• 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

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Examples of Biological Evidence

• 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

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Hair morphology Differences in human hair color
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Hair morphology Absence (above) or presence
(below) or medulla
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Hair morphology Distinct cuticle layering
patterns
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Pulled dreadlocks recovered from a crime scene
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Examples of Biological Evidence

• 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

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Anatomy of a bone
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Examples of Biological Evidence

• 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

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Examples of Biological Evidence

• 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

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An adult human tooth
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Examples of Biological Evidence

• In addition to DNA analysis, teeth can be used
  to
• Identity a person
• Characteristics of teeth
• Alignment
• Overall structure
• X-rays
• Compare suspects with bite marks on victim
• What else might be present on a bite mark that
  could help identify a criminal?