The Biology Behind DNA Fingerprinting

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The Biology Behind DNA Fingerprinting

• Mark Bailey

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Outline

• Basic structure of DNA
• VNTRs and sequence variations
• Procedures used in isolating samples
• Inheritance
• Alleles and population frequency (Hardy-Weinberg
  equation)

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DNA - Basics

• DNA- deoxyribonucleic acid
• Exists within the nucleus of every cell
• Packaged in molecules called chromosomes

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DNA - Basics

• DNA helix composed of repeating units called
  nucleotides- ( a sugar phosphate molecule and a
  base- Adenine, Cytosine, Guanine, Thymine)
• 2 strands connected together by hydrogen bonds
• Unit of length base-pair

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DNA - Basics

• Sample piece of DNA would look like this
• 5' T-T-G-A-C-T-A-T-C-C-A-G-A-T-C3'3'
  A-A-C-T-G-A-T-A-G-G-T-C-T-A-G5'

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DNA - Basics

• Its estimated that the human genome contains 3
  billion base-pairs
• Its also estimated that 3 million bases differ
  from person to person

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VNTRs and sequence variations

• DNA evidence uses variations of sequences found
  in non-coding regions
• variations known as polymorphisms
• single nucleotide polymorphisms
• short tandem repeats (STRs)
• variable number of tandem repeats (VNTRs)

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VNTRs
The number of repeats varies by individual from
as few as 7 to more than 40.
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Procedures used to Isolate DNA

• Restriction Fragment Length Polymorphism (RFLP)
  analysis
• Restriction enzymes (restriction endonucleases)
  cut the surrounding regions of VNTRs at certain
  loci
• Regions amplified using PCR (polymerase chain
  reaction)
• Run through gel electrophoresis
• Sample lengths are then compared

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An Example of VNTR analysis
Lets consider 3 different loci on 3 pairs of
chromosomes from 3 different individuals.
The repeat sequence motif is GTGT in the first
chromosome and the motif on the second chromosome
is CACA
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PCR
Primers are attached around VNTR regions and
begin to make copies of the sequences
Process repeat over many times to get a larger
product sample
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PCR
Product is then captured for use in gel
electrophoresis
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Gel Electrophoresis

• Samples run through agarose gel which is
  electrically charged
• larger fragments move slower
• smaller fragments move faster and further

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VNTR Analysis

• After RFLP analysis, DNA samples from suspect(s)
  can then be compared to samples of DNA left at
  scene of the crime

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VNTR Analysis
Lengths of VNTR regions are also compared and
deemed whether they are statistically close
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Inheritance

• VNTRs, just like genes, are inherited
• Alleles- one copy from the mother one from the
  father

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Alleles and Population Frequency

• Hardy-Weinberg equation
• (p q)2 1 (squared because people have two
  alleles at a gene)
• Or more simply
• p2 (AA) 2pq (Aa) q2 (aa) 1
• where

p2 is the frequency of people with genotype (AA)
in a population. 2pq is the frequency of people
with genotype (Aa) in a population.q2 is the
frequency of people with genotype (aa) in a
population.
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Hardy-Weinberg example
As an example, lets say that in a population of
1000 people, 650 people have the AA genotype,
300 have the Aa genotype, and 50 have the aa
genotype. The frequency of the A allele is
determined by summing the number of A alleles in
the population and then dividing by 2 times the
total. (2 650) (1 300)/ (2 1000) .8
p So, q is equal to .2 (1-p) Let's check (2
50) (1 300)/(2 1000) .2 q p2 (AA)
2pq (Aa) q2 (aa) 1 .64 2 .16 .04 1