Download The Biology Behind DNA Fingerprinting

The Biology Behind DNA
Fingerprinting
Mark Bailey
Outline
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Basic structure of DNA
VNTRs and sequence variations
Procedures used in isolating samples
Inheritance
Alleles and population frequency (HardyWeinberg equation)
DNA - Basics
• DNA- deoxyribonucleic acid
• Exists within the nucleus of every cell
• Packaged in molecules called chromosomes
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”
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-C…3'
3' …A-A-C-T-G-A-T-A-G-G-T-C-T-A-G…5'
DNA - Basics
• It’s estimated that the human genome
contains 3 billion base-pairs
• It’s also estimated that 3 million bases
differ from person to person
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)
VNTRs
The number of repeats varies by individual from as few as 7 to more than 40.
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
An Example of VNTR analysis
Let’s 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’
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
PCR
Product is then captured for use in gel electrophoresis
Gel Electrophoresis
Samples run through agarose gel which is electrically charged
• larger fragments move slower
• smaller fragments move faster and further
VNTR Analysis
• After RFLP analysis, DNA
samples from suspect(s) can
then be compared to samples
of DNA left at scene of the
crime
VNTR Analysis
Lengths of VNTR regions
are also compared and deemed
whether they are statistically close
Inheritance
• VNTRs, just like genes, are inherited
• Alleles- one copy from the mother one from
the father
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.
Hardy-Weinberg example
As an example, let’s 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