Transcript Chapter 8 DNA Fingerprinting and Forensic Analysis

DNA Fingerprinting and Forensic
Analysis
What is a DNA fingerprint?
• Every cell of an individual carries a copy of
the DNA
– a cell collected from a person’s skin or hair
follicle contains the same DNA as from that
persons heart tissue or white blood cells
• Order of base pairs in the DNA of every
individual is different except identical twins
How do we distinguish one person’s
DNA from another?
• Don’t need to sequence all 3 billion base pairs of a
person’s DNA to distinguish it from another person’s
DNA
• Intron regions of DNA (junk DNA) contain sequences
that are 20-100 bp in length that are repeated at
different locations (loci) along the chromosome.
CGGCTACGGCTACGGCTA (repeated 3 times at this
location; at another location, it may be repeated 9 times)
• These sequences are called Short Tandem Repeats
(STRs) or VNTRs
STRs
• Each person has some STRs that were inherited
from mother and some from father
• No person has STRs that are identical to those of
either parent
• The number of repeats at each loci on
chromosome is highly variable in the population,
ranging from 4 to 40.
• The length of the DNA after cutting the
chromosome with a restriction enzyme, and its
position after electrophoresis will depend on the
exact number of repeats at the locus
• The uniqueness of an individual’s STRs provides
the scientific marker of identity known as a DNA
fingerprint.
• In the United States the FBI has standardized a
set of 13 STR assays (13 different locations on
the chromosomes) for DNA typing, and has
organized the CODIS database for forensic
identification in criminal cases.
• The United States maintains the largest DNA
database in the world: The Combined DNA Index
System, with over 60 million records as of 2007.
Preparation of a DNA fingerprint
Step 1
• Specimen collection
– blood, semen, etc
– easy to contaminate a DNA sample with DNA from
other sources (bacteria, DNA of person collecting
sample)
– DNA is not stable for very long-it degrades
• sunlight
• heat
• moisture
• DNA fingerprinting is a comparative
process:
– DNA from crime scene is compared with DNA
of a suspect
– So minimum of two samples must be prepared
Step 2
• DNA extraction
– standardized methods have been developed
– need to separate DNA from other cell material
and debris from crime scene.
Step 3
PCR using primers targeting STRs at
different loci
• PCR amplify STRs using target sites on
chromosome
Step 3
PCR amplification of DNA
1 strand
of DNA
Heat to
denature
doublestranded
DNA
Design primers that anneal to STR locus
Amplify all the regions of the chromosome
where the STRs exist.
STR locus
STR locus
PCR allows you to make
millions of copies of the STR
region from a single copy of
DNA you recovered from crime
scene.
• Since the # of times sequence is repeated is
different for each person, fragment size will be
different.
• This is done for 13 different STR sequences at
this one locus
• Differences occur among individuals at each of
the 13 loci on the chromosome where the STRs
occur
• This allows for a lot of variation
Restriction Fragment Length Polymorphism
For 1 STR sequence at 1 locus
Person A
Forensic sample
STR
STR
G-G-C-C-X-X-X-G-G-C-C-X-X.. G-G-G-C-C-X-X-G-G-C-C-X-X…..
PCR amplify
STR region
C-C-X-X-X-G-G
well
C-C-X-X-G-G
well
Gel
electrophoresis
• If you do this for 13 different repeat sequences
at 13 different loci on the chromosome, each
person produces a different band pattern when
the fragments are separated by gel
electrophoresis
Different
STRs at
other loci
STR1
STR2
STR3
Do any of the
individuals
compare with
forensic sample?