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Notes 13.2
DNA typing
Tandem repeats
• Portions of the DNA molecule that
contain sequences of letters that are
repeated numerous times that act to fill
the space between coding regions of
DNA
– All humans have the repeats
– There is variation in the number of repeats
that each of us have
• Restriction fragment length
polymorphisms (RFLPs) – repeat
segments cut from a strand of DNA
Tandem repeats cont….
• Certain numbers of the repeats have been
chosen for DNA typing
– Example: core sequence 15-35 bases which repeats
itself about 1000 times
– Remember, you inherit 1 chromosome of each pair
from each parent
• RFLP’s are created, which contain repeating
sequences.
• Length difference allows a scientist to
distinguish the individual
• Length difference determined using
electrophoresis gel
Tandem repeats cont….
Southern Blotting
• Used to transfer double stranded DNA from
electrophoresis which have been separated
and transferred to a nylon membrane for
visualization of the RFLP’s.
• Nylon membrane is treated with radioactive
probes containing one base sequence
complementary to the RFLP (hybridization)
• Nylon sheet is place against X ray film,
exposed and processed which causes bands to
appear which can be compared to test
specimens
RFLP
• DNA typing is the first scientifically
accepted protocol in the US for
forensic characterization of DNA
– Read Paragraph 3 on page 372
Polymerase Chain Reaction
• The newest technology to replace RFLP
1. Add a primer
2. Then reduce temperature allowing
combination
3. Add nucleotides
• Best used with sequences that are a
couple of hundred bases
PCR continued
• Has the advantage because it is
short and less vulnerable to
degeneration
• Can amplify small amount of DNA
• Can be used to ID saliva residues
on envelopes, stamps, soda cans
and cigarette butts
Short Tandem Repeats
(STR)
• Latest method of DNA analysis
• Locations on the chromosome that
contain short sequence elements that
repeat
• Less than 400 base pairs
• Shorter than RFLP
• Less susceptible to degradation
• Can be recovered from bodies or stains
that have been subject to decomposition
STR cont…
• Can be easily multiplied using PCR
• Only 1/1,000,000,000th of a gram is
needed
– ex: AATG (known as STR TH01)
• Every person has two STR types for
TH01
• Hundreds of different types are found
in human genes
STR cont…
• The more STR one can ID, the smaller
the percentage of the population that
have them
• Currently there are 13 STRs in the
national data base known as CODIS
– Combined DNA Index System
– See page 379
• Sequence of bases flanking the repeats
must also be known
STR cont…
• Commercial STR kits contain the
capacity to identify the sex of the
individual by the amelogenin gene
– Found on the X and Y chromosome
coding for tooth pulp
– 6 bases shorter on the X chromosome
than the Y chromosome
• There are more than 20 Y-STR markers
13.2 Questions
1.
2.
3.
4.
5.
6.
What are tandem repeats and how are they used in
forensic science?
What are the dominant DNA typing procedures in the
US until the mid 1990s?
What is short tandem repeat and why is it so
attractive to forensic scientists?
How does the number of STRs characterized relate to
the frequency of occurrence of the analyzed sample
in the general population?
(p. 380) Name two process by which a forensic
scientist can separate STRs for characterization.
Which of the two processes is the preferred process?
What gene is often used to determine the sex of a
DNA contributor? What characteristic of the gene
allows the forensic scientist to make this
determination?