Transcript Chapter 7 : DNA Fingerprinting

Bio II: Forensics
 DNA
molecules are found in the nucleus of
cells in the human body in chromosomes.
 People have 23 pairs of chromosomes,
with an allele for each trait on each
chromosome.

So, 2 alleles for each trait
 Junk
DNA is the non-coding portion of DNA
that contains different DNA sequences in
different people.

Most of the DNA in members of the same
species is identical.
 Within
junk DNA, sequences are repeated
multiple times, and the number of times
varies among individuals (called VNTR’s)


Some are 9-80 bases in length
Some are only 2-5 bases in length and are
becoming the preferred sequences for analysis.
(called STR’s)
 In
a human population, these differences
among the junk DNA of individuals are called
polymorphisms.
 DNA Fingerprinting is a technique that was
developed for isolating and analyzing these
variable areas.
 Developed in 1984
 appears as a pattern of bands on X-ray film.
 VNTR and STR data are analyzed for

(a) tissue matching and

(b) inheritance matching.
 Population genetics is the study of
variation in genes among groups of
individuals.

Probability calculations are used to determine
the chance of a random person would having
the same allele as

(a) a suspect in a crime or

(b) an alleged father in a paternity case.
 A perpetrator may leave biological
evidence, such as saliva, blood, hair,
skin, or semen at a crime scene.


 In
is individual evidence
may be trace evidence
1993 the polymerase chain reaction
(PCR) technique was invented. It
generates multiple copies of DNA
evidence when there is only a small
amount available.
1. Use disposable gloves and collection
2.
3.
4.
5.
instruments.
Avoid physical contact, talking, sneezing,
and coughing in the evidence area.
Air-dry evidence and put it into new
paper bags or envelopes.
If evidence cannot be dried, freeze it.
Keep evidence cool and dry during
transportation and storage.
 1.
Cells are isolated
from biological evidence
 2. The cells then are
disrupted (lysed) to
release the DNA from
proteins and other cell
components.
 Once released, the cell
parts are “spun down”
in a centrifuge and DNA
can be precipitated
from the solution
Amplification:
 PCR may be needed
to make more
copies of the DNA
for testing if the
sample was small



To verify that the DNA in
the bands is not the
same, DNA probes are
used to identify the
unique sequences in a
person’s DNA.
The probe binds to
complimentary bases in
the DNA bands.
In most criminal cases,
6-8 probes are used.
 Band
position, width, and probe binding are
significant in matching samples of DNA.
 DNA fingerprinting can



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
(a) match crime scene DNA with a suspect,
(b) determine maternity, paternity, or match to
another relative,
(c) eliminate a suspect,
(d) free a falsely imprisoned individual, and
(e) identify human remains.
Child
shares
this
allele
with
mother
Child
shares
this
allele
with
father
 Polymerase chain reaction (PCR) for DNA
amplification has largely eliminated the problem
resulting from the tiny samples usually available.
 DNA evidence must be collected carefully to
avoid contamination with other DNA.
 DNA analysis involves extraction,
electrophoresis, and visualization.
 DNA profiles are kept by police agencies in
electronic databases.