DNA, or Deoxyribonucleic Acid, is
the genetic material in our cells.
• No two people (except
identical twins) have the
exact same DNA.
DNA patterns from four sets of
twins – which are identical?
DNA fingerprinting is also known as DNA profiling. It is a
technique used by scientists to distinguish between
individuals of the same species using only samples of their
DNA.
i. The process of DNA
fingerprinting was invented
by English geneticist Alec
Jeffreys at the University of
Leicester in 1985.
DNA fingerprinting has been used
to help solve crimes by linking
suspects to crimes and by helping
identify bodies of victims.
In addition to crime investigation, DNA
fingerprinting can also be used to:
1.establish paternity and parentage
DNA paternity
testing can indicate
that a man is highly
likely to be the
father with about
99.9% accuracy or
that he is excluded
as being the father
with 100% accuracy.
In addition to crime investigation, DNA fingerprinting
can also be used to:
1.establish paternity and parentage
2.identify victims of war and large scale disasters
3.study biodiversity of species
4.track genetically modified crops
5.settle immigration disputes
Most lab techniques used for DNA fingerprinting were not intended for
these purposes, but instead were developed for use in the medical field for
diagnosis and treatment of diseases.
Small amounts of biological evidence left at crime scenes,
called trace evidence, are the source of DNA needed for
DNA fingerprinting.
• Biological evidence includes saliva, blood,
semen, skin, hair roots, body tissue cells,
and even urine.
DNA is a nucleic acid,
found in chromosomes,
in the nucleus of your
cells.
• Most human cells have 23 pairs
of chromosomes, or 46 total
• Exception is sperm and egg cells
which have only 23 chromosomes
because they are not in pairs
Each chromosome pair can be broken
into smaller segments called genes.
• Genes control the traits of the
organism, and therefore can
vary between individuals.
i. DNA has four different nitrogenous
bases:
1.Adenine (A),
2.Thymine (T),
3.Guanine (G),
4.Cytosine (C).
i. These bases form pairs according to the base pairing rule:
1.Adenine binds only with Thymine
2.Cytosine binds only with Guanine
3.These pairs are considered to be ‘complementary’.
i. If one strand of DNA has the genetic code: A T C T G C
its complementary strand would read:
DNA is made when these two
strands twist together in a shape
called the double helix.
The sides of the helix, referred to as the
backbone of DNA, are made up of
alternating sugar and phosphate
molecules.
a. The sugar in DNA is called deoxyribose,
from which DNA got its name.
The rungs of the helix are made up of the
paired nitrogenous bases (A-T, C-G),
which help to code the DNA with
instructions for the cell.
DNA in chromosomes is called nuclear DNA
• Nuclear DNA is inherited from both the mother and
father, and is virtually identical in all cells of an
individual’s body
Mitochondrial DNA is in the form of a circular
loop, and unlike nuclear DNA, is inherited only
from the mother
i. The human genome is the total amount of DNA in
a cell. It is contained in both the nucleus and
mitochondria.
ii.The human genome consists of approximately 3
billion base pairs.
iii.The purpose of the genome is to code the
blueprint for the human body and how it works. It
is responsible for telling the body to make proteins
and other needed molecules.
However, not all of the 3 billion bases are useful
code.
a. Exons are encoded DNA, with directions to build molecules
i. Exons made up only 1.5% of the entire genome.
ii. Together, they code for about 24,000 different genes.
iii.The rest of the genome is made of introns.
b.Introns are un-encoded DNA, that do not code for the production of
molecules.
i. These segments make up 98.5% of DNA and are often referred to as
‘junk DNA’.
ii. Despite its nickname, introns may function in gene splicing and
therefore may have an important role in biology after all.
iii.Introns are also very useful in forensic science!
Most of the human genome is the same in all humans,
but some variation exists among individuals.
• Most of the variation in DNA is found in the non-coding
segment, or introns.
• Much of the non-coding DNA is in the form of repeated
base sequences, and some of these sequences can be
repeated many times.
Polymorphisms are the non-coded DNA
segments that contain unique patterns
of repeated base sequences that that
are unique to individuals.
Within the non-coding sections of DNA are short sequences
of bases repeated multiple times.
• The number of times the sequence repeats varies
between individuals.
Two types of repeating DNA sequences:
• Variable Numbers of Tandem Repeats (VNTR) are 9 to
80 bases in length
• Short Tandem Repeats (STR) are 2 to 5 bases in length
VNTR and STR data from DNA fingerprints can be
analyzed for two main purposes:
i. Tissue matching
1. Comparing DNA evidence from a crime scene with DNA
taken from a suspect
2. How it looks: Two samples that have the same band
pattern are from the same person
i. Inheritance matching
1. Comparing family members’ DNA for proof of familial
relationships
2. How it looks: Each band in a child’s DNA fingerprint must
be present in at least one parent (50% from mom, 50%
from dad)
a. Often the amount of evidence left at a crime scene is very
small and therefore considered to be trace evidence.
b. A problem with analyzing trace evidence is that many forensic
tests will destroy the evidence sample, as is the case with
DNA.
c. Therefore, prior to DNA fingerprinting, PCR is used.
PCR stands for
Polymerase
Chain Reaction.
•
PCR is a technique
that makes
thousands of copies
of segments of DNA
that investigators
want to analyze.
Crime scene (template)
DNA is mixed with
nucleotides, an enzyme
known as DNA
polymerase, and primers.
•
Primers are short segments
of complimentary DNA that
base-pair with the template
DNA upstream of the region
of interest and serve as
recruitment sites for the
polymerase
Cycles of denaturation,
annealing and extension are
repeated to achieve
exponential amplification of
the target sequence, allowing
for billions of DNA copies to
be produced in just a few
hours.
Because DNA is found in all cellular
material and cells are very small,
contamination is an important
issue to address when collecting
and preserving DNA evidence.
To avoid DNA evidence contamination, crime scene investigators should:
i. Wear disposable gloves and change them often
ii. Use disposable instruments for handling each sample
iii.Avoid talking, coughing, or sneezing over evidence
iv.Do not touch your face or body when collecting or packaging evidence
v. Air-dry evidence before packaging. If evidence cannot be dried, it may
be frozen.
vi.Avoid using plastic bags to store evidence that contains DNA; use paper
bags or envelopes
vii.Keep evidence cool and dry during transportation and
storage. Avoid direct sunlight which can damage DNA.
Collect DNA
(from scene or
known)
Step One:
Extraction
of the
DNA
Step Two:
Cutting the DNA into restriction fragments
• RFLPs, Restriction Fragment Length Polymorphisms
Step Three: Amplification (making many copies of those fragments)use PCR
Step Four: Electrophoresis
Gel Electrophoresis
• a process that separates RFLPs
according to their length,
creating a DNA Fingerprint.
Video: Gel Electrophoresis
Video: Loading the Gel
Video: Reading the Results
What the above
DNA looks like
after gel
electrophoresis
i. Note: one well should contain a
control, a solution containing
DNA fragments of known
lengths, called Marker or
Standard DNA.
ii. Other wells should contain DNA
from crime scene, victim, and
suspects.
DNA fingerprints look like a striped column. In order for
DNA fingerprints to match, the columns must have bands
in the exact same places with the exact same widths.
•
Each state keeps DNA profiles of
individuals who have been convicted
of certain kinds of crime such as rape,
murder, and child abuse.
•
The military maintains DNA profiles of all service men
and women.
The United States’ Electronic database of DNA profiles
is called CODIS, Combined DNA Index System.
•
As of February 2011, CODIS contained
9.4 million profiles from DNA from
convicted felons of violent crimes. Of
360,000 profiles from DNA at crime
scenes, CODIS made 138,700 hits.