DNA – What it is?
Deoxyribose Nucleic Acid
DNA contains genetic information.
DNA codes the proteins that our bodies
make which are necessary for survival.
Thus DNA is a code for making proteins.
DNA also determines how much of
these proteins each cell makes.
The order of amino acids determines
what type of protein is made.
Some Common proteins are:
Hemoglobin - carries oxygen from lungs to cells
Insulin - regulates metabolism
Many types of enzymes - catalyze reactions in the
body, such as the breakdown of sugar for energy.
When human cells are present in biological evidence, their chromosomes can be
examined to determine whether the evidence comes from a male or a female.
The analysis of chromosomes is known as karyotyping.
DNA fingerprinting, also known as DNA profiling, is used in criminal and legal cases
to determine identity or parentage, Trace the inheritance of genetic disorders,
Identify the origin of a blood, semen, or saliva in a sample, and identify victims of
war and large-scale disasters such as plane crashes, tsunamis, and hurricanes.
Three billion bases in human DNA, 99% of DNA is identical among individuals. 1%
contains significant variation.
Each person’s DNA Profile is unique, Except Identical Twins
DNA - STRUCTURE
Double Helix – Twisted Ladder
The DNA ladder is made up of
building blocks called nucleotides.
The two DNA strands are antiparallel.
The two strands are held together by
hydrogen bonds formed between
the complementary bases.
Sugar Phosphate Backbone
(Sides of Ladder)
(Rungs of Ladder)
molecules that form the building
blocks of nucleic acids (DNA and
They serve to carry packets of
energy within the cell (ATP).
Nucleotides play central roles in
A nucleotide is composed of a
nucleobase (nitrogenous base), a
five-carbon sugar (either ribose
or 2-deoxyribose), and one or
more phosphate groups.
Pearson Education, Inc
4 BASES OF DNA
The nitrogenous bases
found in nucleotides are
as pyrimidines or purines.
Purines have a two ring
Pyrimidine has one ring.
PAIRING OF BASE PAIRS
WHERE IS DNA
DNA in the nucleus is packaged into
Chromosomes (23 Pairs).
DNA can be recovered from any
substance that contains cells.
All types of cells in our body contain
a copy of the same DNA.
DNA holds the instructions to make
all things in your body work
WHERE IS DNA
IT IS EVERYWHERE
Examples: Blood – WBC, Semen, Saliva, Tissue, Bone,
Teeth, Hair, Maggot Corps
HOW DNA DIFFER AMONG INDIVIDUALS
One of the bases (letters) can
Person 1 AGCTAGATCGTTATTCCGAG
Person 2 AGCTAGATCGTCATTCCGAG
Bases (letters) can be added or
Person 1 AGCTAGATCGTTATTCCGAG
Person 2 AGCTAGATCGTATTCCGAG
Person 3 AGCTAGATCGTTTATTCCGAG
Regions of DNA can be repeated
a different number of times.
Person 1 GCCAGCTAGCTAGCTAGCTAGCTAGCTTTCAT
Person 2 GCCAGCTAGCTAGCTAGCTAGCTTTCAT
Person 3 GCCAGCTAGCTAGCTAGCTAGCTAGCTAGCTT
DNA – FORENSIC ANALYSIS
Collection of Evidence:
Types of Unknown Samples: Blood, Semen, Stains, Saliva, Hair, Tissue, Bones, Teeth
Types of Known Samples: Blood / buccal swabs from suspect / victim / other known person.
Avoid Contamination of DNA Evidence:
Use disposable gloves and disposable instruments for handling each sample.
Avoid touching the area where you believe DNA may exist.
Avoid talking, sneezing, and coughing over evidence.
Avoid touching your face, nose, and mouth when collecting and packaging evidence.
Air-dry evidence thoroughly before packaging as moisture destroys DNA.
If wet evidence cannot be dried, it may be frozen.
Put evidence into new paper bags or envelopes.
Keep samples at room temperature and out of sun.
STEPS INVOLVED IN SAMPLE PROCESSING
Sample Obtained from
Crime Scene or Paternity
of Multiple STR markers
Separation and Detection of PCR
Comparison of Sample
Genotype to Other Sample
If match occurs, comparison of
DNA profile to population
Generation of Case Report
with Probability of Random
VARIATIONS IN DNA PROFILE
Mini-satellites - repeated sequences, 10–100 base pairs
Short Tandem Repeats (STRs) – repeated sequences, 2–9 base pairs
Single Nucleotide Polymorphisms (SNPs) - Single Nucleotide — A, T, C or G —
in the genome differs between members of a biological species or paired
When the amount of evidence
left at a crime scene is very
small, it is considered to be
The use of the polymerase
chain reaction (PCR) technique
we can generate multiple
identical copies from trace
amounts of original DNA
This enables forensic scientists
to make billions of DNA copies
from small amounts of DNA in
just a few hours.
The DNA produced with PCR
can be analyzed using DNA
Thermal Cycling Temperatures
The denaturation time in the first cycle is
lengthened to ~10 minutes when using
AmpliTaq Gold to perform a “hot-start” PCR
Typically 25-35 cycles
performed during PCR
STEPS OF DNA FINGERPRINTING
Extraction: DNA is extracted from cells or tissues of the body.
Restriction Fragments: DNA is cut by restriction enzymes. Restriction enzymes
recognize a unique pattern of DNA bases (restriction sites) and will cut the DNA at
that specific location. Restriction fragments of varying lengths are formed when
the DNA is cut.
Amplification: Specifically chosen DNA fragments are amplified using polymerase
Electrophoresis: DNA is loaded into the wells found in an agarose gel. When an
electric current is passed through the gel, the negatively charged DNA fragments
(pieces of DNA) migrate toward the positive end of the gel. DNA fragments are
separated by size, with the smallest DNA fragments moving the fastest through the
Transfer DNA to Nylon sheet by soaking them overnight.
Probing is done by adding radioactive or coloured probes to nylon sheet to
produce a pattern called DNA fingerprint.
DNA Fingerprint is built using several probes (5-10) probes symaltaneously.
DNA - PROFILING
Short Tandem Repeat (STR)
Restriction Fragment Length
In order to study the structure of
DNA, the molecules are broken up
into smaller fragments by enzymes
called restriction enzymes .
Restriction enzymes do not break up
the DNA molecule randomly but ‘cut’
it at particular sites producing
restriction enzymes cut the DNA in
different places and so produce
fragments which are easier to analyse
base on their length.
Polymorphism means many forms.
STR technology is used to evaluate
specific regions (loci) within nuclear
DNA. Variability in STR regions can be
used to distinguish one DNA profile
Short because usually 1-4 nucleotides
Tandem because they occur one
Repeat because they are repeats of
same DNA sequence.
RFLP - ELECTROPHORESIS
Electrophoresis is a separations technique that is based on the mobility of ions in
an electric field. Positively charged ions migrate towards a negative electrode and
negatively-charged ions migrate toward a positive electrode.
Electrophoresis was made possible by the discovery that nucleotide fragments can
be separated by moving them through a porous material (agarose) within an
electric field and DNA bands must be stained to make them visible. Ethidium
bromide-stained DNA will fluoresce when illuminated with UV light.
RFLP - ELECTROPHORESIS
The smallest fragments will move the fastest because they are able to move
through the pores in the gelatin faster. Bands will be produced on the gelatin
where the fragments accumulate. The shortest fragments will accumulate
near one end of the gelatin and the longer, slower-moving ones will remain
near the other end.
In the diagram below, four samples of DNA were placed on the gelatin. After
an electric current was applied for a period of time, the fragments separated.
Notice that sample D on the right does not match the other three samples.
DNA PROFILING USING STRS: AN OVERVIEW
STRs are Short Tandem Repeats of patterns of nucleotides spread throughout our
The number of repeats at a certain distinct region (locus, plural=loci) of DNA is
highly variable from person to person allowing their use in human identity testing
The number of nucleotides involved in the repeats can vary between 9 and 80
(called variable number of repeats, VNTRs, or minisatellites) or between 2 and 5
(called microsatellites, SHORT tandem repeats, STRs)
Several loci along our DNA have been identified as possessing STRs (thanks in part
to the Human Genome Project), and the DNA profiling community has selected 13
regions for identity analysis
These 13 loci ALL contain 4 nucleotide (tetrameric) repeats
Through population studies, the numbers and types (nucleotides involved) of
these repeats at these loci have been analyzed affording probability estimates in
7 short, tandem (back to back) repeats of the nucleotide sequence AATG
SHORT TANDEM REPEAT (STR)
It can start with a much smaller sample of DNA. STR analysis examines how often base
pairs repeat in specific loci, or locations, on a DNA strand. These can be dinucleotide,
trinucleotide, tetranucleotide or pentanucleotide repeats -- that is, repetitions of two,
three, four or five base pairs.
The Federal Bureau of Investigation has chosen 13 specific STR loci to serve as the
standard for DNA analysis. The likelihood that any two individuals (except identical
twins) will have the same 13-loci DNA profile can be as high as 1 in 1 billion or greater.
Variable Number of Tandem Repeat (VNTR) loci are chromosomal regions in which a short DNA
sequence motif (such as GC or AGCT) is repeated a variable number of times end-to-end at a single
location (tandem repeat). In this example, Locus A is a tandem repeat of the motif GC: there are
four alleles, with two, three, four, or five repeats (A2, A3, A4, and A5, respectively). Locus B is a tandem
repeat of the motif AGCT: there are only two alleles, with two or three repeats (B2 and B3,
respectively). The example shows a DNA fingerprint that includes both loci simultaneously.
Individual #1 is heterozygous at Locus A (A2 / A5) and homozygous at Locus 2 (B2 / B2: note that
this genotype gives a single-banded phenotype in the fingerprint). Individual #2 is heterozygous at both
loci: (A4 / A3 and B3 / B2) respectively). The two individuals are distinguishable at either locus.
DNA fingerprinting using STRs. The DNA of two suspects is compared to DNA recovered from the crime scene.
MITROCONDRIAL - DNA
Sometimes, a sample can be old and will no longer
have nuclear material in the cell, which poses a
problem for the other types of DNA analysis. With
mitochondrial DNA can be removed, thus having
important ramifications for cases that were not
solved over many years.
Inherited from the mother only
More sensitive (less DNA needed), degrades slower
than nuclear DNA
Can be used in cases where nuclear DNA cannot
(hair without root, skeletal remains)
All people of same maternal line will be
indistinguishable (less discriminatory)
More work, more time consuming, more costly
1) Diagnosis and Developing cures for inherited disorders:
• DNA fingerprinting is used to diagnose inherited disorders in both prenatal and
newborn babies in hospitals around the world.
These disorders may include cystic fibrosis, hemophilia, Huntington's disease,
familial Alzheimer's, sickle cell anemia, thalassemia, and many others.
Early detection of such disorders enables the medical staff to prepare themselves
and the parents for proper treatment of the child.
In some programs, genetic counselors use DNA fingerprint information to help
prospective parents understand the risk of having an affected child.
In other programs, prospective parents use DNA fingerprint information in their
decisions concerning affected pregnancies.
2)Biological Evidence to Identify Criminals:
• Where fingerprints are not available but biological specimens are available like
blood or semen stains, hair, or items of clothing at the scene of the crime then
these items may prove to be valuable sources of DNA of the criminal.
• Since the year 1987, innumerable cases have been solved with the help of DNA
3) Paternity disputes :
• Another important use of DNA fingerprints in the court system is to establish
paternity in custody and child support litigation. In these applications, DNA
fingerprints bring an unprecedented, nearly perfect accuracy to the determination.
4) Personal Identification :
• DNA maybe the best way to identify a person as all body tissues and organs
contain the same DNA type. The specimen required also is very small. In fact the
US army has been doing DNA fingerprinting of all its soldiers and has a huge