BIOLOGY
ASSIGNMENT
DNA FINGERPRINTING
BY
CHOWHAN.R.S
CLASS : XII-B3
CERTIFICATE:
INDEX
SNo CONTENTS
1 INTRODUCTION
2 DNA FINGERPRINTING
3 HOW WAS THE FIRST DNA
FINGERPRINT PRODUCED
4 DNA PROFILING
5 HOW IS A DNA PROFILE
PRODUCED TODAY
6 SOLVING CRIME
7 HOW ARE DNA
PROFILES STORED
8 LINKING BLOOD RELATIVES
INTRODUCTION
DNA fingerprinting is a method used to identify an individual from a sample of
DNA by looking at unique patterns in their DNA.
• Almost every cell? in our body contains our DNA?.
• On average, about 99.9 per cent of the DNA between two humans is the
same.
• The remaining percentage is what makes us unique (unless you are an
identical twin!).
• Although this might sound like a small amount, it means that there are
around three million base pairs that are different between two people.
These differences can be compared and used to help distinguish you
from someone else.
• Minisatellites are short sequences (10-60 base pairs long) of repetitive
DNA that show greater variation from one person to the next than other
parts of the genome?. This variation is exhibited in the number of
repeated units or ‘stutters’ in the minisatellite sequence.
• The first minisatellite was discovered in 1980.
DNA fingerprinting
• DNA fingerprinting was invented in 1984 by Professor Sir Alec Jeffreys
after he realised you could detect variations in human DNA, in the form
of these minisatellites.
• DNA fingerprinting is a technique that simultaneously detects lots of
minisatellites in the genome to produce a pattern unique to an
individual. This is a DNA fingerprint.
• The probability of having two people with the same DNA fingerprint that
are not identical twins is very small.
• Just like your actual fingerprint, your DNA fingerprint is something you
are born with, it is unique to you.
How was the first DNA fingerprint
produced
1. The first step of DNA fingerprinting was to extract DNA from a sample
of human material, usually blood.
2. Molecular ‘scissors’, called restriction enzymes, were used to cut the
DNA. This resulted in thousands of pieces of DNA with a variety of
different lengths.
3. These pieces of DNA were then separated according to size by a process
called gel electrophoresis:
• The DNA was loaded into wells at one end of a porous gel, which
acted a bit like a sieve.
• An electric current was applied which pulled the negatively-
charged DNA through the gel.
• The shorter pieces of DNA moved through the gel easiest and
therefore fastest. It is more difficult for the longer pieces of DNA
to move through the gel so they travelled slower.
• As a result, by the time the electric current was switched off, the
DNA pieces had been separated in order of size. The smallest DNA
molecules were furthest away from where the original sample was
loaded on to the gel.
4. Once the DNA had been sorted, the pieces of DNA were transferred or
‘blotted’ out of the fragile gel on to a robust piece of nylon membrane
and then ‘unzipped’ to produce single strands of DNA.
5. Next the nylon membrane was incubated with radioactive probes.
• Probes are small fragments of minisatellite DNA tagged with
radioactive phosphorous.
• The probes only attach to the pieces of DNA that they are
complementary to – in this case they attach to the minisatellites in
the genome.
6. The minisatellites that the probes have attached to were then visualised
by exposing the nylon membrane to X-ray film.
• When exposed to radioactivity a pattern of more than 30 dark
bands appeared on the film where the labelled DNA was. This
pattern was the DNA fingerprint.
• To compare two or more different DNA fingerprints the different
DNA samples were run side-by-side on the same electrophoresis
gel
Illustration showing the steps in DNA fingerprinting.
DNA profiling
• Modern-day DNA profiling is also called STR analysis and relies on
microsatellites rather than the minisatellites used in DNA fingerprinting.
• Microsatellites, or short tandem repeats (STRs), are the shorter relatives
of minisatellites usually two to five base pairs long. Like minisatellites
they are repeated many times throughout the human genome, for
example ‘TATATATATATA’.
How is a DNA profile produced today
1. DNA is extracted from a biological sample. STR analysis is incredibly
sensitive so it only needs a tiny amount of someone’s DNA to produce an
accurate result. As a result the DNA can be extracted from a wider
range of biological samples, including blood, saliva and hair.
2. Unlike the original DNA fingerprinting method, DNA profiling does not
use restriction enzymes to cut the DNA. Instead it uses the polymerase
chain reaction (PCR) to produce many copies of specific STR sequences.
• PCR is an automated procedure that generates lots of copies of a
specific sequence of DNA. It only requires small amounts of DNA
to start with and can even make copies from a DNA sample that is
partially degraded.
• In PCR small bits of DNA called primers? bind to complementary
sequences of the DNA of interest and mark the starting point for
the copying of the DNA of interest.
• In STR analysis the primers used in the PCR are designed to
attach to either end of the STR sequence of interest.
• The primers for each STR is labelled with a specific coloured
fluorescent tag. This makes it easier to identify and record the STR
sequences after PCR.
3. Once enough copies of the sequence have been produced by PCR,
electrophoresis is used to separate the fragments according to size.
4. Each fragment passes by a laser which causes the fragments with
fluorescent tags to glow with a specific colour. The output is displayed as
a series of coloured peaks (as shown in the image below) highlighting
the colour and length of each STR sequence.
Illustration showing the steps in DNA profiling.
• The more STR sequences that are tested, the more accurate the test is
at identifying someone.
• Other STRs used for forensic purposes are called Y-STRs, which are
derived solely from the male Y chromosome?. This is useful for
identifying a male perpetrator from mixed DNA samples.
• Only one person in every 10 million million (10,000,000,000,000) will
have a particular STR profile. With the world human population
estimated at only 7,100 million (7,100,000,000) it is therefore extremely
unlikely you will share the same profile as someone else, unless you are
an identical twin.
Solving crime
• DNA profiles are very useful in forensics because only a tiny sample of
human material left behind after a crime may be sufficient to identify
someone.
• In the UK, a complete DNA profile consists of 11 STR sequences plus a
sex determiner to confirm if the profile is from a man or a woman. Now
all new profiles include an additional five STR sequences to provide
consistency across borders in Europe.
• In the USA, the Federal Bureau of Investigation (FBI) recommends that
13 STR sequences are tested. Many states are increasing the number of
STR sequences tested to enable more efficient investigations across
state borders.
• A match made between a crime scene profile and an individual profile
identifies a possible suspect.
• A match made between different crime scene profiles indicates a repeat
offender at work.
• The police may use this DNA evidence to support other evidence to help
prosecute someone for a crime. Complete DNA profiles give very reliable
matches and may provide strong evidence that a suspect is guilty or
innocent of a crime.
Illustration showing a comparison of a DNA fingerprint from a crime scene
and DNA fingerprints from two suspects. The DNA fingerprint from suspect 2
matches that taken from the crime scene.
How are DNA profiles stored?
• The UK was the first country to set up a national database of DNA
profiles in 1995.
• The UK National DNA Database holds the DNA profiles from a select
number of UK individuals, most of which are linked to serious crimes.
• The Protection of Freedom Act 2013 ensured that 1,766,000 DNA
profiles taken from innocent adults and children were deleted from the
UK National DNA Database.
• Most countries now have a national DNA database.
Linking blood relatives
• You get half of your DNA from your mother and half from your father.
STRs are therefore passed down from parents to their children.
• DNA profiling can be used to help confirm whether two people are
related to one another and is commonly used to provide evidence that
someone is, or is not, the biological parent of a child.
• DNA profiling can also be used to identify victims of crime or major
disasters and help bring separated families back together.
• DNA profiling has a high success rate and very low false-positive rate.
Illustration comparing the DNA profiles of two parents and their child. You
can see which STRs in the child have been inherited from which parent.
