DNA Fingerprinting

Content

• The structure of DNA

• History of DNA fingerprinting

• DNA Electrophoresis

• DNA fingerprinting application

The structure of DNA• The characteristics of all living

organisms, including humans, are essentially determined by information contained within DNA that they inherit from their parents. The molecular structure of DNA can be imagined as a zipper with each tooth represented by one of four letters (A, C, G, or T), and with opposite teeth forming one of two pairs, either A-T or G-C.

• The letters A, C, G, and T stand for adenine, cytosine, guanine, and thymine, the basic building blocks of DNA.

• The information contained in DNA is determined primarily by the sequence of letters along the zipper. For example, the sequence ACGCT represents different information than the sequence AGTCC in the same way that the word "POST" has a different meaning from "STOP" or "POTS," even though they use the same letters. The traits of a human being are the result of information contained in the DNA code.

• Living organisms that look different or have different characteristics also have different DNA sequences. The more varied the organisms, the more varied the DNA sequences. DNA fingerprinting is a very quick way to compare the DNA sequences of any two living organisms.

History

• DNA fingerprinting was developed in 1984 by Alec. J. Jeffrey at the University of Leicester while he was studying the gene for myoglobin

• He found that myoglobin genes contain many segments that vary in size and composition and have no apparent functions

• Jeffrey called these segments minisatellites because they were small and surrounding the part of the gene.

• In 1987, British baker Colin Pitchfork was the first criminal caught using DNA fingerprinting

• 1990 California establishes DNA fingerprint banks for sex offenders.

• Electrophoresis is a technique that can be used to separate the DNA and RNA fragments from one another according to their sizes.

DNA Electrophoresis Experiment

• It can be done by loading the fragments(碎片 ) in a well at one end of a piece of gel,which is immersed in a buffer solution inside a gel tray.

• An electrical current is then passed through the gel.

Diagram showing gel electrophoresis

• As DNA or RNA molecules contain many phosphate groups,they are highly negatively charged.The fragments are attracted towards the positively charged electrode.

• The fragements pass through a gel at a rate that is inversely proportional to their size.Small fragments move faster than large fragments through the gel.In other words,they travel further through the gel.This procedure can be used to separate different DNA or RNA fragments into bands

• These bands can be visualized by staining the gel with ethidium bromide.It is a dye that intercalates between the nitrogenous bases and gives a pinkish orange colour under UV light.Usually a marker is loaded together with the sample that contains bands with known sizes.In this way,the sizes of unknown fragments can be estimated.

• The DNA sequences of DNA fragments can be known by means of an automated DNA sequencing machine.

DNA sequencing result

Home Made Gel for Electrophoresis

Procedure of DNA fingerprinting

1: Isolation of DNA.DNA must be recovered from the cells or tissues of the body. Only a small amount of tissue - like blood, hair, or skin - is needed. For example, the amount of DNA found at the root of one hair is usually sufficient.

2: Cutting, sizing, and sorting.Special enzymes called restriction enzymes are used to cut the DNA at specific places. For example, an enzyme called EcoR1, found in bacteria, will cut DNA only when the sequence GAATTC occurs. The DNA pieces are sorted according to size by a sieving technique called electrophoresis. The DNA pieces are passed through a gel made from seaweed agarose (a jelly-like product made from seaweed). This technique is the biotechnology equivalent of screening sand through progressively finer mesh screens to determine particle sizes.

3: Transfer of DNA to nylon.The distribution of DNA pieces is transferred to a nylon sheet by placing the sheet on the gel and soaking them overnight.

4-5: Probing.Adding radioactive or colored probes to the nylon sheet produces a pattern called the DNA fingerprint. Each probe typically sticks in only one or two specific places on the nylon sheet.

6: DNA fingerprint.

The final DNA fingerprint is built by using several probes (5-10 or more) simultaneously. It resembles the bar codes used by grocery store scanners

DNA fingerprinting application

• parentage test

• Forensic science

• DNA ID Card

Uses in parentage test

• DNA fingerprinting is widely used in parentage testing around the world. By comparing different DNA sequences, we can analyse the relationship between any two individuals.

• The test will be conducted using the latest in PCR (Polymerase Chain Reaction) technology. The PCR method is the most widely used and trusted DNA testing technology today.

• Polymerase chain reaction (PCR) is a molecular biology technique for enzymatically replicating DNA without using a living organism. Like amplification using living organisms, the technique allows a small amount of the DNA molecule to be amplified exponentially.

• The results that you receive will look like a table full of numbers. There are three columns: one for the Mother, one for the Child and one for the possible Father. Each row in the table represents a different DNA locus, which represent that specific area on their individual DNA .

Uses in forensics

• Because human DNA is unique to individuals, DNA isolated from blood, hair, skin cell, or other genetic evidence left at the scene of crime can be compared, using the DNA fingerprinting technology, with the DNA of a criminal suspect to determine guilt or innocence.

• In criminal cases, this generally involves obtaining samples from crime-scene evidence and a suspect, extracting the DNA, and analyzing it for the presence of a set of specific DNA regions (markers).

• Scientists find the markers in a DNA sample by designing small pieces of DNA (probes) that will each seek out and bind to a complementary DNA sequence in the sample.

• Forensic scientists compare these DNA profiles to determine whether the suspect's sample matches the evidence sample. If the sample profiles don't match, the person did not contribute the DNA at the crime scene.

DNA ID Card

• The DNA ID Card contains your personal information and DNA profile. The information recorded on the card includes photograph, Name, Gender, Date of Birth and DNA personal profile.