Who Committed the Crime? Let's Find Out!

INTRODUCTION

It would be quite safe for me to say that no two people share the same DNA sequence. However, if I were to tell you that your DNA sequence is 99.9% similar to mine, would you still believe me?

Well, if you are a human reading this article, then I can assure you that we do have something in common. Now, you must be thinking then what accounts for individual differences, and if you guessed 0.1% of our DNA sequence. You are absolutely correct, and surprisingly enough that 0.1% accounts for more than 3 million differences between your genome and mine or anyone else for that matter. In fact, I will go as far as to say that if we altered 1% of your DNA in the desired manner it would be really hard for your mom to distinguish you from an ape.

SHORT TANDEM REPEATS(STR)

1984, Alec Jeffery observed that some introns or intervening sequences (which not code for any protein) showed short units of nucleotides that repeated over and over. The portion of DNA which shows these repeating units are called STR's(Short Tandem Repeats). The number of times the unit of nucleotide repeats itself in an STR is called an allele.

Example- STR system D3S1358 has 13 to 17 repeating units of [AGAT] and [AGAC]. Therefore, there are 8 different alleles possible for this STR.

All individuals will have the STR system D3S1358 present on chromosome number 18. However, each individual will have different numbers of repeats(alleles) at a given satellite DNA locus. We can use this to our advantage and distinguish between two individuals based on the number of repeats. This shows that variation also occurs at portions of DNA that don't code for any protein.

HOW DOES IT WORK?

In the above situation, we know the child's DNA sequence and we are sure about who the mother is. There are three 3 people who claim to be the father. Can you tell who is the father?

HINT-1

The child inherits half of its DNA from the mother and another half from the father.

HINT-2

Compare the pattern of the bands.

Drop your answer in the comments section!

PROCEDURE

1) Isolating the DNA particular DNA

DNA can be extracted from blood, skin cells, semen, or even hair. The source of DNA is then purified and treated to remove any contaminants that can deteriorate the results. The treatment is carried out until pure DNA emerges as a white solid. DNA can be stored in water at cryoscopic temperatures.

2) Amplification

This is done by a process known as PCR( Polymerase Chain Reaction). This step is necessary to increase the readability and accuracy of the results by making multiple copies of a particular sequence of DNA. In our case, PCR would be used to make several copies of STRs. ( This is done by utilizing the enzyme polymerase, primers, and extra-bases dissolved in a solution)

3) Digestion and separation of DNA

DNA is chopped into fragments of varying lengths. The enzyme used to perform this step is endonucleases (restricting enzyme). Then the fragments are separated depending upon the length. This is process is called electrophoresis. Blue dye can be added to locate DNA strands in the translucent gel medium. In a nutshell, it creates an electric potential causing negatively charged DNA fragments to move along the direction of the electric field. The gel is checked by shining ultraviolet light on it to check for a nice strong DNA smear

4) Blotting of Arranged DNA Fragments

DNA fragments are then transferred on a membrane (e.g. nylon) by a technique called Southern Blotting. Before they are transferred, they are converted to a single strand exposing the base pairs. The blot is left overnight.

5)Addition of Radioactive Probes

Probes help us see the desired sequences of DNA at the end of the experiment. A Probe is a short radioactive strand of DNA that has a particular base pair sequence. The probe binds to the fragments of the DNA that has the matching sequence. In our case, It contains repeating units of nucleotides belonging to a particular STR.

6)Getting the results.

The results are obtained on an X-ray sheet which can record the patterns of radioactivity.

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BIBLIOGRAPHY

Nature News, Nature Publishing Group,

www.nature.com/scitable/topicpage/forensics-dna-fingerprinting-and-codis-736/.

“DNA Fingerprinting.” Genome.gov,

www.genome.gov/genetics-glossary/DNA-Fingerprinting#:~:text=DNA fingerprinting is a laboratory, DNA sample from a suspect.

The Editors of Encyclopaedia Britannica. “DNA Fingerprinting.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 21 Nov. 2019, www.britannica.com/science/DNA-fingerprinting

“What Is a DNA Fingerprint?” Facts, The Public Engagement Team at the Wellcome Genome Campus, 2 June 2016, www.yourgenome.org/facts/what-is-a-dna-fingerprint.