4.2 Helical Structures that Tell Us Everything

Welcome to part 2 of question 5. (:

Besides forensic anthropology, we can also identify victims with DNA Profiling.

The term DNA profiling - or genetic fingerprinting - is applied to the scientific process whereby samples of DNA are collected, collated and used to match other samples of DNA, which may have been found at the crime scene.

 James Watson & Francis Crick

Founder of DNA ,1962

James Watson in 2003

Examining sequencing gel as a part of DNA profiling.

http://science.howstuffworks.com/dna-profiling.htm

Why use DNA Profiling?

This process is used as one means of identification when an attacker or assailant has left some kind of bodily fluid or blood at the scene of a crime and when no visual identification is possible.

DNA profiling relies heavily on the principle that no two individuals share the same genetic code - except for identical twins and statistically those elements of DNA that are examined and used to obtain a match will be unique.

The process of DNA fingerprinting was first used during the 1980's and its application was quickly to become that of identification of suspects involved in serious crimes including murder. The premise that most attackers or killers will leave some measure of bodily fluid at a crime scene - be it saliva, blood, semen or other such fluid - was quickly accepted as common place and it became a staple of many criminal investigations.

Blood stain at crime scene

Collecting Evidence

Just like normal fingerprints taken from a suspect they are so unique that only in the case of identical twins - as we have already mentioned - could they be the same.

http://www.cartoonstock.com/lowres

Statistically there is a one in sixty-four billion chance that any two unrelated individuals would have comparable DNA: comparable DNA is DNA that has certain attributes similar to that of another person but is not identical. This can be used as evidence in a court of law however most defence or prosecution counsels would tend not to rely heavily on it if there are only certain similarities.

Procedures used in DNA Profiling

Restriction Fragment Length Polymorphism Restriction enzymes are used to cut segments of a sample from which DNA is extracted. The procedure of RFLP focuses on repetitious sequences of DNA 'bases' which vary greatly from individual to individual.

These segments are then separated using a technique called Electrophoresis, which measures each of them by length. Once they have been sorted in this way a visual representation of the results is created using a procedure known as Autoradiography: basically producing an x-ray of the results that can be compared by overlapping the x-rays.

http://science.howstuffworks.com/dna-profiling1.htm

Steps for RFLP:

1.                 Separate white and red blood cells with a centrifuge.

2.                 Extract DNA nuclei from the white blood cells. This is done by bathing the cells in hot water, then adding salt, and putting the mixture back into the centrifuge [source: University of Arizona].

3.                 Cut DNA strand into fragments using a restriction enzyme.

4.                 Place fragments into one end of a bed of agarose gel with electrodes in it. Agarose gel is made from agar-agar, a type of seaweed that turns into gelatin when dissolved in boiling water.

5.                 Use an electric current to sort the DNA segments by length. This process is called agarose gel electrophoresis. Electrophoresis refers to the process of moving the negatively-charged molecules through the gel with electricity. Shorter segments move farther away from their original location, while longer ones stay closer. The segments align in parallel rows.

6.                 Use a sheet of nitrocellulose or nylon to blot the DNA. The sheet is stained so the different lengths of DNA bands are visible to the naked eye. By treating the sheet with radiation, an autoradiograph is created. This is an image on x-ray film left by the decay pattern of the radiation. The autoradiograph, with its distinctive dark-colored parallel bands, is the DNA profile.

Short Tandem Repeat Test

A more recent form of test is the STR Test (Short Tandem Repeat Test), which looks at DNA segments and counts the number of repeats at a number of different DNA sites - normally around thirteen.

http://www.hawgoodfamily.co.uk/wpimages/wpa5479d92.jpg

Polymerase Chain Reaction

Another more recent test - can create multiple copies of the DNA sequence using as little as fifty molecules; this procedure can help to produce a usable DNA sample from a single human hair.

It is worth noting that all of these tests have proven very reliable over the years and continual advances in forensic science - and genetic fingerprinting as a whole - have made these tests almost foolproof!!!!

The chances of any of these results leading to the misidentification of a suspect is one in several billion and over the years since their inception these tests have also not only proved the guilt of many suspects but have also proven the innocence of many people wrongly convicted of a crime at a time when DNA fingerprinting was not available.

http://users.ugent.be/~avierstr/principles/pcrsteps.gif

Ok guys, let's end this topic with a video. (:

That's it. Happy Learning. 

DI

References:

http://www.exploreforensics.co.uk/dna-fingerprinting.html (last visited 13 Oct 2013)

http://science.howstuffworks.com/dna-profiling1.htm