10th lesson- DNA fingerprinting

For this lesson, we learnt about a very important part of forensic science, which is DNA FINGERPRINTING.

Basically, we learnt many complex ideas that we never really learnt before... like what DNA was made up of. DNA= Double helix :)

The sides/ backbone of the DNA molecule are made up of sugar (DEOXYRIBOSE) and phosphate molecules.
The rungs in the middle are made up of pairs of nucleotides or nitrogen bases. A-> T and G-> C (vice versa) A= adenine. T=Thymine. G=Guanine. C=Cytosine.
Importantly, the order of the bases determines the genetic code.

The sides or backbone of the DNA molecule are made up of sugar (deoxyribose) and phosphate molecules.
The rungs that form the middle of the molecule are made up of pairs of nucleotides or nitrogen bases.
Adenine pairs with Thymine (A with T) and Guanine pairs with Cytosine (G with C)
The order of the bases determine the genetic code.

DNA structure

DNA fingerprinting is a test to identify and evaluate the genetic information in a person's cells. 

It is called a "fingerprint" because it is very unlikely that any 2 people would have exactly same DNA information, in the same way that it is very unlikely that any 2 people would have exactly same physical fingerprint. 

It is used to determine whether a family relationship exists between two people, to identify organisms causing a disease, and to solve crimes. 

Only a small sample of cells is needed for DNA fingerprinting. A drop of blood or the root of a hair contains enough DNA for testing. Semen, hair or skin scrapings are often used in criminal investigation. 

The first step in DNA fingerprinting is to break open the sample cells and collect the DNA. Next many copies of the DNA are made using polymerase chain reaction (PCR). 

To get the DNA fingerprint: 

1.) All of the DNA is broken into pieces at certain locations by restriction enzymes that break each DNA strand at the same place.

2.) The DNA pieces are placed at the top of a special gel held in a frame. 

3.) AN electric current is applied to the gel. The current separates the DNA into bands of identical pieces. This process is called gel electrophoresis. 

4.) The band pattern is transferred to a nylon membrane containing a radioactive chemical. The chemical makes the bands show up clearly. 

For a criminal investigation, photos of the criminal and suspect DNA are compared. The results as shown in the figure above show that the suspect is not the criminal. 

Further detail on gel electrophoresis...

Illustrated process of gel electrophoresis

1.) A gel is prepared by pouring a liquid containing either melted agarose or unpolymerized acrylamide between two glass plates a few millimetres apart. 

2.) As the agarose solidifies or the acrylamide polymerises into polyacrylamide, a gel matrix (orange ovals) forms, consisting of long, tangled chains of polymers. 

3.) The dimensions of the interconnecting channels, or pores, depend on the concentration of the agarose gels than in polyacrylamide gels, the former are used to separate large DNA fragments (around 500 bp to 20 bp) and the latter to separate small DNA fragments (1 nucleotide to around 2 kb) 

4.) The mixture of DNA fragments to be separated is layered in a well at the top of the gel and an electric current is passed through the gel. 

5.) DNA fragments move toward the positive pole at a rate inversely proportional to the log of their length, forming bands that can be visualised by autoradiography (if the fragments are radiolabeled) or by addition of a fluorescent dye such an ethidium.