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Biology-Dna Fingerprinting And Polymerase Chain Reaction

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In this coursework I will be exploring two issues, my major issue being DNA Fingerprinting and my minor issue is PCR (Polymerase Chain Reaction).

DNA Fingerprinting

(Obtained from

(The diagram above shows that the defendant had the victim’s blood on his clothes)

Web Description: A method of comparing the genetic similarities or differences between individuals. This technology is often used as a forensic tool to identify the source of blood and tissue samples found at crime scenes. (

Science behind DNA

DNA- Deoxyribonucleic Acid is a long chain molecule made up of many nucleotides. A Nucleotide contains 3 molecules held together by condensation reactions, these molecules are: Phosphate group, Deoxyribose and an Organic base.

These nucleotides are linked between then the sugar group of one nucleotide and the phosphate group of another. The base containing nitrogen is the only part of the nucleotide which changes.

They are interchangeable by the 4 bases, Adenine, Cytosine, Guanine and Thymine. These bases form the double helix backbone of a DNA strand.

The sequence of these bases is what determines our characteristics. Each individual has 99.9% of the same DNA; it is only identical twins that have exactly the same pattern. It is because of this 0.1% we can identify which person a piece of DNA has come from. This is what DNA fingerprinting works from, it screens for certain DNA sequences which are found in some persons but not others.

What Is DNA Fingerprinting?

DNA fingerprinting is a procedure which can distinguish the difference in DNA from certain persons without having to observe the complete 3 billion bases in the whole genome. It is called fingerprint as it is improbable that any two people will have the same fingerprint.

How Is It Done?

The way in which the cells are obtained depends on what is being tested. For example if it is for a paternity test blood will be drawn from the veins. Also DNA can be obtained from saliva which is taken using a cotton swab wiped against the inside of the cheek.

Firstly the DNA is removed from the cells using the detergents on the cell membrane. Cell membranes are coated in detergents and this dissolves the lipids of the cell membrane exposing the protein and nucleic acids. Restriction enzymes are added and each recognize a precise sequence and cut it away. Restriction enzymes will cut the DNA at a specific sequence. Restriction enzymes are naturally in bacteria where they are used to cut up attacking viral DNA, the enzymes hold the value that they will only cut the DNA at certain base sequences. For example the enzyme EcoRI comes from the bacteria Escherichia coli strain RY13 and the DNA is split wherever the base sequence is GAATTC. This then produces different lengths of sequences which then need to be ordered according to their lengths.

Agarose Gel Electrophoresis is used to do this. DNA is a negatively charged particle so it will shift in the direction of a positive electrode in an applied electric field. A charged particle is a particle which is either positive or negative. The fragments travel in this electric field in accordance with their size. The gel slows down the larger particles but the smaller particles move quicker therefore the sequences are arranged in order.

Southern Blotting is then used to transfer the fragments to a nitrocellulose filter. The filter is placed over the gel and the DNA fragments are then absorbed to. The paper acts as a wick to extract an alkaline buffer solution all the way through the gel. The strands then break up revealing the base sequences.

Chemical probes are then added and they contain the complementary bases for the bases which have been separated. The probe is made using radioactive atoms which in turn makes the DNA fragments radioactive so when the sample is exposed to x-ray film then sequences will show up.

A fluorescent probe can be used as an alternative, and then it is placed under an ultraviolet light to see the sequences.

What Is It Used For?

• Maternity & Paternity Tests

This is to determine paternity or to also identify the parents of babies who may have been switch at birth, which is very rare.

The DNA sequence of the parents and that of the child’s are compared and analysed. From this it able to determine who the mother and father is. To determine the mother of a child the mitochondrial genome can be looked at as this only gets passed on from the mother without any changing. A father’s paternity can be established through the blood type of the father compared with the child’s.

• Abnormalities of an Embryo

It can be used for prenatal testing of an embryo. There are two ways of acquiring the sample of cells needed before a child is born. These are Amniocentesis and Chronic Villus Sampling.

1. Amniocentesis

This is when a needle is inserted into the amniotic fluid to gather cells that have fallen off the placenta and foetus. Amniocentesis carries between a 0.5% and 1% chance of miscarriage. It can be carried out at around 15-17 weeks of pregnancy.

2. Chronic Villus Sampling

A small piece of the placenta’s tissue is removed through the abdomen or vagina. This can take place between 8-12 weeks since no amniotic fluid is needed. The risk of miscarriage is between 1 and 2%.

• Solving Crimes & Identification

People who commit crimes can expect to be found out using the technique of DNA Fingerprinting.



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