What's the Connection?

What does skin, hair or blood left at a crime scene have in common with Echinacea?

DNA is the chemical that embodies the genetic makeup of all living things and acts as a blueprint for physical development. The chemical structure of DNA for plants, animals and humans is virtually identical. The only difference is the order of the nucleotide bases A, T, C and G. Different DNA sequences result because there are so many millions of base pairs that every person’s DNA sequence is different. Characteristics of all living organisms are determined by information contained within DNA that is inherited from each parent. Although some areas of human DNA are universal, certain sections contain different variations resulting in a distinctive DNA sequence for each individual. These variations of the base sequence are referred to as polymorphisms. Scientists use these variations to create a genetic map, or barcode, to compare different samples to.

The method used to amplify trace amounts of DNA at a crime scene is the same method used to amplify DNA from rare plant specimens. When working with rare plant specimens, and in the forensic sciences, it is not uncommon to have only a small amount of sample to work with. With the invention of PCR it is now possible to amplify an unusable trace amount of DNA into a larger, testable pool. PCR is a very simple technique that can also be thought of as “molecular photocopying”. It is possible to achieve exponential amplification of a target gene because the products of each synthesis reaction, or each cycle, can because used as templates for further DNA synthesis. After 25 cycles of a PCR, or approximately 2 hours, there are 25 000 copies of the desired gene.

Molecular Photocopying

Double stranded DNA is split apart into two single stranded DNA pieces. Then, the two DNA strands are mixed with all the chemicals necessary for DNA replication. When all of the components of replication are in the correct ratio and the temperature of the reaction mixture is optimal, single stranded DNA molecules combine with their complementary counterparts to become double stranded DNA molecules. After one PCR cycle there is double the amount of DNA sample you started with. The process is repeated until enough DNA is amplified to complete whatever lab work necessary. For more information, click here.

After DNA amplification is complete gel electrophoresis is used to separate the DNA fragments resulting in a very characteristic DNA fingerprint. The exact number and size of fragments produced is dictated by the polymorphisms within the DNA sample. Once the analysis is complete, it can be concluded that samples with an identical barcode are from the same sample, or in the case of Echinacea, from the same species.

Come Investigate With ISI

Below is a pictorial image of a common agarose gel used to separate RAPD-PCR amplification products. Agarose gels are used to visualize the DNA sample after it has been amplified. A marker, or DNA ladder, is a mixture of DNA fragments of known size to compare our amplification products to. This helps us to compare each species unique barcodes and determine if they are the similar.

SEE BELOW FOR ANSWER