Question

What kind of samples when used as evidence may require the separation of the sample into its various chemical components? Try to think of one unique example and what a separation would tell you.

Answer 1

Blood samples (to identify DNA) when used as evidence may require the separation of the sample into its various chemical components. This method is mostly used by Forensic sciences laboratories.

Red blood cells do not have any DNA, as they lose their nuclei (the compartment in a cell that contains the DNA) as they mature. So the DNA in your blood is in your white blood cells. To get at it, scientists first spin a small sample of your blood at high speed, to separate the cells from the blood fluid. Next, they release the DNA from the cells using a detergent and a special enzyme. Finally, they add alcohol, which makes your DNA appear as sticky blob in the mixture.

There are three basic and two optional steps in a DNA extraction:

• Cells which are to be studied need to be collected.
• Breaking the cell membranes open to expose the DNA along with the cytoplasm within (cell lysis).
  • Lipids from the cell membrane and the nucleus are broken down with detergents and surfactants.
  • Breaking proteins by adding a protease (optional).
  • Breaking RNA by adding an RNase (optional).
• The solution is treated with concentrated salt solution to make debris such as broken proteins, lipids and RNA to clump together.
• Centrifugation of the solution, which separates the clumped cellular debris from the DNA.