|MadSci Network: Molecular Biology|
Dear Lisa Hughes, Despite the fact that I have never done calf thymus DNA extraction, I readily believe you when you are saying that the DNA is visible as strands as long as two meters. It is generally observed that amongst many common DNA sources, the most impressive amount is from the calf thymus. Lets just consider some basic facts (for a single human cell): -Humans have about 3,000,000,000 bp of DNA per genome -The rise in the B-DNA helix is very close to 0.34 nm per bp So, 3x10(9) bp x 0.34x10(-9) m/bp => 1 meter (2 meter if you consider the second set of chromosomes). Therefore, for a single human cell, the DNA helix would be around 1-2 meters. Having this example in mind and also the huge quantity of DNA coming from your extraction, there is no need for multiple DNA molecules to be stuck together vertically to make them so visible. Instead, we can view all DNA strands arranged in a more or less side-by-side fashion resulting in some very thick strands that are visible to the naked eye. For the following part of your question, you are basically right. DNA precipitation is based on the non-solubility of DNA in alcohol (ethanol in your experiment). The purpose of adding salt is to neutralize the negative charges of DNA backbones so that it will be easier for them to clump together. It also has to be said that since DNA is less dense than water and the rest of the cell debris (cell walls, membranes, organelles, proteins, ...), it floats up at the surface of the liquid. This is why upon gently pouring a layer of ethanol, you can see precipitation of DNA at the interface between water and ethanol. At this stage, it should be kept in mind that residual RNA that has not been degraded by intracellular enzymes (ribonucleases) can also readily precipitate with DNA. By the way, if we assume that an average human contains about 0.5 grams of DNA (which is approximatively correct) and that DNA weighs about 1x10(-21) grams/bp, all the DNA stretched out in the B-DNA double helical form would extend a little more than 400 times the distance earth-moon...! Daniel Lafontaine. My principal references: DNA extractions: http://www.accessexcellence.org/AE/AEC/CC/DNA_extractions.html http://raven.umnh.utah.edu/secondlevel/core/dnaextraction/index/dnaindex.html
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