MadSci Network: Molecular Biology

Re: How are the DNA molecules arranged after an extraction lab?

Date: Fri Nov 26 05:06:45 1999
Posted By: Daniel Lafontaine, Post-doc/Fellow, Biochemistry, University of Dundee
Area of science: Molecular Biology
ID: 941844381.Mb

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 

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:

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