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During our discussions on genetics and DNA, I recalled that the amount of DNA in a human is quite substantial, and if the strands were linked end to end, they would cover a distance of "X". I forgot how long human DNA was, and my class is now curious themselves. Can you tell us how long human DNA is?
Answer 1:

I did not know the answer to this off the top of my head, but there is a way to do a rough calculation.
There are about 3 billion nucelotides in human DNA. The average length of a human nucleotide is 0.6 nanometers, or 0.0000000006 meters, so human DNA is about 1.8 meters (5 feet) long. This doesn't sound that impressive, but keep in mind that 5 feet of DNA is being packed into each cell in your body, and the average diameter of the a nucleus in a human cell is only 10 microns, or 0.00001 meters. Clearly, DNA is packaged (twisted, wrapped and folded) so that it is very compact. Another question might be how is this DNA replicated if it's packaged so tightly?

There are about 50 to 75 trillion cells in a human body, so if the DNA from each cell were placed end to end, the chain would be from 90 to 135 trillion meters long.

Answer 2:

If you know how many base pairs a DNA molecule has than you can calculate the length - so I hope you like to do some math!

If we assume that when you stretch the DNA out into a straight line, it still retains the typical B-form helix structure and that a single human genome contains about 3 billion base pairs (3 x 109), then we get the following:
(3,000,000,000 base pairs) x (3.4 angstroms per base pair) = 10,200,000,000 angstroms long.
Now, 1 angstrom is equal to 0.0000000001 meter. So, the length of the human genome is about: 1.02 meters or around 40 inches

On the other hand, the diameter of DNA is around 2 nanometers (nm). That's two billionths of a meter, or 2x10-9 m.
Some comparisons: many atoms are 0.1-0.2 nm in diameter.
The common cold virus is a sphere 30 nm across.
Brightly colored soap bubbles are about 100-400 nm thick.

As you know eucaryotic cells contain a very large quantity of DNA (human cells have at least a thousand times more DNA than a typical bacteria cell).
E.coli chromosomes have about 4.7 x 106 base pairs which results in a length of 1.6 mm. An E.coli cell, however is only 0.002 mm long . That explains why the DNA must be folded in order to fit into the cell.
Now, the length of DNA in eucaryotic cells is so great that the risk of entanglement and breakage becomes severe.
That is the reason why histones (proteins unique to eucaryotes) bind to DNA and wrap it up into compact and manageable chromosomes. (a human egg cell, which is a large cell actually, is about 0.08 mm in diameter. That's why the 1000 mm long DNA is wrapped up and divided into 23 chromosomes to make it fit).
I hope this answers your question.


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