|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?
I did not know the answer to this off the top of
my head, but there is a way to do a rough
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
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.
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:
pairs) x (3.4 angstroms per base pair) =
10,200,000,000 angstroms long.
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
Some comparisons: many
atoms are 0.1-0.2 nm in diameter.
cold virus is a sphere 30 nm across.
colored soap bubbles are about 100-400 nm
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
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.
length of DNA in eucaryotic cells is so great that
the risk of entanglement and breakage becomes
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).
hope this answers your question.
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