Re: How big is DNA?

Dear Katy,

Thank you for your question!

DNA (deoxyribonucleic acid, a tongue-twister) is like a two-stranded rope 
that is around 2 nanometers (nm) in diameter.  That's three billionths of a 
meter, or 3x10-9 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.

How long is it?  Well, that depends on how much you have.  :-)  The 
complete set of genes in each of your cells is 3 billion base pairs. 
 This corresponds to a length of one meter of DNA!  Good thing it's so 
thin, or it wouldn't fit in a cell, let alone the nucleus of the cell.

How much does it need to be magnified to be seen with the human eye?  Well, 
most people can see things that are a tenth of a millimeter wide, or 
1x10-4 m.  So you would need to magnify it at least

1x10-4
------  =  33,000 times.
3x10-9

You can't do that with an ordinary light microscope.  You need a 
transmission electron microscope or an atomic force microscope to do it.  
However, you can see DNA in a light microscope if it is balled up into 
chromosomes, as when a cell divides.

Some links for you:

DNA Structure  [Note: 1 A is 0.1 nm]


Atomic force microscopy of DNA


How atomic force microscopy works

I wasn't able to find a good transmission electron microscope image of DNA 
on the web.  Consult your library for a book on molecular biology.

Hope this makes things clearer for you!

Deron Walters
Rice University