Re: how long is the DNA of each human cell?

This is a great question. We can start by looking up the answer:
 http://www.nature.ca/genome/
03/a/03a_11a_e.cfm

Diploid length = "Close to two meters"

That's no fun, in science we want to figure it out ourselves. Here are some links to web sites 
about DNA structure:
 http://www.blc.arizona.edu/Molecular_Graphics/DNA_Structure/
DNA_Tutorial.HTML
 http://
www.accessexcellence.org/AE/AEC/CC/DNA_structure.html
 http://
www.accessexcellence.org/AB/GG/structure.html
 http://
www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/dnast.htm

The last site has this statement:

"E. coli  has a single circular DNA molecule of 4,600,000 base pairs.  The total length is 1.4 mm."

We also learn from the other sites that DNA has "about" 10 base pairs per turn, and that the 
length of a turn is 3.4 nm (a nonometer is one billionth of a meter).

This gives us two estimates of the length of a DNA molecule with a given number of base pairs. 

The E. coli estimate gives:

4,600,000 base pairs/1.4 mm = 3,285,714 bases/mm = 3.29 Gbp/m

The 10 base pairs per turn estimate gives:

10 base pairs/3.4 nm = 2.94 bp/nm = 2.94 Gbp/m

A "Gbp" or gigabasepair is 1 billion base pairs.

For an estimate of the size of the human genome in base pairs we go to Ensembl:
 http://www.ensembl.org/Homo_sapiens/


Build 32 at Ensembl = 3,223,443,491 base pairs in the human genome = 3.2 Gbp

Using the first estimate we get:

3.2 Gbp x 1 meter/3.29 Gbp = 0.973 meter/haploid human genome

Using the second estimate we get:

3.2 Gbp x 1 meter/2.94 Gbp = 1.088 meter /haploid human genome

So the correct answer is "about a meter" for the length of the haploid human genome or "about 
two meters" for the length of the diploid human genome that is in each of your cells.

At Ensembl you can get estimates of the length of each chromosome in base pairs; working out 
this part is left as an exercise for you.

Yours,

Paul Szauter
Mouse Genome Informatics