Ask A Scientist Molecular Biology Archive

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Question -   If you spread out a strand of DNA, how long would it be?
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As described in the "Chromosomal DNA and Its Packaging" section of the on-line textbook, Molecular 
Biology of the Cell @ NCBI 
( http://www.ncbi.nlm.nih.gov/books/bv.fcgi?tool=bookshelf&call=bv.View..Show Section
&searchterm=human&rid=cell.section.1607#1608 ):

"Every million "letters" (nucleotides) take up a linear distance of only 3.4 × 10E5 nm 
(0.034 cm)....The [haploid] human genome...contains about 3 x 10E9 nucleotide pairs, organized 
as 24 chromosomes (22 different autosomes and 2 different sex chromosomes), and thus consists 
of 24 different DNA molecules - each containing from 50 × 10E6 to 250 x 10E6 nucleotide pairs 
of DNA. DNA molecules of this size are 1.7 to 8.5 cm long when uncoiled, and even the slightest 
mechanical force will break them once the chromosomal proteins have been removed."

Thus, using the conversion factor of 0.034 cm per million (10E6) nucleotides, the uncoiled length 
of the total human genome would be approximately 102 cm, or 40 inches.  This is pretty impressive 
if you consider that "A typical [dipoid] human cell thus contains ...about 6 × 10E9 nucleotide 
pairs of DNA....By comparison, 6 x 10E9 letters..would occupy more than a million pages [of a 
typical book].", as described in the Molecular Biology of the Cell at NCBI.  The "Chromosomal 
DNA and Its Packaging" section also explains the nature of haploid vs. diploid chromosomal 
organization, if you need some clarification 
( http://www.ncbi.nlm.nih.gov/books/bv.fcgi?tool=bookshelf&call=bv.View..Show Section
&searchterm=human&rid=cell.section.1607#1608 ).

Thanks for the great question,

Jeff Buzby, Ph.D.
CHOC Research Institute

NEWTON AAS
Division of Educational Programs
Argonne National Laboratory
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