Expert: Dana Krempels, Ph.D. - 10/1/2009

Question
Dr. Krempels

As you suggested I am resubmitting my question.

"When a human cell divides, all the DNA in the chromosomes unzips. The approx. 3 billion base pairs now expose 6 billion bases. To replicate the DNA, the exposed bases must find 6 billion each of: matching base, sugar, and phosphate molecules.  When the two cells divide again, 12 billion new sets of molecules must be found.  Where do these molecules come from."


Answer
Dear Gunther,

You can see a nice movie of the process of DNA replication here:

http://www.youtube.com/watch?v=teV62zrm2P0

The one misleading thing about this video is that it makes it appear that the DNA polymerase makes the nucleotides *de novo*, and links them to the existing DNA template strand.  In reality, the nucleotides diffuse to the area of replication, have an affinity for the polymerase, and link to an active site on the enzyme so that they can be bonded (covalently to the previous new nucleotide and hydrogen-bonded to its older neighbor across the DNA helix) to the growing strand.

The cell manufactures nucleotides from raw materials the organism obtains from its environment.  In the case of a photosynthetic organism, the earliest precursors would be carbon dioxide and water from the environment, which are made into sugar via photosynthesis.  The organic sugar molecules are not only used to store energy, but are also used as raw materials to help build all the other organic substances in the cell--including nucleotides--via a long series of complex, enzymatic reactions.

Animals use organic molecules from the organisms they eat as raw materials to manufacture the nucleotides they need to build DNA (and other intracellular structure), also via complex series of enzymatic pathways of many different kinds.

To get a detailed description of nucleotide biosynthesis, you'll have to do a little biochemistry, which may be farther than you want to go. But there's an overview here:

http://www.biochem.arizona.edu/classes/bioc461/Chapter25Notes.htm

if you are interested.

I hope this helps.

Dana