Expert: Dana Krempels, Ph.D. - 11/30/2011

Question
Dear Expert,

 I am Mr Washington. Since this is my first time learning about DNA, I hope you can help me. During DNA replication of the lagging strand, why can't the DNA polymerase  synthesize the lagging strand continuously in one piece? I know that the DNA polymerase can only replicate in one direction so they replicate in a different direction(since the lagging strand has different ends), but why must they form Okazaki fragments ? Can the polymerase just replicate the lagging strand continuously in a opposite direction,without forming fragments, like it did on the leading strand?
Regards,
Washy

Answer
Dear Mr. Washington,

The problem is that the enzymes that facilitate DNA replication can add incoming nucleotides only to the existing 3' end of the growing strand.  They *cannot* add them to the 5' strand.  They simply are not configured that way.  Adding the new nucleotide to the 3' end also allows the new nucleotide to provide energy for the reaction when the phosphate groups on the 5' end are hydrolyzed.

For a very nice animation of why Okazaki fragments are necessary, have a look here:

http://www.bio.miami.edu/dana/mov/DNAreplication1.mov

http://www.bio.miami.edu/dana/mov/cont_discont_replication.mov

If you don't want to wait so long for that to load, here's one on YouTube that's also pretty good:

http://www.youtube.com/watch?v=teV62zrm2P0&feature=results_video&playnext=1&list

I hope it helps you understand the process.

The main reason, though, is that the new nucleotides can be added ONLY to an existing 3' (-OH) end of the growing strand, not to the 5' (phosphate) end.  


Dana