A reason for DNA's accuracy? - (Sep/01/2008 )

A cell so accurately replicates DNA that there is less than one mistake per billion nucleotides added to the growing chains. Suggest a reason for this, please help me.

DNA polymerase adds bases from 5'->3', but can also remove bases from the 3' end=exonuclease ,the 3'->5' exonuclease can remove the last base that was added, if it was added incorrectly=proofreading, then fill in that sequence with the correct base ,proofreading makes replication extremely accurate at producing an exact copy .

if your questions mean why DNA's accuracy, then the answer is to maintain the genetic stability. as for instance if it mutated frequntly then different species emerge form one species. human offspring should be human.

The DNA should stay the fidelity of the chain to conserved the genes,as party said if got lots of error could drive to mutations. The coding part of genes need to stay conserved so the proteins will be functional. As the cell age will have an acumulation of nucleotide error, but the genes that make the characteristic of the specie will remain the same or at least the mutations will no interfere with the protein functionability. Thats why we have a system that corrects the errors the Mismatch Repair Proteins will remove several hundreds of bases before and after the error, not the incorrect one only (as many think) then the replication will resume. If the MMR proteins don't function well the accumulation of mutation will be higher leading to diseases like cancer.

Bonus points: why isn't the accuracy dramatically higher?

hi thx everyone
the replies have really helped me
and Spanishflower, thx i have my answer and thx to others i have more knowledge on DNA's accuracy.