When DNA is replicated the two strands come apart and tend to get intertwined, which can complicate many other biological processes including trying to divide the DNA equally between daughter cells. Thankfully there is an enzyme, topoisomerase , which changes the topology of DNA circles, in that it will break a DNA chain, take out the chain which is caught, and then rejoin the first chain. All this unwinding of old strands and creation of new strands occurs at a remarkably fast rate. For each origin of replication, roughly 500-1000 nucleotides are added per second. In the absence of topoisomerases, this process would burn up the cell.
Replication also contains built-in error checking .
Over the entire human genome, that works out to roughly 30 errors every single time the genome replicates. There are really only around three errors per replication because of DNA repair . If a repair enzyme finds a mistake, it can fix it, and it can tell which strand is wrong because it can tell which strand is the newly synthesized strand by at the extent of cytosine methylation. As DNAs exist in cells, many of the cytosines have a methyl group added to them by enzymes called methylases . A new DNA will have relatively few methylated cytosines because it has not been around long enough to have picked up that many methyl groups Without DNA repair there can be some major problems.