What is an end replication problem?

Answer 1

The two strands of a DNA molecule are antiparallel to one another (the backbone of one strand runs from 5'-3' while the complimentary strand runs 3'-5'). Unfortunately, DNA polymerase, the enzyme responsible for replicating DNA, can only make DNA in a 5'-3' direction (and read DNA in the 3'-5' direction). Also, it needs a "primer" to give it a place to bind and start replication. So this creates a problem when synthesizing the 3'-5' stand because your enzyme will only synthesize 5'-3'. During replication this is solved by synthesizing small pieces of DNA ahead of the replication fork on the 5'-3' mother strand. Thus we have one daughter strand which is synthesized as a continuous piece of DNA (called the leading strand) and one daughter strand which is synthesized in small, discontinuous pieces (called the lagging strand). However, at the extreme end of the DNA, we run into another problem. The leading stand can be made to the very end, but the lagging strand cannot because you need the RNA primer upstream to begin each piece of the lagging strand DNA but at the end of the DNA there is nothing for this piece to attach to. Thus, the last section of the lagging strand cannot be synthesized and after several rounds of DNA replication, the DNA molecule gets smaller and smaller. This is "the end of replication problem" and it is solved by putting a DNA cap on the ends of DNA called a telomere which does not code for any protein, thus when this information is lost it does not have severe consequences for the cell.