Final answer:
DNA replication produces a leading strand synthesized continuously and a lagging strand in segments called Okazaki fragments. The lagging strand requires a primer for each segment, which poses a challenge at the chromosome end that is resolved by telomerase extending the telomeres.
Step-by-step explanation:
The process by which DNA is replicated involves two types of strands: the leading strand and the lagging strand. The leading strand is synthesized continuously in the 5' to 3' direction, following the replication fork. In contrast, the lagging strand is synthesized in a series of segments known as Okazaki fragments, each requiring a primer to begin synthesis. The fragments are extended away from the replication fork in the 5' to 3' direction, contrary to the direction of fork movement. This is because DNA polymerase can only add nucleotides in the 5' to 3' direction.
When the replication fork reaches the end of a chromosome, there's a problem for the Okazaki fragments: without a new primer, the last fragment cannot be replaced by DNA nucleotides. This is resolved by the enzyme telomerase, which extends the telomeres of the lagging strand to prevent the DNA from shortening during replication. Telomerase adds a short sequence of DNA nucleotides to the 3' end, allowing space for a primer to be added and for DNA polymerase to fill in the last gap left by the previous primer removal.