Final answer:
The end replication problem in linear DNA molecules occurs when there is no place to make a primer for the DNA fragment at the end of the chromosome, resulting in unpaired telomeres that may get shorter over time.
Step-by-step explanation:
The end replication problem in relation to a linear DNA molecule arises because DNA polymerase can only add nucleotides in the 5' to 3' direction. During DNA replication, the leading strand is synthesized continuously, but the lagging strand is synthesized in short stretches called Okazaki fragments. When the replication fork reaches the end of the linear chromosome, there is no place to make a primer for the DNA fragment at the end of the chromosome. As a result, these ends, known as telomeres, remain unpaired and may get progressively shorter with each round of replication.