Final answer:
The ends that could not be replicated by DNA polymerase are those with '3’' on the leading strand and '5’' on the lagging strand due to the end-replication problem, which is counteracted by the telomerase enzyme.
Step-by-step explanation:
The ends that could not be replicated by DNA polymerase are those with '3’' on the leading strand (5’_ACTG…GTCA_3’) and '5’' on the lagging strand (3’_CATG…TCAG_5’). In eukaryotic chromosomes, which are linear, DNA replication poses a challenge at the chromosome ends due to the inability of DNA polymerase to initiate synthesis de novo. It can only add nucleotides to the 3’ end of a pre-existing primer or DNA strand.
On the leading strand, replication continues to the end. However, on the lagging strand, there is no place to lay down a new primer for the DNA segment at the very end after the previous primer is removed, leading to the well-documented end-replication problem. As a cell divides, these ends, known as telomeres, may progressively shorten. This is counteracted by the action of telomerase enzyme, particularly in germ cells and stem cells, which adds nucleotide repeats to the telomeres, lengthening them and allowing for the complete replication of chromosome ends.