text: During DNA replication, DNA polymerase cannot replicate the sequences present at the 3 ends of the parent strands. This is a consequence of its unidirectional mode of DNA synthesis: it can only attach new nucleotides to an existing 3-end (that is, synthesis progresses 5-3) and thus it requires a primer to initiate replication. On the leading strand (oriented 5-3 within the replication fork), DNA-polymerase continuously replicates from the point of initiation all the way to the strands end with the primer (made of RNA) then being excised and substituted by DNA. The lagging strand, however, is oriented 3-5 with respect to the replication fork so continuous replication by DNA-polymerase is impossible, which necessitates discontinuous replication involving the repeated synthesis of primers further 5 of the site of initiation (see lagging strand replication). The last primer to be involved in lagging-strand replication sits near the 3-end of the template (corresponding to the potential 5-end of the lagging-strand). Originally it was believed that the last primer would sit at the very end of the template, thus, once removed, the DNA-polymerase that substitutes primers with DNA (DNA-Pol δ in eukaryotes) would be unable to synthesize the "replacement DNA" from the 5-end of the lagging strand so that the template nucleotides previously paired to the last primer would not be replicated. It has since been questioned whether the last lagging strand primer is placed exactly at the 3-end of the template and it was demonstrated that it is rather synthesized at a distance of about 70–100 nucleotides which is consistent with the finding that DNA in cultured human cell is shortened by 50–100 base pairs per cell division.
  If coding sequences are degraded in this process, potentially vital genetic code would be lost. Telomeres are non-coding, repetitive sequences located at the termini of linear chromosomes to act as buffers for those coding sequences further behind. They "cap" the end-sequences and are progressively degraded in the process of DNA replication.
  The "end replication problem" is exclusive to linear chromosomes as circular chromosomes do not have ends lying without reach of DNA-polymerases. Most prokaryotes, relying on circular chromosomes, accordingly do not possess telomeres. A small fraction of bacterial chromosomes (such as those in Streptomyces, Agrobacterium, and Borrelia), however, are linear and possess telomeres, which are very different from those of the eukaryotic chromosomes in structure and function. The known structures of bacterial telomeres take the form of proteins bound to the ends of linear chromosomes, or hairpin loops of single-stranded DNA at the ends of the linear chromosomes.