<P> Nevertheless, in the 1970s there was no recognition that the telomere - shortening mechanism normally limits cells to a fixed number of divisions, nor was there any animal study suggesting that this could be responsible for aging on the cellular level . There was also no recognition that the mechanism set a limit on lifespans . </P> <P> It remained for a privately funded collaboration from biotechnology company Geron to isolate the genes for the RNA and protein component of human telomerase in order to establish the role of telomere shortening in cellular aging and telomerase reactivation in cell immortalization . </P> <P> Telomeres are repetitive nucleotide sequences located at the termini of linear chromosomes of most eukaryotic organisms . For vertebrates, the sequence of nucleotides in telomeres is T T A G G G. Most prokaryotes, having circular chromosomes rather than linear, do not have telomeres . Telomeres compensate for incomplete semi-conservative DNA replication at chromosomal ends . A protein complex known as shelterin serves to protect the ends of telomeres from being recognised as double - strand breaks by inhibiting homologous recombination (HR) and non-homologous end joining (NHEJ). </P> <P> In most prokaryotes, chromosomes are circular and, thus, do not have ends to suffer premature replication termination . A small fraction of bacterial chromosomes (such as those in Streptomyces, Agrobacterium, and Borrelia) are linear and possess telomeres, which are very different from those of the eukaryotic chromosomes in structure and functions . 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 . </P>

Repetitive dna at the end of a eukaryotic chromosome