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<span style="font-size: 13.3333px;">link&nbsp;:&nbsp;[https://en.wikipedia.org/wiki/Tetrahymena https://en.wikipedia.org/wiki/Tetrahymena]</span><br/> &nbsp;
 
The "end replication problem" is exclusive to linear chromosomes as circular chromosomes do not have ends lying without reach of DNA-polymerases. Most&nbsp;[https://en.wikipedia.org/wiki/Prokaryote prokaryotes], relying on circular chromosomes, accordingly do not possess telomeres.<sup id="cite_ref-11">[https://en.wikipedia.org/wiki/Telomere#cite_note-11 [10]]</sup>&nbsp;A small fraction of&nbsp;[https://en.wikipedia.org/wiki/Bacteria bacterial]&nbsp;chromosomes (such as those in&nbsp;''[https://en.wikipedia.org/wiki/Streptomyces Streptomyces]'',&nbsp;''[https://en.wikipedia.org/wiki/Agrobacterium Agrobacterium]'', and&nbsp;''[https://en.wikipedia.org/wiki/Borrelia 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&nbsp;[https://en.wikipedia.org/wiki/Proteins proteins]&nbsp;bound to the ends of linear chromosomes, or hairpin loops of single-stranded DNA at the ends of the linear chromosomes.<sup id="cite_ref-12">[https://en.wikipedia.org/wiki/Telomere#cite_note-12 [11]]</sup><br/> &nbsp;
 
=== Telomerase ===
 
'''Telomerase''', also called&nbsp;'''terminal transferase''',<sup id="cite_ref-1">[https://en.wikipedia.org/wiki/Telomerase#cite_note-1 [1]]</sup>&nbsp;is a&nbsp;[https://en.wikipedia.org/wiki/Ribonucleoprotein ribonucleoprotein]&nbsp;that adds a species-dependent&nbsp;[https://en.wikipedia.org/wiki/Telomere#Sequences telomere repeat sequence]&nbsp;to the&nbsp;[https://en.wikipedia.org/wiki/Directionality_(molecular_biology)#3.27-end 3']&nbsp;end of&nbsp;[https://en.wikipedia.org/wiki/Telomere telomeres]. A telomere is a region of repetitive&nbsp;[https://en.wikipedia.org/wiki/Sequence_(biology) sequences]&nbsp;at each end of the&nbsp;[https://en.wikipedia.org/wiki/Chromosome chromosomes]&nbsp;of most&nbsp;[https://en.wikipedia.org/wiki/Eukaryote eukaryotes]. Telomeres protect the end of the chromosome from&nbsp;[https://en.wikipedia.org/wiki/DNA_damage_(naturally_occurring) DNA damage]&nbsp;or from fusion with neighbouring chromosomes. The fruit fly&nbsp;''[https://en.wikipedia.org/wiki/Drosophila_melanogaster Drosophila melanogaster]''&nbsp;lacks telomerase, but instead uses&nbsp;[https://en.wikipedia.org/wiki/Retrotransposon retrotransposons]&nbsp;to maintain telomeres.<sup id="cite_ref-pmid21821789_2-0">[https://en.wikipedia.org/wiki/Telomerase#cite_note-pmid21821789-2 [2]]</sup>
 
Telomerase is a&nbsp;[https://en.wikipedia.org/wiki/Reverse_transcriptase reverse transcriptase]&nbsp;[https://en.wikipedia.org/wiki/Enzyme enzyme]&nbsp;that carries its own&nbsp;[https://en.wikipedia.org/wiki/Telomerase_RNA_component RNA molecule]&nbsp;(e.g., with the sequence 3′-[https://en.wikipedia.org/wiki/Cytosine C]CC[https://en.wikipedia.org/wiki/Adenine A]A[https://en.wikipedia.org/wiki/Uracil U]CCC-5′ in&nbsp;''[https://en.wikipedia.org/wiki/Trypanosoma_brucei Trypanosoma brucei]'')<sup id="cite_ref-pmid10097086_3-0">[https://en.wikipedia.org/wiki/Telomerase#cite_note-pmid10097086-3 [3]]</sup>&nbsp;which is used as a template when it elongates telomeres. Telomerase is active in&nbsp;[https://en.wikipedia.org/wiki/Gamete gametes]&nbsp;and most&nbsp;[https://en.wikipedia.org/wiki/Cancer cancer]&nbsp;cells, but is normally absent in most&nbsp;[https://en.wikipedia.org/wiki/Somatic_cell somatic cells].<br/> <br/> &nbsp;
 
The existence of a compensatory mechanism for telomere shortening was first found by Soviet biologist&nbsp;[https://en.wikipedia.org/wiki/Alexey_Olovnikov Alexey Olovnikov]&nbsp;in 1973,<sup id="cite_ref-pmid4754905_4-0">[https://en.wikipedia.org/wiki/Telomerase#cite_note-pmid4754905-4 [4]]</sup>&nbsp;who also suggested the telomere hypothesis of&nbsp;[https://en.wikipedia.org/wiki/Aging aging]&nbsp;and the telomere's connections to cancer and perhaps some neurodegenerative diseases.<sup id="cite_ref-pmid16247010_5-0">[https://en.wikipedia.org/wiki/Telomerase#cite_note-pmid16247010-5 [5]]</sup>
 
Telomerase in the ciliate&nbsp;''[https://en.wikipedia.org/wiki/Tetrahymena Tetrahymena]''&nbsp;was discovered by&nbsp;[https://en.wikipedia.org/wiki/Carol_W._Greider Carol W. Greider]&nbsp;and&nbsp;[https://en.wikipedia.org/wiki/Elizabeth_Blackburn Elizabeth Blackburn]&nbsp;in 1984.<sup id="cite_ref-pmid3907856_6-0">[https://en.wikipedia.org/wiki/Telomerase#cite_note-pmid3907856-6 [6]]</sup>&nbsp;Together with&nbsp;[https://en.wikipedia.org/wiki/Jack_W._Szostak Jack W. Szostak], Greider and Blackburn were awarded the 2009&nbsp;[https://en.wikipedia.org/wiki/Nobel_Prize Nobel Prize]&nbsp;in&nbsp;[https://en.wikipedia.org/wiki/Nobel_Prize_in_Physiology_or_Medicine Physiology or Medicine]&nbsp;for their discovery.<sup id="cite_ref-Nobel_Prize_2003_7-0">[https://en.wikipedia.org/wiki/Telomerase#cite_note-Nobel_Prize_2003-7 [7]]</sup>&nbsp;Later the&nbsp;[https://en.wikipedia.org/wiki/Cryogenic_electron_microscopy cryo-EM]&nbsp;structure of telomerase was first reported in&nbsp;''T. thermophila'', to be followed a few years later by the cryo-EM structure of telomerase in humans.<sup id="cite_ref-pmid31451513_8-0">[https://en.wikipedia.org/wiki/Telomerase#cite_note-pmid31451513-8 [8]]</sup>
 
The role of telomeres and telomerase in&nbsp;[https://en.wikipedia.org/wiki/Cellular_aging cell aging]&nbsp;and&nbsp;[https://en.wikipedia.org/wiki/Cancer cancer]&nbsp;was established by scientists at&nbsp;[https://en.wikipedia.org/wiki/Biotechnology biotechnology]&nbsp;company&nbsp;[https://en.wikipedia.org/wiki/Geron_Corporation Geron]&nbsp;with the cloning of the&nbsp;[https://en.wikipedia.org/wiki/RNA RNA]&nbsp;and catalytic components of human telomerase<sup id="cite_ref-9">[https://en.wikipedia.org/wiki/Telomerase#cite_note-9 [9]]</sup>&nbsp;and the development of a&nbsp;[https://en.wikipedia.org/wiki/Polymerase_chain_reaction polymerase chain reaction]&nbsp;(PCR) based assay for telomerase activity called the TRAP assay, which surveys telomerase activity in multiple types of cancer.<sup id="cite_ref-10">[https://en.wikipedia.org/wiki/Telomerase#cite_note-10 [10]]</sup>
 
The&nbsp;[https://en.wikipedia.org/wiki/Negative_stain negative stain]&nbsp;electron microscopy (EM) structures of human and&nbsp;''Tetrahymena''&nbsp;telomerases were characterized in 2013.<sup id="cite_ref-11">[https://en.wikipedia.org/wiki/Telomerase#cite_note-11 [11]]</sup><sup id="cite_ref-12">[https://en.wikipedia.org/wiki/Telomerase#cite_note-12 [12]]</sup>&nbsp;Two years later, the first cryo-electron microscopy ([https://en.wikipedia.org/wiki/Cryo-EM cryo-EM]) structure of telomerase holoenzyme (''Tetrahymena'') was determined.<sup id="cite_ref-13">[https://en.wikipedia.org/wiki/Telomerase#cite_note-13 [13]]</sup>&nbsp;In 2018, the structure of human telomerase was determined through cryo-EM by UC Berkeley scientists.<sup id="cite_ref-14">[https://en.wikipedia.org/wiki/Telomerase#cite_note-14 [14]]</sup><br/> <br/> full text link&nbsp;:&nbsp;[https://en.wikipedia.org/wiki/Telomerase https://en.wikipedia.org/wiki/Telomerase]
=== DNA replicate ===
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