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<span style="font-size: 13.3333px;">link : [https://en.wikipedia.org/wiki/Tetrahymena https://en.wikipedia.org/wiki/Tetrahymena]</span><br/>
The "end replication problem" is exclusive to linear chromosomes as circular chromosomes do not have ends lying without reach of DNA-polymerases. Most [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> A small fraction of [https://en.wikipedia.org/wiki/Bacteria bacterial] chromosomes (such as those in ''[https://en.wikipedia.org/wiki/Streptomyces Streptomyces]'', ''[https://en.wikipedia.org/wiki/Agrobacterium Agrobacterium]'', and ''[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 [https://en.wikipedia.org/wiki/Proteins proteins] 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/>
=== Telomerase ===
'''Telomerase''', also called '''terminal transferase''',<sup id="cite_ref-1">[https://en.wikipedia.org/wiki/Telomerase#cite_note-1 [1]]</sup> is a [https://en.wikipedia.org/wiki/Ribonucleoprotein ribonucleoprotein] that adds a species-dependent [https://en.wikipedia.org/wiki/Telomere#Sequences telomere repeat sequence] to the [https://en.wikipedia.org/wiki/Directionality_(molecular_biology)#3.27-end 3'] end of [https://en.wikipedia.org/wiki/Telomere telomeres]. A telomere is a region of repetitive [https://en.wikipedia.org/wiki/Sequence_(biology) sequences] at each end of the [https://en.wikipedia.org/wiki/Chromosome chromosomes] of most [https://en.wikipedia.org/wiki/Eukaryote eukaryotes]. Telomeres protect the end of the chromosome from [https://en.wikipedia.org/wiki/DNA_damage_(naturally_occurring) DNA damage] or from fusion with neighbouring chromosomes. The fruit fly ''[https://en.wikipedia.org/wiki/Drosophila_melanogaster Drosophila melanogaster]'' lacks telomerase, but instead uses [https://en.wikipedia.org/wiki/Retrotransposon retrotransposons] 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 [https://en.wikipedia.org/wiki/Reverse_transcriptase reverse transcriptase] [https://en.wikipedia.org/wiki/Enzyme enzyme] that carries its own [https://en.wikipedia.org/wiki/Telomerase_RNA_component RNA molecule] (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 ''[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> which is used as a template when it elongates telomeres. Telomerase is active in [https://en.wikipedia.org/wiki/Gamete gametes] and most [https://en.wikipedia.org/wiki/Cancer cancer] cells, but is normally absent in most [https://en.wikipedia.org/wiki/Somatic_cell somatic cells].<br/> <br/>
The existence of a compensatory mechanism for telomere shortening was first found by Soviet biologist [https://en.wikipedia.org/wiki/Alexey_Olovnikov Alexey Olovnikov] in 1973,<sup id="cite_ref-pmid4754905_4-0">[https://en.wikipedia.org/wiki/Telomerase#cite_note-pmid4754905-4 [4]]</sup> who also suggested the telomere hypothesis of [https://en.wikipedia.org/wiki/Aging aging] 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 ''[https://en.wikipedia.org/wiki/Tetrahymena Tetrahymena]'' was discovered by [https://en.wikipedia.org/wiki/Carol_W._Greider Carol W. Greider] and [https://en.wikipedia.org/wiki/Elizabeth_Blackburn Elizabeth Blackburn] in 1984.<sup id="cite_ref-pmid3907856_6-0">[https://en.wikipedia.org/wiki/Telomerase#cite_note-pmid3907856-6 [6]]</sup> Together with [https://en.wikipedia.org/wiki/Jack_W._Szostak Jack W. Szostak], Greider and Blackburn were awarded the 2009 [https://en.wikipedia.org/wiki/Nobel_Prize Nobel Prize] in [https://en.wikipedia.org/wiki/Nobel_Prize_in_Physiology_or_Medicine Physiology or Medicine] 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> Later the [https://en.wikipedia.org/wiki/Cryogenic_electron_microscopy cryo-EM] structure of telomerase was first reported in ''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 [https://en.wikipedia.org/wiki/Cellular_aging cell aging] and [https://en.wikipedia.org/wiki/Cancer cancer] was established by scientists at [https://en.wikipedia.org/wiki/Biotechnology biotechnology] company [https://en.wikipedia.org/wiki/Geron_Corporation Geron] with the cloning of the [https://en.wikipedia.org/wiki/RNA RNA] and catalytic components of human telomerase<sup id="cite_ref-9">[https://en.wikipedia.org/wiki/Telomerase#cite_note-9 [9]]</sup> and the development of a [https://en.wikipedia.org/wiki/Polymerase_chain_reaction polymerase chain reaction] (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 [https://en.wikipedia.org/wiki/Negative_stain negative stain] electron microscopy (EM) structures of human and ''Tetrahymena'' 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> 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> 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 : [https://en.wikipedia.org/wiki/Telomerase https://en.wikipedia.org/wiki/Telomerase]
=== DNA replicate ===