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Many eukaryotic organisms like yeasts, hydras , planarias, plants etc reproduce asexually.

Replication of End of linear DNA pose a limit to the number of cell divisions.

My question : Do asexually reproducing organisms have telomerase in all their cells ? Is the telomerase activated at specific times at specific places ?

If they don't, how can they reproduce infinitely ?

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I'm not sure I can answer generically for all asexual organisms, but would an example of why this isn't an issue in E. Coli and yeast work? –  Atl LED Jan 28 at 2:18
    
@AtlLED Thanks! but I think I know why it works in Yeast and E.Coli - Isn't it because they have round chromosomes and so there is no telomeres. –  biogirl Jan 28 at 7:41
    
E. coli is really a bacterium and has only a circular chromosomes. On the other hand, yeast like S. cerevisiae is an eukaryotic organism and has 16 chromosome pairs if it is diploid. The division in yeast is limited. It is done by budding, where there is a clear difference, which cell is a mother and which one is a daughter. Mother can have only certain number of daughters and stops budding afterwards. I do not know, if this is due to telomeres, or budding scars or both. –  Barbara Jan 28 at 8:19
    
@AtlLED Ok, so I was wrong about the yeasts :) You could tell me about them. –  biogirl Jan 28 at 8:24

2 Answers 2

up vote 2 down vote accepted

Short answer:

No. Eukaryotes have more ways of maintaining telomere length than via telomerase alone and all organisms with circular genomes do not need to worry about telomere length anyway.

Long answer:

Firstly, the telomerase system is not the only observed mechanism in Eukaryotes that elongates telomeres. Other mechanisms such as the transposition of retrotransposons (e.g. Drosophila) or recombination (e.g. Yeast) can also be used in some species. Yeast has been observed to use several of these mechanisms. So no, telomerase itself is not always necessary for immortality.

Ofcourse prokaryotes do not need to worry about shortening chromosome ends at all since their genome is circular - i.e. it has no ends to shorten with each cell division!

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Here is a study of planarian worms, which are immortal in asexual reproduction and mortal in sexual reproduction.

Hydras also become mortal after they reproduce sexually.

Relevant to your question: Cells within planarian worm differ in expression of telomerase active subunit depending on body part. Immortal (asexual) worms have more expression in the area around the pharynx, those cells there are "the proliferative stem cells". There is a nice figure in the article.

I assume it is similar for hydra, that outer body coat has less expression than the cells inside. “Hydra is a bag of stem cells”

(Less relevant (a version of previous version of my answer. But does not answer the "every cell" inquiry well): In planarian worms "We demonstrate that this difference is reflected in the expression and alternate splicing of the protein subunit of the telomerase enzyme." The mentioned telomerase subunit was 1. comparatively more transcribed in immortal (asexual) worm and 2. within the resultant trancripts, there were possibilities of functional and non-functional isoforms due to alternate splicing. Immortal worms had higher ratio of functional isoforms. Summary: immortals have more transcripts and higher proportion of functional versions of them.)

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Interesting! But I would wait for other answers before accepting yours. –  biogirl Jan 28 at 8:59
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Yeah, I know, I am offering 2 bounties myself. Watch my answer for edits, I might find more clear data when I have time. –  Barbara Jan 28 at 9:03
    
I think the worm aspect of my answer is now correct. I must check later, whether really hydra cells have different telomerase expression in the coat versus inside the body. It interests me, but takes time to look at this thoroughly. If you are impatient, try to read the article "Hydra is the bag of stem cells" plus the original un-popularised reference. –  Barbara Feb 2 at 12:38

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