Telomeres and Telomerase

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slinquii

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There were a couple of similar threads on this, however they didn't help me.

I understand what telomeres are and why the ends of chromosomes cannot be polymerized without the enzyme telomerase adding new nucleotides to the parent strand...allowing the lagging strand to be copied.

However, I do not understand why this would cause shortening instead of lengthening over time.

It seems to me that if telomerase is adding nucleotides to the parent strand each time, the DNA would lengthen over time instead of shorten.

Help?

Thanks.
 
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Telomeres at the 3'end of chromosomes are unreplicated at the overhang. If there were no teleomerase, then telomeres and so the parent strand since would shorten over time, yes. This would be a problem, but telomerase overcomes this shortening by adding nucleotides to the 3' end of the strand with each replication.
 
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Thank you for your replies. However, the information that I've been reading and the videos that I've been watching still communicate that telomerase ADDS nucleotides to the parent strand, in excess of the original number of nucleotides. With telomerase doing this, how is it that DNA molecules still shorten over time? It would make sense that they would actually lengthen over time. There has got to be some puzzle piece that I'm missing.
 
I found this: "Telomerase is active during embryonic development, enabling the rapid cell division that supports normal growth. During the latter stages of human fetal development and in adulthood, telomerase is repressed in most cells, and telomere length gradually decreases during a lifetime."
 
The above poster hints at the key. The key difference that you're missing is that human somatic cells have turned telomerase activity off (http://www.nature.com/onc/journal/v21/n4/full/1205083a.html). This makes them mortal. Only cells that divide very regularly, i.e. germ cells, stem cells, or cancer cells, express telomerase strongly. That's why DNA in somatic cells shortens and why we die.
 
Totally agree, current research supports the link between reduced telomerase activity and aging, and likewise excessive telemorase activity and cancer. My initial response was just a generalisation to the growth phase.
 
NO telomerase activity = shorter each division, acts as a "sort of" aging time clock
Telomerase activity = longer each division, allowing perpetual division

Telomerase activity = good OR bad. Good in the case of an embryo or a stem cell or a bone marrow cell...BAD if you have telomerase activity in a cancer cell.

In your original post you asked why telomerase would cause shortening instead of lengthening...I think THAT is your confusion. It is the EXISTING PROBLEM of shortening DNA strands which is addressed by telomerase. Telomerase would NOT result in shortening, it prevents shortening.

In the case of cancer cells that's not a good thing, so the solution ends up causing a problem...kind of the take home message for cancer generally. All of the cancers occur when the normal regulatory mechanisms (p53, Rb, etc.) which normally SOLVE/PREVENT problems by controlling cell division/cell cycle/apoptosis get messed up. Ironically, it is the knockout or disregulation of the regulatory molecules that normally prevent cancer which causes it.
 
In your original post you asked why telomerase would cause shortening instead of lengthening...I think THAT is your confusion. It is the EXISTING PROBLEM of shortening DNA strands which is addressed by telomerase. Telomerase would NOT result in shortening, it prevents shortening.

I think the OP understands that telomerase prevents shortening. I think he/she is asking why, despite telomerase adding on to the DNA strand, the DNA strand still shortens over a lifetime.
 
From my understanding, while the telomerase still adds on extra bases and protects the DNA, they still lose a very small amount every time it divides. So even though DNA is repaired fairly well, with there being hundreds of divisions, it makes a significant change. At the end, the strand becomes too short and the cell dies.

It seems like you read somewhere that telomerase adds extra bases. You were probably reading something on germline telomerases. Telomerase is much more abundant in germline cells, while in somatic cells, they are few. The germline cells do not experience a loss in base pairs due to high number of telomerases but somatic cells do.
 
Thank you to everyone. Your combined responses really helped me to understand this better. I was wrong in thinking that all cells had the same telomerase activity :/
There will probably be more questions to come as I study for the MCAT... hopefully I'll be able to help with some too
 
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