Earlier this year in February, a certain Dr Aziz Aboobaker and his friends from the University of Nottingham published a paper called Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms. These guys were trying to prove that a certain type of planarian flatworm is immortal (Haran 2012). In 2008, they already demonstrated this when they chopped up these worms, and the individual pieces actually grew and regenerated their respective heads and tails (Aboobaker 2008). Except this time, they were trying to find some solid biological evidence.
|The regenerative capacity of the planarian flatworm (Aboobaker 2012)|
Aziz defined this immortality as the ability to maintain the length of DNA in a cell (Aboobaker 2012). Let me explain what he meant. Remember that during DNA replication, in the lagging strand, DNA polymerase III attaches itself to the 3’ end of the primer to produce an Okazaki fragment, and then DNA polymerase I attaches to the fragment in front of it, and replaces the primer with DNA. Well, once we get to the end of the DNA strand, we have a problem, because there’s nothing in front of the very last primer for DNA polymerase I to attach to, and so we’re left with a gap there. This happens every time our DNA replicates during cell division, and so our DNA strands get shorter and shorter (Reece et al. 2012).
|DNA replication and DNA shortening (Reece et al. 2012)|
The reason this isn’t a huge problem for us is because we have sequences of DNA at the ends of our strands that essentially code for nothing. These are called telomeres, and in effect, the telomeres act as a buffer that protects our important coding DNA from being eroded away. What eventually happens, though, is that the telomeres themselves become dangerously short, and that’s when the cell stops dividing. This, theoretically, is the cause of ageing. We do have something that restores the length of our telomeres, and that’s an enzyme called telomerase (Reece et al. 2012). The thing is, in sexual animals such as us, telomerase only activates during reproduction, so you could say that our germ cells are immortal, but our somatic cells still age. Since the worms we’re dealing with are asexual, they reproduce clonally through fission. So, our friend Aziz said that, for the worm to be immortal, it must have a somatic mechanism that indefinitely restores its telomere lengths (Aboobaker 2012).
So they went and did some tests on these worms, and they discovered several things. Firstly, they found that in the absence of any reproductive event, the telomere lengths in both asexual and sexual worms decreased over time, but importantly, these were restored when the asexual worms fissioned or regenerated. This basically showed that there is something that allows asexual worms to somatically restore its telomere lengths- the researches just needed to find out what that is. They did some more tests, and they found that telomerase activity is influenced by the gene, Smed-Tert, and that this activity happens mainly in a certain type of adult stem cell, for asexual animals, but not sexual animals. In the end, it really came down to the way this gene was expressed, and it was the small differences in transcription ratios between asexual and sexual worms that made the first supposedly immortal (Tan et al. 2012).
To summarise, the study showed that a somatic telomere-restoring mechanism is indeed present in asexual animals. This is a pretty important advance in genetics, because understanding how this stuff works can lead to further breakthroughs in the field, and perhaps eventually solve age-related problems in humans.
Aboobaker, A 2008, Immortal Worms, Podcast, University of Nottingham, Nottingham, viewed 20 March 2012, < http://www.test-tube.org.uk/videos/pages_aziz_immortal_worms.htm>
Aboobaker, A 2012, Immortal Worms Breakthrough, Podcast, University of Nottingham, Nottingham, viewed 20 March 2012, < http://www.test-tube.org.uk/videos/pages_aziz_breakthrough.htm>
Haran, B 2012, Who wants to live forever? This immortal worm knows how, viewed 20 March 2012, < http://www.guardian.co.uk/science/blog/2012/feb/29/live-forever-immortal-worms?INTCMP=SRCH>
Reece, JB, Meyers, N, Urry, LA, Cain, ML, Wasserman, SA, Minorsky, PV, Jackson, RB & Cooke, BN 2012, Campbell Biology, 9th ed. (Australian version), Pearson Australia Group, China.
Tan, TCJ, Rahman, R, Jaber-Hijazi, F, Felix, DA, Chen, C, Louis, EJ & Aboobaker, A 2012, ‘Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms’, Proceedings of the National Academy of Sciences of the United States of America, vol. 109, no. 11, pp. 4209-4214, viewed 20 March 2012, < http://www.pnas.org/content/109/11/4209.full>