Pharyngula

Life will find a way

Creationists sometimes try to argue that what we consider straightforward, well-demonstrated cytological and genetic events don’t and can’t occur: that you can’t get chromosome rearrangements, or that variations in chromosome number and organization are obstacles to evolution, making discussions of synteny, or the rearrangement of chromosomal material in evolution, an impossibility. These are absurd conclusions, of course—we see evidence of chromosomal variation in people all the time.

For example, A friend sent along (yes, Virginia, there is a secret network of evilutionists busily sharing information with one another) a remarkable case study of a radical chromosome arrangement in a mother and daughter. When you see how these chromosomes are scrambled, you’ll wonder how they ever managed to sort themselves out meiotically to produce viable offspring…but life will find a way.

First, here’s a partial karyotype to show the affected chromosomes 6, 9, 11, and 20. These are four pairs of chromosomes, and in a normal karyotype, each member of the pair should be basically identical to the other; here you can see that they’re of different lengths, and even the pattern of bands shows some variations that are difficult to make out.

i-2bc639a31b5487e85571421cbe09f365-cfcr_karyo.jpg
(a, b) Partial high-resolution GBG-banded karyotypes showing the rearranged chromosomes 6, 9, 11 and 20 of mother (a) and daughter (b).

After a fair amount of work, the investigators sorted out what was wrong. There were 12 breakpoints in these four chromosomes, and the fragments had been scrambled about to form these rearranged chromosomes, color-coded to help sort out who is who.

i-22c6badd690fd54c89b1c72f0271a2c2-cfcr_chrom.jpg
Summary of all breakpoints after completion of all mapping efforts. The chromosome fragments are drawn to scale.

This is amazing stuff. This is what cells do, though: when chromosomes are damaged, there are repair enzymes that struggle to put them back together. These enzymes are not smart or guided in any way, so they just do the best job they can…and sometimes that means they are reassembled so they can function, but it may not be the usual order of things.

Now the mother in this pair had the above arrangement of chromosome bits. The father’s chromosome set was normal (as was that of the woman’s parents), except that he did have a small deletion (again, these kinds of minor variations in chromosomes are common). In meiosis, when the woman produces haploid eggs, one step requires that chromosomes pair up—both orange chromosomes line up together, as do both blue ones, both green ones, and both yellow ones. Then they separate in an orderly fashion to guarantee that each egg receives exactly one orange chromosome, one blue, one green, and one yellow.

Umm, wait a minute…both orange chromosomes? In this woman, the orange chromosome is scattered in fragments across four chromosomes. How could this woman’s cells arrange and sort out their contents in a well-distributed way?

Well, one way is for the chromosomes to contort themselves into strange yoga positions that allow most of the color-coded bits to pair up appropriately. Here, for example, is the most likely solution that allows the majority of the chromosomal homologs to pair up in meiosis. That’s impressive.

i-e7630f5cd4042729aa3c9f86bfaa371b-cfcr_tetravalent.jpg
(b–d) Putative pachytene configurations in meiosis. (b) One
possible pachytene configuration could consist of two tetravalents.

Even more impressive is a truly maximal pairing that allows all of the homologous portions to be in register, a structure called an octavalent that brings all 4 pairs of chromosomes together in a very specific tangle. This is an optimal arrangement for pairing, but is less likely to have occurred simply because getting that many chromosomes into an ideal arrangement is difficult.

i-5150ec534f586e9d8712fc49e26fd007-cfrc_octavalent.jpg
(c) Possible octavalent pachytene figure allowing more regions to
synapse. (d) An alternative octavalent figure allowing even more
regions to synapse.

The real test of whether this can happen, of course, is that the woman successfully reproduced. It was difficult, and she had three miscarriages first—probably a result of occasions when her chromosomes did not sort out properly, and so her egg had an unbalanced arrangement—but on the fourth try she had a healthy daughter. The daughter had some developmental abnormalities, developmental delays, some retardation (although she did graduate from school), and later was diagnosed with diabetes. These problems are not a direct consequence of the chromosome arrangement, however, since the mother had the same arrangement without the developmental problems. Both mother and daughter shared one unusual trait, the prolonged expression of fetal hemoglobin, which could be correlated with one of the breakpoints.

This is an extreme example of a scrambled chromosome; lesser variations, like single translocations or fusions, are far less problematic.


Fauth C, Gribble SM, Porter KM, Codina-Pascual M, Ng BL, Kraus J, Uhrig S, Leifheit J, Haaf T, Fiegler H, Carter NP, Speicher MR (2006) Micro-array analyses decipher exceptional complex familial chromosomal rearrangement.
Hum Genet 119(1-2):145-53.

Comments

  1. #1 Russ
    February 28, 2006

    Thank you for this. I’ve been on the lookout for a good example of chromosomal rearrangement that I could share for a quite a while. The public needs to see how pliable the evolutionary mechanisms can be.
    Thanks again.
    Russ

  2. #2 Timothy Chase
    February 28, 2006

    Ah — rearrangements! Interesting topic.

    You may be interested in the following articles that I have organized in chronological order, beginning with the most recent. Every non-technical or semi-technical article is paired with corresponding technical articles written by the same researchers describing the research which was the basis for the non-technical articles.

    These deal principally with genomic rearrangement, including:

    1. the genomic differences in genomic structure which naturally exist in the human population and are not disadvantageous (in which similar differences of genomic structure within the populations of other species has undoubtedly facilitate adaptation and speciation during throughout the history of the evolution of life);
    2. the causes of genomic rearrangement in terms of alternate DNA structures (Z-DNA and H-DNA) which place stress on the genome and result in breakages at specific points;
    3. diseases (particularly cancer) which may result from such breakages;
    4. the manner in which genomic rearrangement resulted in the mammalian radiation approximately 60 MYA;
    5. the origins of this rearrangement and gene duplication potential in primarily in retrotransposons which are relics of retroviral infections over the past 150 MYA;
    6. the apparent role of one class of retrotransposons (Type III LTR) in the regulation and orchestration of gene expression during embryonic development.

    The last of these suggests that the evolution of life in eukaryotes and the pace at which it took place has been significantly influenced by the the endogenization of retroviruses and subsequent co-adaption of retroelements to the host genome in which at least one class of retroelements (the Type III LTR retrotransposons) regulatory roles in gene expression. One indication of the significance of this lies in the fact that while genes which are transcribed into proteins constitute only 1.5 percent of the human genome, Type III LTR retrotransposons (much of which may play a fairly significant role in the regulation of gene expression) constitute 30 percent.

    All but one technical article (“Patchwork People” in “Nature”) is accessible without subscription.

    Enjoy!

    Chromosome rearrangements not as random as believed
    Genetic predisposition may raise risk of rare disabling syndrome
    Public release date: 16-Feb-2006
    http://www.eurekalert.org/pub_releases/2006-02/chop-crn021506.php

    A palindrome-mediated mechanism distinguishes translocations involving
    LCR-B of chromosome 22q11.2
    Anthony L. Gotter, Tamim H. Shaikh, Marcia L. Budarf, C. Harker Rhodes
    and Beverly S. Emanuel
    Human Molecular Genetics, 2004, Vol. 13, No. 1 103-115
    http://hmg.oxfordjournals.org/cgi/content/abstract/10/23/2605

  3. #3 Timothy Chase
    February 28, 2006

    When Good DNA Goes Bad
    Article Date: 15 Feb 2006 – 1:00am (UK)
    http://www.medicalnewstoday.com/medicalnews.php?newsid=37639

    Naturally occurring H-DNA-forming sequences are mutagenic in mammalian cells
    Guliang Wang, and Karen M. Vasquez
    Published online before print September 1, 2004, 10.1073/pnas.0405116101
    PNAS | September 14, 2004 | vol. 101 | no. 37 | 13448-13453
    http://www.pnas.org/cgi/content/abstract/101/37/13448

  4. #4 Timothy Chase
    February 28, 2006

    The Human Genome is a Mess
    Friday, October 21, 2005
    http://evolgen.blogspot.com/2005/10/human-genome-is-mess.html

    Human genome: Patchwork people
    Erica Check
    20 October 2005
    http://www.nature.com/nature/journal/v437/n7062/edsumm/e051020-05.html

  5. #5 Timothy Chase
    February 28, 2006

    Comparative chromosome study finds breakage trends, cancer ties
    Article Date: 23 Jul 2005 – 0:00am (UK)
    http://www.medicalnewstoday.com/medicalnews.php?newsid=27844

    Insight Into DNA’s ‘Weakest Links’ May Yield Clues To Cancer Biology
    Posted: March 24, 2005
    http://www.sciencedaily.com/releases/2005/03/050323131423.htm

    Dynamics of Mammalian Chromosome Evolution Inferred from Multispecies
    Comparative Maps
    William J. Murphy,Denis M. Larkin,
    Annelie Everts-van der Wind, Guillaume Bourque, Glenn Tesler,
    Loretta Auvil, Jonathan E. Beever, Bhanu P. Chowdhary,
    Francis Galibert, Lisa Gatzke, Christophe Hitte,
    Stacey N. Meyers, Denis Milan, Elaine A. Ostrander, Greg Pape,
    Heidi G. Parker, Terje Raudsepp, Margarita B. Rogatcheva,
    Lawrence B. Schook, Loren C. Skow, Michael Welge,
    James E. Womack, Stephen J. O’Brien,
    Pavel A. Pevzner, Harris A. Lewin.
    http://www.sciencemag.org
    22 February 2005; accepted 1 June 2005
    http://www.swinegenomics.com/documents/1/37/Murphy%20et%20al,%202005%20Science.pdf

  6. #6 Timothy Chase
    February 28, 2006

    ‘Junk’ DNA May Be Very Valuable To Embryos
    Posted: October 12, 2004
    http://www.sciencedaily.com/releases/2004/10/041012085830.htm

    Retrotransposons Regulate Host Genes in Mouse Oocytes and
    Preimplantation Embryos
    Anne E. Peaston, Alexei V. Evsikov,
    Joel H. Graber,1 Wilhelmine N. de Vries,
    Andrea E. Holbrook, Davor Solter,
    and Barbara B. Knowles
    Developmental Cell, Vol. 7, 597606, October, 2004, Copyright 2004 by Cell Press
    http://www.zi.ku.dk/evolbiology/courses/ME05_projekter/Transposable%20elements/Peaston.pdf

  7. #7 Timothy Chase
    February 28, 2006

    Human, Mouse, and Rat Genome Large-Scale Rearrangements: Stability
    Versus Speciation
    Shaying Zhao, Jyoti Shetty, Lihua Hou, Arthur Delcher, Baoli Zhu,
    Kazutoyo Osoegawa, Pieter de Jong, William C. Nierman, Robert L.
    Strausberg and Claire M. Fraser
    Genome Research 14:1851-1860, 2004
    http://www.genome.org/cgi/content/full/14/10a/1851

    Reconstructing large regions of an ancestral mammalian genome in silico
    Mathieu Blanchette1, Eric D. Green, Webb Miller and David Haussler
    Genome Research 14:2412-2423, 2004
    http://www.genome.org/cgi/content/full/14/12/2412

  8. #8 Timothy Chase
    February 28, 2006

    Some 400 ‘Fragile Regions’ Of Genome More Vulnerable To Evolutionary Breaks
    New Theory Could Have Implications For Cancer Research
    Posted: June 18, 2003
    http://www.sciencedaily.com/releases/2003/06/030618081448.htm

    Genome Rearrangements in Mammalian Evolution: Lessons From Human and
    Mouse Genomes
    Pavel Pevzner and Glenn Tesler
    Genome Research
    Vol 13, Issue 1, 37-45, January 2003
    http://www.genome.org/cgi/content/abstract/13/1/37

  9. #9 Timothy Chase
    February 28, 2006

    “Jumping Genes” Create Ripples In The Genome – And Perhaps Species’ Evolution
    Posted: August 16, 2002
    http://www.sciencedaily.com/releases/2002/08/020816072241.htm

    Human L1 Retrotransposition Is Associated with Genetic Instability In Vivo
    David E. Symer, Carla Connelly, Suzanne T. Szak, Emerita M. Caputo,
    Gregory J. Cost, Giovanni Parmigiani, and Jef D. Boeke
    Cell, Vol 110, 327-338, 9 August 2002
    http://www.cell.com/content/article/abstract?uid=PIIS0092867402008395

  10. #10 Andy Groves
    February 28, 2006

    (Said in teh manner of a clinching argument):

    “But they’re still humans!!

  11. #11 Jeffery Keown
    February 28, 2006

    “Vary-a-shuns with-in ah ky-nd” say it in Southern Baptist just once for me…

  12. #12 Dave Thomas
    February 28, 2006

    While not directly on topic, I did a little graphic last week on the theme “Life will find a way.”

    http://www.nmsr.org/beacon206.JPG

    Cheers, Dave

  13. #13 Kristine
    February 28, 2006

    I find these particular creationist arguments to be especially curious (“Mutations have NEVER produced additional DNA structures…” “The lightning in the early earth environment would more likely KILL the cute little amino acid than form it..” “It TAKES MORE FAITH to believe in evolution than creationism…” etc., ad nauseum). Their originators seem to think that matter sits around, waiting to be acted upon, and has to be cajoled, as it were, to interact–as if matter were a sulky teen-ager dawdling at the task of cleaning up his bedroom and must be yelled at 15 times by his father before he does it.

    Perhaps there is a kernel of truth among the layers of BS in the argument that observing evolution involves more faith than does clinging to creationism, since it is the scientific mind that honors the awesome powers of matter, whereas the religious mind assumes matter merely to be–well, immaterial, unimportant, and impotent in itself. Religion seems to assume a pessimistic view of nature in the first place–and this, I’ve never understood. I shall never understand it.

  14. #14 Timothy Chase
    February 28, 2006

    Here are a few more articles related to rearrangements. In particular, in smaller populations, rearrangements may be more common which are slightly deleterious to the population, but which later become stablized through additional mutations and act as a positive, creative force in evolution.

    Mutations, Population Size, and Near-Neutral Evolution

    Fishing for the Origins of Genome Complexity
    Deciphering a paradox of evolution( non-techical)
    December 15, 2005
    http://www.gatech.edu/news-room/release.php?id=801

    Quantifying the Slightly Deleterious Mutation Model of Molecular Evolution
    Adam Eyre-Walker, Peter D. Keightley, Nick G. C. Smith, and Daniel Gaffney
    Mol. Biol. Evol. 19(12):2142?2149. 2002
    http://mbe.oxfordjournals.org/cgi/reprint/19/12/2142

    The Origins of Genome Complexity
    Michael Lynch and John S. Conery
    http://www.sciencemag.org SCIENCE VOL 302 21 NOVEMBER 2003
    http://www.zoology.ubc.ca/veg/Lynch&Conery2003Science.pdf

  15. #15 Timothy Chase
    February 28, 2006

    In a previous post (currently on hold for approval) I listed a few articles on how retrotranspositions and rearrangements may be tolerated in smaller populations. In what follows, I have a couple more articles which may be of interest…

    The view that genomic complexity is inversely related to population size is largely an outgrowth of the near-neutral theory of molecular evolution put forward by Tomoko Ohta, a student of Motoo Kimura, the originator of the neutral theory of evolution. While Kimura’s theory is no longer considered a viable theory, it still provides a theoretical basis for the molecular clock and the null hypothesis through which we test for selection as opposed to mere drift. To learn more about Ohta’s near neutral theory, you may wish to see her article:

    Near-neutrality in evolution of genes and gene regulation
    Tomoko Ohta
    PNAS | December 10, 2002 | vol. 99 | no. 25 | 16134-16137
    http://www.pnas.org/cgi/content/full/99/25/16134?maxtoshow=&HITS=&hits=&RESULTFORMAT=&author1=Tomoko+Ohta&andorexactfulltext=and&searchid=1135100538334_18794&FIRSTINDEX=0&resourcetype=1

    I tend to focus on on combinatorial regulation, particularly as it is closely related to one of my interests — how viruses in general and retroviruses in particular are an important engine in the evoluition of eukaryotic life. However, this should be balanced with a recognition of the importance of downstream regulation, particularly in terms of the evolutionary change in introns. You may want to check:

    Intron evolution as a population-genetic process
    (exon shuffling|genome complexity|genome evolution)
    Michael Lynch
    http://www.pnas.org/cgi/content/abstract/99/9/6118

  16. #16 Ithika
    February 28, 2006

    That was really amazing! Thanks. I was struggling a bit at first (a lot of technical terms that weren’t in my ken) but when the coloured diagrams appeared it all became very clear.

    I’d never really understood before what things go wrong to cause a miscarriage. That was very informative, and helped to slide a few more things into place.

    I don’t really know cos I didn’t take high school biology, but is this kind of thing ever covered at that stage? I don’t necessarily mean in as much technical detail, but the diagrams and the concepts are still comprehendible.

  17. #17 Lemony
    February 28, 2006

    Fascinating posting, I’m passing it on to my biologist housemate who will no doubt demonstrate that i don’t understand it at all.

    One little request though, is it by any chance possible to have diagrams that are clickable for enlargements? or even just gif/png compression on the flat colour diagrams? it would make the annotations a little easier to read for those of us on high resolutions.

    cheers,
    sam

  18. #18 Timothy Chase
    February 28, 2006

    I hope that no one minds the links. I figure they are relevant enough, though, and will provide some interesting additional reading.

    However, back to the paper, in humans, chromosomal anomalies occur in roughly 6 out of 1000 births. Approximately half of these are not clinically significant. In smaller populations, they more easily become widespread simply as the result of genetic drift. In anycase, when they are not clinically significant, they may be regarded as a kind of genomic polymorphism which nevertheless results plasticity and contributes to the evolvability of the species. Even when the chromosomal anomalies are initially slightly deleterious, if the species is fragmented into smaller populations, this would undoubtedly be a significant causal factor in allopatric speciation.

  19. #19 clvrmnky
    February 28, 2006

    Nice. Going into a bookmark file for my anthro friend who also has a strong interest in evolutionary science (yes, those nasty cultural anthropologist who are always asking uncomfortable questions about your studies often get _some_ grounding in biology).

    It was groovy trying to remember how meiosis works.

  20. #20 saurabh
    February 28, 2006

    WOW. Never mind meiosis, how does this woman manage mitosis without major problems? Her cells must be replete with aneuploidy… I’m completely thrown.

  21. #21 Jim Hu
    March 1, 2006

    In mitosis the rearranged chromosomes only pair with copies of themselves after S phase. Why do you think there’d be a problem?

  22. #22 Dustin
    March 1, 2006

    Funny, I somehow expected the Wagnerian Inquisition to make an appearance here. I’m sure Larsson will point out that my expectation of the Wagnerian Inquisition was just the problem.

    Maybe the journal linking spree up there scared the little tyke off.

  23. #23 Timothy Chase
    March 1, 2006

    Dustin wrote:

    Funny, I somehow expected the Wagnerian Inquisition to make an appearance here. I’m sure Larsson will point out that my expectation of the Wagnerian Inquisition was just the problem.

    His chief weapon is stupidity
    … and the drool dripping from his chin.

    His two chief weapons are stupidity, the drool dripping from his chin
    … and the overwhelming, nauseating bordom which he induces in his enemy.

    His three chiefs weapons are…

  24. #24 Peter Ellis
    March 1, 2006

    Very rarely, someone will wind up with a damaged Y chromosome so they turn out female instead of male despite being XY. Even more rarely, they’ll retain some fertility.

    There’s at least one known example of a girl who inherited her X chromosome from Dad and her Y chromosome from Mum.

  25. #25 Torbjorn Larsson
    March 1, 2006

    Dustin, Timothy,

    LOL! You made my day a good one. Even if I too expect the Wagnerian Inquisition, always…

  26. #26 Steve LaBonne
    March 1, 2006

    Timothy- what a great collection of links. Thank you very much.

  27. #27 Timothy Chase
    March 1, 2006

    Torbjorn Larsson wrote:

    Dustin, Timothy,

    LOL! You made my day a good one. Even if I too expect the Wagnerian Inquisition, always…

    The truly inspired idea was Dustin’s. I just added to it a little in the way of a response.

    Incidentally, Kent Hovind inspired a non-technical piece Thoughts on the Intelligent Design Inference a while back. Currently, I am working up to being able to write something semi-technical. Gathering information, organizing notes.

    And oddly enough, the Wagnerian opera/comedy(?) which we keep getting treated to (does seem a bit ad lib for an opera — doesn’t it?) has given me the inspiration for a semi-technical piece which might be something I could write at this point. Not sure how much the fellow who inspired it will be able to follow — may be a wee bit over his head if I manage to get it done…

  28. #28 Timothy Chase
    March 1, 2006

    PZ —

    I personally didn’t know specifically about the enzymes which are responsible for piecing back together the fragments of chromosomes. Essentially, what I had thought was simply that there exist transposons which are responsible for breaking chromosomes up, or at least for removing pieces of them, then integrating them into other chromosomes. Likewise, there are also retrotransposons which duplicate and then integrate segments into different places in the same chromosome or into other chromosomes.

    Although there is very little information (from what I have been able to gather so far), a likely origin for transposons appears to be infection by DNA viruses. However, there appears to be a great deal of evidence that retroelements have their origin in the endogenization of retroviruses. For example, retrotransposons are more or less simply the proviral form of retroviruses which have lost their envelope gene.

    Now I realize, for example, that the enzyme telomerase (which exists on chromosome 5 in humans) is responsible for the lengthening of telomeres which is essential for meiosis. Moreover, the enzyme telomerase is essentially functionally homologous to reverse transcriptase and the template for a telomere. Moreover, in at least one species of eukaryote (drosophila, I believe) there is considerable structural homology between the reverse transcriptase found in some species of retrovirii and the telomerase employed by this eukaryote.

    Given this, I was wondering whether the enzymes responsible for reconstituting the chromosomes after breakages (due to weaknesses in Z/H-DNA, etc.) are at least functionally homologous to either the enzymes of DNA viruses, or to the integrase of retroviruses? Are there significant differences between the integration enzymes from one species to another, such that some of these enzymes may have originated in DNA viruses, whereas others are far more likely to have arisen in retroviruses?

  29. #29 Timothy Chase
    March 1, 2006

    Steve LaBonne wrote:

    Timothy- what a great collection of links. Thank you very much.

    No problem. Just trying to piece things together for myself, and maybe at some later point an article. What these sources would be for however is a little beyond my current level of understanding, but they might help others, and they did seem relevant.

    Part of what is slowing me down right now is the lack of the appropriate writing environment. I had created something a while back, a program I called BrainStorm. It was on a Mac. It wound up helping me organize over two thousand pages of material — on a different subject. Currently I am working on recreating it on a PC, this time written in C-Sharp, only bigger, badder. It is now to the point that I can begin to use it for organizing my notes (hierarchically, with cross-referencing within a dynamic structure) and structuring larger papers. When the program is sufficiently polished, I will make it available for free. Might help others with their personal studies, learning and more.

  30. #30 Torbjorn Larsson
    March 1, 2006

    “The truly inspired idea was Dustin’s. I just added to it a little in the way of a response.”

    Yes. But I’m a simple mind, so I enjoyed both. And as is abundantly clear, it started in a similar vein, was a ripoff, and dumbed down to fit the reduced bandwith of the web. (The original sketch contrasts dialog and action.)

    Crackpots are fascinating in their odd behaviour, and I see I’m not the only one who sometimes get stimulated by them. Unfortunately for them, the stimulation isn’t quite as they envision…

  31. #31 Timothy Chase
    March 2, 2006

    An Alternative to Reverse Transcriptase in Drosophilia

    Since I mentioned the fact that telomerase includes a reverse trascriptase a little earlier on, I thought people might be interested in these two articles regarding an interesting alternative to the usual telomerase/telomere system employed by most multicellular organisms — in the drosophilia. (It involves two retrotransposons. PZ might have mentioned it at some point, but even then it is worth mentioning as some might have missed it.)

    TAHRE, a Novel Telomeric Retrotransposon from Drosophila melanogaster, Reveals the Origin of Drosophila Telomeres
    Jos預. Abad, Beatriz de Pablos, Kazutoyo Osoegawa, Pieter J. de Jong, Antonia Mart�Gallardo and Alfredo Villasante
    Mol. Biol. Evol. 21(9):1620-1624. 2004

    http://mbe.oxfordjournals.org/cgi/content/abstract/21/9/1620

    Annual Review of Genetics
    Vol. 37: 485-511 (Volume publication date December 2003)
    (doi:10.1146/annurev.genet.38.072902.093115)
    First published online as a Review in Advance on August 6, 2003
    RETROTRANSPOSONS PROVIDE AN EVOLUTIONARILY ROBUST NON-TELOMERASE
    MECHANISM TO MAINTAIN TELOMERES
    Mary-Lou Pardue and ?P.G. DeBaryshe?
    http://www.url.fm/183
    http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.genet.38.072902.093115

  32. #32 Timothy Chase
    March 2, 2006

    As for the reverse transcriptase in most eukaryotic telomerase complexes, you may wish to check:

    Telomeres and telomerase
    Elizabeth H. Blackburn
    UC San Francisco
    University of California
    Postprints
    Year 2004 Paper 366
    http://repositories.cdlib.org/cgi/viewcontent.cgi?article=1841&context=postprints

    The telomerase reverse transcriptase: components and regulation
    Genes and Development
    Vol. 12, No. 8, pp. 1073-1085, April 15, 1998
    Constance I. Nugent, and Victoria Lundblad
    http://www.genesdev.org/cgi/content/full/12/8/1073

  33. #33 Timothy Chase
    March 2, 2006

    This may also be of interest:

    Reversing Time: Origin of Telomerase
    Cell, Vol. 92, 587-590, March 6, 1998
    Toru M. Nakamura and Thomas R. Cech
    Howard Hughes Medical Institute
    Department of Chemistry and Biochemistry
    University of Colorado
    Boulder, Colorado 80309-0215
    http://www.uic.edu/labs/nakamura/pdf/4.pdf

  34. #34 David Harmon
    March 16, 2006

    Timothy Chase: Any chance you’ll release your new program as Open Source for us Linux & BSD weenies? And yeah, that’s an awesome mass of data — possibly enough for a textbook (hint, hint 😉 ).

  35. #35 Paper Han
    July 14, 2008

    Fascinating! Thank you very much. I have often wondered how descendant species could have different numbers of chromosomes, since it would seemingly be a barrier to reproduction. Now I begin to understand. I knew there must be a solution, since it obviously happened. 🙂

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