quote:
If a circular chromosome broke, one of two things would happen to the cell: either it would survive, or it would die. There is no better/worse, faster/slower, it is pass/fail. If an enzyme was present which was not functional, even if it was extremely close, the cell would still die. There would be no co-evolution - since the cell would die if the enzyme was not working all the way, there would be no selective pressures able to keep around almost functional proteins.
I think someone mentioned stem-loops earlier, but I found an article (not primary) that discussed this process. In
this article Dr. de Lange speaks about the use of telomeric loops (t-loops) as a means of stopping degradation of linear DNA. This type of end protection does not require telomerase or any enzymes at all. The mechanism relies on DNA bending back on itself forming a type of stem-loop. A few excerpts from the article:
But things started out very simply, de Lange says. She suggests that tiny structures called telomeric-loops (t-loops), which she and her collaborators discovered four years ago, are actually remnants of the original telomere system that served to protect the ends of the first linear chromosomes found in early microorganisms. The investigators have shown that without these little loops, cells mistake the exposed chromosome ends for sites of DNA damage and when they attempt to repair them, the cells die.
If the chromosomes of a microorganism, such as the bacterium that gave rise to eukaryotes, were linear, a t-loop could easily be formed from just a few repeats at the end of the chromosome, de Lange says. All the enzymes to make t-loops were already available; they were used for regular DNA replication in bacteria. When E. coli is replicating its DNA, occasionally the newly synthesized fork collapses, leaving the end of the new DNA hanging out of a half-replicated E. coli genome.
So it would seem that the telomeres and telomerases were not necessary for the formation of linear DNA. Also, de Lange also mentions that telomerases could have developed later as a more effecient and dynamic system (control by committee as de Lange puts it).
Also, we see other solutions in higher taxa, such as in Drosophila.
Annu Rev Genet. 2003;37:485-511.
Retrotransposons provide an evolutionarily robust non-telomerase mechanism to maintain telomeres.
Pardue ML, DeBaryshe PG.
Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. mlpardue@mit.edu
Telomere molecular biology is far more complex than originally thought. Understanding biological systems is aided by study of evolutionary variants, and Drosophila telomeres are remarkable variants.
Drosophila lack telomerase and the arrays of simple repeats generated by telomerase in almost all other organisms; instead, Drosophila telomeres are long tandem arrays of two non-LTR retrotransposons, HeT-A and TART. These are the first transposable elements found to have a bona fide role in cell structure, revealing an unexpected link between telomeres and what is generally considered to be parasitic DNA. In addition to providing insight into the cellular functions performed by telomeres, analysis of HeT-A and TART is providing insight into the evolution of chromosomes, retrotransposons, and retroviruses. Recent studies show that retrotransposon telomeres constitute a robust system for maintaining chromosome ends. These telomeres are now known to predate the separation of extant Drosophila species, allowing ample time for elements and hosts to coevolve interesting mechanisms. emphasis mine
The fruit fly actually uses viral genetic sequences to insert repetitive sequences at the end of their linear DNA. This is more evidence that telomerases are not needed and other solutions exist.
My conclusion is that assuming telomeres and telomerases are NECESSARY for linear DNA formation is not a correct assumption. Two other systems are found within nature, and possibly more exist that have not yet been discovered. Also, with a t-loop system in place it is very possible that another system was able to coevolve and replace the less ellegant system. As metazoans evolved they needed to control cell replication to prevent certain tissue types from dominating the organism. This requires a level of control not seen in the t-loop system, but is seen within a telomerase control system.
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