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Topic: Dermott J. Mullan: Probabilities of randomly assembling a primitive cell on Earth
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posted 23. November 2002 17:36
Probabilities of randomly assembling a primitive cell on Earth
by Dermott J. Mullan
mullan@bartol.udel.edu
ABSTRACT—We evaluate the probability Pr that the RNA of the first cell was assembled randomly in the time available (1.11 billion years [b.y.]). To do this calculation, we first set a strict upper limit on the number of chemical reactions nr which could have occurred before the first cell appeared.
To read the entire paper, please click here.
Response to Art
Dermott Mullan has replied to Art (a Brainstorms participant) via a PDF file because certain formatting could not be performed in the Bulletin Board.
To read the reply, please click here.
[ 02. December 2002, 13:39: Message edited by: Moderator ]
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Art
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posted 23. November 2002 19:28
I think the author of this manuscript would be well-served to review some threads here on ISCID. Of particular relevance is http://www.iscid.org/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=6;t=000145
One very pertinent point that comes out of this thread is the fact that the frequency with which functional sequences (protein or RNA) occur in the totality of sequence space is not a function (for the most part) of polymer length. Without bothering to quote a lot of specific passages, I find that the opposite conclusion is reached by Dr. Mullan - e.g., (from p. 20)
quote:
The overall probability f12 that all twelve proteins arise as a result of random processes is the product of the probability for the twelve separate proteins. That is, f12 is roughly equal to f1^12, i.e. f12 is roughly (1/10)^y where y = 15.6Na.
This conclusion is at odds with direct experimental observation. As I noted in the cited thread (for our purposes here, Nf/N can be taken as a similar quantity as f12):
“Hi Kirk,
You asked about examples where Nf/N was independent of the size of the polymer being studied. Two specific examples come to mind: one from Bartel’s group (Ekland et al., Science 269, 365-370, 1995) and one from Szostak’s lab (Wilson et al., PNAS 98, 3750-3755, 2001). Briefly, Bartel found that the frequency of RNA ligases in a random population of 200-mers was fairly high, and that the frequency of long catalysts was roughly equal to short ones (I haven’t the details in front of me, but the longer ones, as I recall, were on the order of 90 or so nts, while the shorter ones only 40-50 nts). The end result was that, if length was a consideration, the longer family of sequences should have been less than 10^-15 as abundant as the shorter one, but this is not what was found. Szostak’s group found that the frequency of ca. 90 amino acid steptavidin binders in a library somewhat analogous to that used in the Keefe and Szostak paper was on the order of 1 in 10^13 - quite comparable to the frequency of much shorter (5-38 amino acids, according to work cited by Wilson et al.) streptavidin binding peptides. As remarkable for this thread is the observation that the longer polypeptides bound with a much greater affinity - IOW, greater “activity” was not accompanied by a significantly lower Nf/N.
How do we make sense of results such as these? (I suspect that they are going to be pretty general - the availability of longer randomized polypeptide libraries is such that not much has been done, and I feel that other targets are going to display similar characteristics.) The easiest way is to hypothesize that functionality in proteins (and RNAs) is not a matter of large, highly-constrained motifs, but rather of loose collections of highly degenerate ones. This “explains” why tools such as the PROSITE collection of motif signatures works in identifying functionality in unknown sequences. It also explains the lack of length dependence of Nf/N that is seen. (Look at things this way - if a functional unit needs only 5 amino acids, then it will be roughly as abundant in a library of 12-mers as in one of 90-mers.)”
I’d have to say that Dr. Mullan’s work is entirely too preliminary to warrant publication in PCID (if that is the intent - apologies if I am assuming too much) or any other journal. Of course, ISCID members who are familiar with this area might have a less negative reaction, and they may be able to help Dr. Mullan in revising the manuscript so that it takes into account some of the problems that I raise here, and that have been discussed here on Brainstorms. The preceding point is but one of many serious issues that need to be resolved. (I apologize for not getting into more detail.)
Art
[ 24. November 2002, 00:17: Message edited by: Art ]
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Frances
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posted 23. November 2002 21:47
Dermott J. Mullan, professor of Astrophysics at the Bartol Research insitute of the University of Delware has provided us with his extensive thoughts on how to calculate the probabilities of randomly assembling a primitive cell on earth. It pleases me to
Lets jump to your conclusions: your calculations suggest that such a hypothesis would remain significantly under the Universal Confidence Bound of 10^-150. Which by itself may be very interesting in that it suggests that the ID inference is not triggered (yet?).
But may I point out some additional suggestions that may help lower these bounds even further?
You are talking about RNA and DNA and proteins. It may be relevant to distinguish between these terms. DNA is what is considered the blueprint of life, RNA can be further subdivided into ribosomal, messenger and transfer RNA. The messenger RNA is a copy of the gene, ribosomal RNA is the copier, transfer RNA uses available amino acids to build the protein.
So when you state that you are evaluating the probability that the RNA of the first cell was assembled randomly, you seem to be following a pathway that suggests you are interested in the RNA world hypothesis?
If that is the case then I suggest that your approach to calculate the purely chance hypothesis of a cell arising may be interesting but it does not seem to reflect the latest ideas on how life may have started.
As I understand it, the hypothesis may look as follows
A self replicating molecule arises, a precursor to the RNA (Leslie Orgel and Stanley Miller seem to support such a view).
quote:
Orgel and his group, at the Salk Institute for Biological Studies in San Diego, have studied a compound known as peptide nucleic acid (PNA) which has the ability to replicate itself and catalyze reactions but is much simpler than RNA. They showed that PNA can act as a template both for its own replication and for the formation of RNA from its subcomponents. Although, Orgel's team has not claimed that PNA itself may have been the primordial replicator (since it is not clear how this substance could have arisen under plausible prebiotic conditions) their work demonstrates that the evolution of a more complex self-replicating molecule from a simpler precursor is at least possible.
Source
Stanley Orgel describes his scenario Here
A relevant potential first step may have been
quote:
Quite recently Szostak found even stronger evidence that an RNA molecule produced by prebiotic chemistry could have carried out RNA replication on the early earth. He started by creating a pool of random oligonucleotides, to approximate the random production presumed to have occurred some four billion years ago. From that pool he was able to isolate a catalyst that could join together oligonucleotides. Equally important, the catalyst could draw energy for the reaction from a triphosphate group (three joined phosphates), the very same group that now fuels most biochemical reactions in living systems, including nucleic acid replication. Such a resemblance supports the idea that an RNA molecule could have behaved like, and preceded, the protein catalysts that today carry out the replication of genetic material in living organisms. Much remains to be done, but it now seems likely that some kind of RNA-catalyzed reproduction of RNA will be demonstrated in the not too distant future.
More interesting findings that may help us formulate an RNA world based hypothesis.
So the question is simple, are there evolutionary precursors of the DNA/Protein as found in cells? In fact the data show that this may be the case, RNA, PNA... There are surely many unknowns and assumptions but the data suggests that the origins of the first cell are likely not to have been purely random. From the abiotic world to DNA proteins, there seem to be many steps. The work of Miller and others have shown how the building blocks for life could have arisen.
Despite these common objections against probability calculations I would like to thank Mr Mullan for his efforts and encourage him to fine tune his models.
Finally a very interesting Powerpoint presentation on some of the evidence and problems and unanswered question with the RNA world hypotheses.
I will be commenting more when I have had a chance to read the article in more depth. These are merely my first impressions.
[ 23. November 2002, 21:48: Message edited by: Frances ]
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yersinia
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posted 23. November 2002 22:53
Brief criticism on a specific point which is not really here or there WRT to evolution/ID but which is worth making:
quote:
Nevertheless, as Yockey (p. 203) points out, the possibility that an organism from the doublet-codon world might have survived the ihbottleneckl⇚ may have some empirical support. According to the endosymbiotic theory (L. Margulis 1970, Origin of Eukaryotic
Cells, Yale Univ. Press, New Haven CT), mitochondria might have been at one time free-living bacteria which now survive in a symbiotic relationship with the cytoplasma of other cells. In mitochondria, the genetic code differs somewhat from the code in other cells. Perhaps mitochondria are representative of organisms which originated in the doublet-codon world, but which could not survive on their own because of the difficulties associated with the hostile zone of parameter space where they originated.
This is a combination of a garbled and/or outdated view of modern endosymbiotic theory.
Points:
1) The bacterial origin of mitochondria is well-accepted and is now about as close to fact as anything gets in science.
2) The variations in the mitochondrial code are derived and (although I'm very far from an expert) mostly transitions from the "canonical" code made possible by the very small genomes of mitochondria (similar patterns are seen in other peculiar genetic systems, e.g. the micronuclei of certain single-celled eukaryotes).
3) Therefore the bacterial ancestors of mitochondria had the canonical code or something very close and aren't separate descendents from a doublet-codon world
4) However the phenomenon of code change in mitochondria and other genomes has been a fertile (technical) field of study recently and does give some insight into how genetic codes (as opposed to sequences) can themselves evolve; so if you wanted to make a similar point to the above paragraph this would a direction to investigate.
Like I said, neither here nor there...
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Cre8ionist
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posted 24. November 2002 10:31
quote:
1) The bacterial origin of mitochondria is well-accepted and is now about as close to fact as anything gets in science.
Yersinia, I must take exception with the above comment. Perhaps you meant to say that
the bacterial origin of mitochondria is well-accepted and is now about as close to fact as
anything gets in evolution theory?
Let's illustrate through the use of scientific knowledge:
Water is made of hydrogen and oxygen.
The average temperature of Venus is higher than that of the Earth.
The Earth is round.
Mitochondria originated from bacteria.
It appears to me that one of these things is not like the others, one of these things just don't
belong. .......................................Cre8
![[Smile]](smile.gif)
[ 24. November 2002, 10:40: Message edited by: Cre8ionist ]
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Frances
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posted 24. November 2002 20:41
Creationist
quote:
Yersinia, I must take exception with the above comment. Perhaps you meant to say that
the bacterial origin of mitochondria is well-accepted and is now about as close to fact as
anything gets in evolution theory?
Which means pretty well established I would say. Indeed the controversial theory has become all but a fact.
Btw the earth is not round, it's an oblate spheroid
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Cre8ionist
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posted 25. November 2002 08:17
Well unless you're prepared to say that only a perfect circle is round, I think you're stretching a
little Frances.
quote:
Models of the Earth
Models are representations of objects that aid in our understanding. If we were to choose a scale model of the Earth, what objects would make a good miniature Earth? We might be tempted to choose an oblate object to show the true shape, but that would be inaccurate! Since the Earth is so slightly oblate, and the Earth's relief is so insignificant in comparison with its size, the best scale models of the Earth would actually be very round and very smooth. Billiard balls, marbles, ping pong balls, and other smooth spheres are the best representation of the Earth's true appearance.
Visit this Regents Prep on Earth Science
http://regentsprep.org/Regents/earthsci/units/introduction/oblate.cfm
for the complete reference.
BTW, the dictionary definition of round is "ball-shaped." FYI
Anyway, your response didn't do much to answer the point of my post.
It may be that most scientists have come to accept the bacterial origin as fact. That doesn't
surprise me at all, what surprises me is that they claim this without being able to back it up in
the same way they would back up other scientific facts. It's overstating in the worst way. In order to back it up in the way they do the other facts, they would need to show the exact steps of the transformation, this is something I would need to see to believe, otherwise
you may rightly say of me, oh ye of little faith. .......................................Cre8
[ 25. November 2002, 08:21: Message edited by: Cre8ionist ]
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yersinia
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posted 25. November 2002 10:36
Cre8ionist,
Why don't you show us just how familiar you are with the evidence for the endosymbiotic origin of mitochondria by listing the (numerous) lines of evidence commonly cited in the scientific literature?
Then, if you have a hypothesis that you think explains these lines of evidence better, please propose it and argue for it.
In the scientific world, this debate happened 20 years ago and even the most vehement scientific skeptics were converted (endosymbiosis violated a lot of gradualist, lineal-descent prejudices, and yet it won the day anyway, because for scientists evidence is primary).
yersinia
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Frances
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posted 25. November 2002 13:07
Dear Creationist,
While the true shape of the earth is an oblate spheroid there may be instances were for modeling purposes it is sufficient to represent it as a smooth ball. However such models remain approximations to the true shape of the earth and did we not discuss scientific knowledge?
As far as your comments about not being able to back it up, it seems to me that you may be unfamiliar with the scientific history of the endosymbiotic origin of the mitochondria and the multiple lines of evidence used to verify this model?
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posted 25. November 2002 13:14
Cre8, Frances and yersina,
Watch out;-) Seriously, the back and forth catfight that is in its early stages should stop.
Posts that are winged off without much thought, also, need to be eliminated. Frances, lately I've noticed you throwing these in quite a bit throughout the forum.
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Kirk Durston
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posted 25. November 2002 14:13
My workload over the next few weeks will almost certainly prevent me from being able to contribute much to this thread but, at least, here are a few comments on Mullan's paper and on some of Art's response:
First, I have only had the time so far to skim just part of Mullan's paper, thus have only two comments. My first comment has to do with the size of his extremely minimalist cell, 12 proteins, each containing 14 amino acids. He does mention later on that a more realistic minimal cell would require close to 250 proteins (which would, by the way, be composed of approximately 300 amino acids each, according to papers dealing with this subject). I really do question the utility of starting with such an extremely unrealistic minimal cell. I realize he does not want to stack the deck in his favor, but there is a point where one can go too far with this. If he wished to be real conservative, why not use 150 proteins of 300 amino acids each, which is already extreme so far as minimal requirements are (Koonin, E.V. (2000). How many genes can make a cell: the minimal-gene-set concept. Annu. Rev. Genomics Hum. Genet. 01, 99-116).
Secondly, in my skim of the paper, I missed it if there is a lower probability bound, so I'm not sure what he is using. Seth Lloyd shows that the upper limit for the number of operations that universe could have performed if it were a quantum computer is about 10^120 (Lloyd, S. (2002). Computational capacity of the universe. Phys. Rev. Lett. 88, no. 23, 237901-1 to 237901-4. Doi: 10.1103/PhysRevLett.88.237901). Keeping in mind that chemical reactions involved in the origin of life do not occur at quantum frequencies, and that most of the matter and energy in the universe would not be available for organic life production, it is safe to say that any probabilities anywhere close to 1 chance in 10^120 for organic life can be dismissed. That is, if the probability of achieving a minimal genome of 150 protein-coding genes is close to 10^-120, then it is not going to happen within any remotely reasonable degree of expectation.
I want to respond to what Art said re. the frequency of functional to non-functional protein sequences (Nf/N). I have suggested that one should use a value of .45 bits/amino acid for proteins with a very simple function, based on Keefe, A.D., Szostak, J.W. (2001). Functional proteins from a random-sequence library. Nature 410, 715-718. For moderately active enzymes, I would suggest .83 bits/amino acid based on Taylor, S.V., Walter, K.U., Kast, P., Hilvert, D. (2001). Searching sequence space for protein catalysts. PNAS 98, no. 19, 10596-10601. Art and I had some discussion earlier on another thread as to whether longer enzymes would have a finer Nf/N. Art argued that there is reason to believe that the frequency Nf/N stays roughly the same regardless of length. I want to say two things about this. First, we need to take a second look at length. Adding more amino acids to an already functional sequence does not necessarily increase the functional information content of the protein. Thus, for a given function, it would be expected that Nf/N would not change too much relative to length. Since Bartel's findings have to do with RNA ligases, I would expect the Nf/N frequency to stay fairly constant, since we are focusing on only one function here. However, if we have a different protein with a different, more complex function, I see no reason to expect that it will have a similar (within a few orders of magnitude) Nf/N frequency. In fact, a protein such as FOXP2 appears to be extremely well conserved which implies a very fine Nf/N ratio. So what I am saying is that for a given function, Nf/N may change very little relative to length, but may change significantly relative to degree of complexity of function.
Anyway, for the average protein, I suggested .83 bits/residue. The second thing I have to say is relevant to this estimation. I was discussing this with Doug Axe. He has just finished some significant work in this area and a paper is forthcoming. Without revealing anything that would be inappropriate at this stage, prior to the publication of his paper, his empirical results indicate that my estimate of .83 bits/amino acid is too low by a significant amount, for the average protein. That's about all I feel comfortable in saying, but my point is that I now think it is very safe to use a figure of .83 bits/amino acid for anything but smaller proteins (smaller protein: any protein that has less than 100 amino acids). One proviso I would put on this, of course, is that one use the shortest sequence that will perform the given function with acceptable efficiency.
All this to say that at .83 bits/amino acid, a bare-bones minimal genome of 150 protein-coding genes of 1000 bp each, has an Nf/N of less than 10^-11,000. Given that we leave the realm of reasonable probability at 10^-120, we have a problem.
I don't see that discussions of an RNA world, or a precurser to an RNA world, is going to be very useful in this context. The reason, I suggest, is that the challenge is not to get self replication up and running. Rather, the challenge is to get more than just a rudimentary amount of functional information in the self-replicating system. It is the information that is the problem, not self replication. I'm prepared to grant any realistic RNA world, or pre-RNA world you wish. The problem will be to achieve the degree of functional, biological information required for the first minimal organic cell.
Kirk
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charlie d.
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posted 25. November 2002 20:05
What, more secret Axe research? Does he work at Fort Detrick or something? ![[Wink]](wink.gif)
Jokes aside, Kirk, the problem with your calculations (and Mullan's) is that, no matter how accurate your estimates of information content/aa residue are, no one is claiming that a minimal genome (of 12, 150, or 250 protein-encoding genes) was assembled randomly. Thus, how much information it may have contained, vis-a-vis the chances of such information being generated randomly, is irrelevant to current understanding of OoL.
[ 25. November 2002, 20:05: Message edited by: charlie d. ]
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Cre8ionist
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posted 25. November 2002 22:46
Point taken mod, I'll finish up here and let someone else have some fun.....
quote:
Why don't you show us just how familiar you are with the evidence for the endosymbiotic
origin of mitochondria by listing the (numerous) lines of evidence commonly cited in the
scientific literature?
First of all, do you think I consider your question as anything other than condescension?
All I'm asking you to do is back up your claim that the bacterial origin of mitochondria is
"as close to fact as anything gets in science."
I hinted that it should probably be changed to "as close to fact as anything gets in evolution theory." I still hold to that.
What leads evolutionists to believe in this pathway anyway, I mean besides a possible bias? Similarity to bacteria. More specifically, similarity of DNA, ribosomes and tRNA etc.
This is what's called circumstantial evidence, not even close to scientific fact.
quote:
Then, if you have a hypothesis that you think explains these lines of evidence better,
please propose it and argue for it.
Well, I might venture a guess......Perhaps the reason that many genes which control mitochondria are found in the host cell (something which you probably won't hear too much about here) is because they were designed as a working unit from the beginning! And perhaps the reason that there are similarities in design with bacteria is that they have the same designer! I know this goes against the grain, but that's why I'm here. I'm not a big believer in Darwin's theory or Lynn Margulis' as you can probably tell...........Cre8
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Frances
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posted 25. November 2002 23:31
Creationist
quote:
All I'm asking you to do is back up your claim that the bacterial origin of mitochondria is
"as close to fact as anything gets in science."
I will give it a try and show what the present scientific understanding of the origins of the mitochondriae are.
A good overview paper on Mitochondrial evolution gives also some useful references.
A useful paper may be "Gray, M.W. and Doolittle, W.F. (1982) Has the endosymbiont hypothesis been proven? Microbiological Revs. 46:1-42"
Evidence in favor of
Changing perspectives on the origin of eukaryotes
A fascinating topic indeed. I would say that the evidence for endosymbiosis is as convincing as lets say the evidence that the sun is powered by fusion. It's fascinating how science has managed to look back into our past.
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yersinia
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posted 26. November 2002 00:41
Well, cre8tionist, that's why I asked the question. You don't appear to be very familiar with the evidence. For example, you didn't mention the fact that the similarities aren't just any old similarities, but some very particular ones that are only expected if mitochondria did indeed descend from bacteria.
E.g., why do mitochondrial sequences nest within those of a particular bacterial group?
quote:
Mitochondrial Evolution
Michael W. Gray, 1* Gertraud Burger, 2 B. Franz Lang 2
The hypothesis of an endosymbiotic origin of the mitochondrion (1, 2), the beginnings of which surfaced over a century ago (3), draws much of its contemporary support from the discovery of a unique genome in this organelle, a relic of the mitochondrion's evolutionary past. Studies of mitochondrial DNA (mtDNA) and its expression have amply affirmed the eubacterial roots of this genome (4); mitochondrial gene sequences have enabled researchers to trace the evolutionary antecedents of mitochondria to a single ancestor related to the division of the Proteobacteria (5). Members of the rickettsial subdivision of the [alpha]-Proteobacteria, a group of obligate intracellular parasites that includes genera such as Rickettsia, Anaplasma, and Ehrlichia, are considered to be among the closest known eubacterial relatives of mitochondria (6).
This same phylogenetic methodology has even been used to help convict a suspect for attempted second-degree murder:
quote:
Proc. Natl. Acad. Sci. USA, Vol. 99, Issue 22, 14292-14297, October 29, 2002
Evolution
Molecular evidence of HIV-1 transmission in a criminal case
Michael L. Metzker*,, David P. Mindell, Xiao-Mei Liu*,§, Roger G. Ptak¶,, Richard A. Gibbs*, and David M. Hillis
A gastroenterologist was convicted of attempted second-degree murder by injecting his former girlfriend with blood or blood-products obtained from an HIV type 1 (HIV-1)-infected patient under his care. Phylogenetic analyses of HIV-1 sequences were admitted and used as evidence in this case, representing the first use of phylogenetic analyses in a criminal court case in the United States. Phylogenetic analyses of HIV-1 reverse transcriptase and env DNA sequences isolated from the victim, the patient, and a local population sample of HIV-1-positive individuals showed the victim's HIV-1 sequences to be most closely related to and nested within a lineage comprised of the patient's HIV-1 sequences. This finding of paraphyly for the patient's sequences was consistent with the direction of transmission from the patient to the victim. Analysis of the victim's viral reverse transcriptase sequences revealed genotypes consistent with known mutations that confer resistance to AZT, similar to those genotypes found in the patient. A priori establishment of the patient and victim as a suspected transmission pair provided a clear hypothesis for phylogenetic testing. All phylogenetic models and both genes examined strongly supported the close relationship between the HIV-1 sequences of the patient and the victim. Resampling of blood from the suspected transmission pair and independent sequencing by different laboratories provided precaution against laboratory error.
Darwin on trial, for real this time. Looks like he won.
Or perhaps you would you like to propose a common designer to explain the similarities in the HIV sequences as well?
C'mon, the mitochondrial sequences could have been virtually anything, and yet they just happened to nest within those of a particular bacterial subgroup? Or perhaps your hypothesized designer was trying to fool us?
Just to emphasize, the sequence nesting isn't the only supporting evidence. For example,
quote:
J Eukaryot Microbiol 1999 Jul-Aug;46(4):320-6
A comparative genomics approach to the evolution of eukaryotes and their mitochondria.
Lang BF, Seif E, Gray MW, O'Kelly CJ, Burger G.
Departement de biochimie, Universite de Montreal, Quebec, Canada. Franz.Lang@Umontreal.ca
The Organelle Genome Megasequencing Program (OGMP) investigates mitochondrial genome diversity and evolution by systematically determining the complete mitochondrial DNA (mtDNA) sequences of a phylogenetically broad selection of protists. The mtDNAs of lower fungi and choanoflagellates are being analyzed by the Fungal Mitochondrial Genome Project (FMGP), a sister project to the OGMP. Some of the most interesting protists include the jakobid flagellates Reclinomonas americana, Malawimonas jakobiformis, and Jakoba libera, which share ultrastructural similarities with amitochondriate retortamonads, and harbor mitochondrial genes not seen before in mtDNAs of other organisms. In R. americana and J. libera, gene clusters are found that resemble, to an unprecedented degree, the contiguous ribosomal protein operons str, S10, spc, and alpha of eubacteria. In addition, their mtDNAs code for an RNase P RNA that displays all the elements of a bacterial minimum consensus structure. This structure has been instrumental in detecting the rnpB gene in additional protists. Gene repertoire and gene order comparisons as well as multiple-gene phylogenies support the view of a single endosymbiotic origin of mitochondria, whose closest extant relatives are Rickettsia-type alpha-Proteobacteria.
Looks like a wildly successful hypothesis to me.
Strangely enough, the closest sequences to those of mitochondria are not just any old alpha-proteobacteria, but:
quote:
source
Accumulating evolutionary data point to a monophyletic origin of mitochondria from the order Rickettsiales. This large group of obligate intracellular alpha-Proteobacteria includes the family Rickettsiaceae and several rickettsia-like endosymbionts (RLEs).
Why did the designer make the closest sequences to mitochondria obligate intracellular (inside-cells) parasites? Quite a coincidence, eh?
As if this weren't enough, there are cases where the mutualistic association of bacteria inside the cells of eukaryotes has been observed to occur and persist in the lab.
And, of course, we have the really peculiar case of secondary endosymbiosis. Check this out:
...in this case (multiple groups have things like this) it looks like a heterotrophic (non-photosynthetic, no chloroplasts) engulfed an autotrophic eukaryote (with chloroplasts), and formed a mutualistic association with the autotrophic eukaryote. This is supported by extra membranes around the chloroplast and a remnant eukaryotic nucleus inside the membrane with the chloroplasts.
Described here:
Here is a page that lists things somewhat thoroughly.
Pretty clever of the designer to leave those extra membranes and vestiges of a eukaryotic nucleus inside, eh?
[ 26. November 2002, 00:46: Message edited by: yersinia ]
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