Posted on 01/14/2002 3:02:24 PM PST by Karl_Lembke
By Barry A. Palevitz
One of the enduring questions in biology is how eukaryotic cells arose from prokaryotic ancestors at least 2 billion years ago. Besides differences in genome organization, eukaryotic animals, plants, and fungi possess a much higher degree of cellular compartmentation in the form of membrane bound organelles than their distant bacterial and Archaean cousins. But how did such a plethora of cellular domains, each with a discrete role in metabolism, evolve?
To the extent that science proves anything, it answered the question for two eukaryotic organelles a long time ago. Mitochondria and chloroplasts evolved from endosymbiotic associations between an ancestral host cell and smaller prokaryotic partners. In the case of chloroplasts, the symbiont was a photosynthetic cyanobacterium; for mitochondria, most likely it was ana-proteobacterium.
The cytoplasm of eukaryotic cells is like chicken soup-it's chock full of organelles suspended like chunks of assorted vegetables and noodles in cytosolic broth. The broth also contains filaments of various dimensions that collectively comprise the cell's cytoskeleton. Like the bones of a large animal, the cytoskeleton provides a structural framework lending shape to cells and against which enzymatic 'muscles' work to elicit movement. That's how amoebae migrate, algae swim, stem cells divide, and cytoplasm streams relentlessly up, down, and across plant cells.
While the cytoskeleton is as much a hallmark of eukaryoticity as any mitochondrion or chloroplast, the origin of its filaments in deep time is more mysterious. Biologists assumed that genes for cytoskeletal proteins arose from prokaryotic precursors, but evidence in favor of the hypothesis was scarce, until recently.
Tubulin First on Stage
Microtubules comprise one component of the cytoskeleton responsible for a variety of movements including mitosis and meiosis. The 25 nm tubes consist of dimerica- and b-tubulin subunits that share about 40 percent sequence homology. Another form,y-tubulin, functions in microtubule formation.
But where did microtubules come from? It now appears that tubulins share a common ancestor with a protein called FtsZ, a key player in bacterial cell division.1 FtsZ is also present in plants, where it functions in chloroplast division,2 and a similar protein associates with mitochondria, at least in one alga.3 FtsZ polymerizes into filaments in the test tube in a process dependent on GTP. The same nucleotide is required for tubulin assembly into microtubules.1
Tubulins and FtsZ are clearly related, judging from similarities in three-dimensional structure. And although the proteins share only about 15 percent amino acid sequence identity overall, they're much more similar at the local level, particularly at the domain responsible for binding and cleaving GTP.4,5
Actin Into the Fold
Like the tubulins, actin-another essential component of the eukaryotic cytoskeleton-is a globular protein that binds nucleotide, in this case ATP. As actin monomers polymerize into 6-nm-wide microfilaments consisting of two helically wound protofilaments, the ATP, situated in a deep enzymatic cleft between two halves of the protein, hydrolyzes to ADP and inorganic phosphate.
It turns out that actin shares its ATPase domain with a family of proteins including hexokinase, the enzymatic kick starter of glycolysis, and several bacterial proteins. One of them is called MreB, a protein essential for generating or maintaining the rod shape of many bacteria. By examining structural similarities between eukaryotic actin and MreB from Thermotoga maritima, a research team at the Medical Research Council in Cambridge, England recently concluded that the two proteins are more closely related to each other than to other members of the family and undoubtedly share a common ancestor.6
The group showed that the three-dimensional shapes of actin and MreB are so similar they can be superimposed. The analogy with tubulin/FtsZ goes even further. Both proteins share considerable amino acid homology at several key sequences surrounding the ATP binding site, again situated deep in a cleft between two halves of the folded polypeptide chain.
Under the right conditions, MreB polymerizes into protofilaments that pair up lengthwise. The protein subunits are spaced about the same distance apart along the filaments as in polymeric actin, but MreB double filaments aren't nearly as helical.
The similarity between MreB and actin doesn't stop at structure and sequence. In a paper published earlier in 2001, a research group led by Jeffrey Errington at the University of Oxford, U.K. visualized MreB in the rod shaped cells of Bacillus subtilis using fluorescence and electron microscopy.7 MreB forms filamentous bands that encircle the cell in low helices, like reinforcing hoops. In an essay accompanying the Cambridge group's article, Duke University cell biologist Harold Erickson calculated that each band contains 10 protofilaments.8
When Errington's team genetically deprived cells of functional MreB, they became spherical. A search of genome databases showed that MreB is present in bacteria with nonspherical shapes, including rods. It's absent in spherical cocci. In other words, MreB has a cytoskeletal function. "I think it is quite convincing that MreB is the actin progenitor," says Erickson. "A key step, still unknown, going from bacteria to vertebrates is to develop a mechanism to make the double-helical actin filament from the single MreB protofilament structure."
More Acts to Follow
The story doesn't end with MreB; there's more to find out. Scientists want to know if MreB is also present in eukaryotes-associated with mitochondria and chloroplasts-as is FtsZ. According to Katherine Osteryoung, a plant biologist at Michigan State University in East Lansing who identified two FtsZ genes in the mustard plant Arabidopsis,2 "there's no obvious indication of MreB in plants that I've found or am aware of."
Actin normally functions along with the motor enzyme myosin to produce cellular motion, while microtubules utilize two other motor families called dynein and kinesin related proteins. Researchers now wonder whether MreB and FtsZ work in conjunction with bacterial motors. According to Erickson, "none have been turned up in genetic screens for cell division (or other activities), and none have been identified by sequence gazing. My bet is that kinesin and myosin evolved in eukaryotes, after the evolution of microtubules and eukaryotic actin filaments."
Still, Osteryoung is pleased with the latest results: "To someone interested in these issues, establishment of the prokaryotic origins of two major eukaryotic cytoskeletal proteins is enormously satisfying. I look forward to the day when evolutionary intermediates... from MreB to actin and FtsZ to tubulin, perhaps awaiting discovery in some obscure and primitive eukaryote, will more fully reveal the evolutionary steps by which key components of the eukaryotic cytoskeleton acquired their present-day structures and functions."
Barry A. Palevitz (palevitz@dogwood.botany.uga.edu) is a contributing editor for The Scientist.
References
If you don't really have any favored position, then you basically don't know. And if you don't know, how can you be so sure? All you seem to know is that God did not have a hand in it. That is faith based on a personal choice.
The 24 hour day thing is a non-starter. I am a fundementalist, but I am not taking the position that the bible teaches 24 hour days in Genesis One. That is an interpretation that is not demanded by the text. By the text, the eventh day is still ongoing. Chapter two says God did the work of the six days in one day! The phrase "Evening and Morning" is used in Daniel to describe the whole of the end times, a period of 3 1/2 years at the least. The New Testament says to God 1,000 years is as a day, and a day as 1,000 years. He is outside of time and this is just a smattering of the scriptures that strongly imply that the universe is far older than 6K years.
You are trying to argue against a position that neither I, nor anyone else on this thread holds. I could come up with some good arguments against a Flat Earth, but what good would that do? It is not a position that you hold.
Hindu cosmology does not stomp the bible flat when considering timescales, unless you hold to the most stompable interpretation of Genesis.
Only one aspect of evolution is actual science (micro-evolution), the rest is simply a way for people to explain our existence without God. Evolution (as used to explain our origins from single-celled organisms) is a religion. It is all speculation with some "evidence" thrown in and twisted around to fit the agenda.
Clearly he will welcome atheists and agnostics into a higher level of heaven than mere believers will attain, for he recognizes that it is they who best applied the gift of reason.
AAAAAAAAAAAHAHAHAHAHAHAHAHAHAHAHAHA!!!!! I haven't laughed that hard in a long time! Oh, it hurts!
Thanks for the laugh.
That will never happen.
Exactly! So anything you infer about how we got here and anything I infer about how we got here is all just speculation. It comes down to what you *BELIEVE* about our origins, thus, evolution as an explanation of our existence is a religion. I can use a lot of the evidence that is supposedly for evolution to support creationism (rock layers, fossils, etc.). I will not waste any more of my time showing you how because I've posted numerous times on other threads about the same thing. Research it if you wish to know.
BTW, you have not told me anything that I haven't heard before, and I'm sure you've already heard everything I've said so far. All these crevo threads are all the same: the same people spouting the same rehearsed lines. I've about given up on them all because in the end, the same questions go unanswered.
The Bible has been scientifically tested for accuracy for the last 2000 years, and it has passed every test. Using the Bible, my own experiences with God, and the "evidence" we have gathered (rock layers, fossils, etc.), I feel like my speculation is VERY informed. You left out the part about how evolution as an explanation of our existence is nothing but religion (a TAX-SUPPORTED religion).
The "evidence" we see can be interpreted several different ways. Rock layers can be seen on earth. Evolution would have it that it settled over millions of years, while creation would say that it formed due to rapid sedimentation during the flood. "Scientists" have set out to test this, and different results have come in. Those who set out to prove that it happened rapidly have found evidence that supports their claim, while those who have set out to show that it happened over millions of years have found evidence to support THEIR claims. So who's evidence is better?
Can God make a rock so big even he can't pick it up? Never have heard a good reply to that one.
That's because it's not a real question (have you stopped beating your wife yet?). It's a loaded paradoxical question dreamed up by God-haters to distract from the real issue. If you really want an answer, ask God Himself, He would know. Maybe read the Bible a little before you start dissing it. You seem to have an opinion on something you know nothing about. Intelligent, eh?
It's a God-given talent.
BTW, "faith" is an interesting word. Have you thought about its meaning? Is your faith that God doesn't exist so strong that you would be willing to place your life on the line? Would you be willing to risk death for your "faith"? If you had to declare right now that you are 100% certain that God does not exist, and if you are right, nothing happens, but if you are wrong, you would be killed, would you have the faith to make that statement?
1. That particular mistake is "easy" to aquire. It happened independently in all three species 'cause that gene just mutates a lot. It is well known that some mutations occur more often than others. Something in the DNA structure is not well protected on that gene, so the same mutation will occure in many species.
If it does in species that don't eat a lot of fruit, the individual dies- that would be the only reason it does not show up in other species (if it doesn't!. I would not be suprised to see the same mutation in fruit eating monkies but not leaf or seed eating monkies.)
2. There was wholesale 'borrowing' of genetic code to make man. Some of the code used was from a chimpanzee. Not all that different than what I used to do when creating computer code. Leaving the extra stuff in does not hurt a bit, but my new program could never have 'evolved' from my previous one without my intelligent design efforts.
3. It may not be the same gene/mutation at all. A while back they made the astounding announcement that bacteria shared exact duplicates of hundreds of genes that humans have, but not most things 'inbetween' us and bacteria. THAT was seen as explosive evidence for DESIGN and the evos were scrambling for other explanations that seemed as implausable as my 1 and 2 no doubt seem to you. Later, they said they were wrong, it was more like 40 genes. Still astounding, but also amazing about how wrong they were at first.
I wonder if it really IS the same mutation, can they demonstrate a 'reverse' mutation that will restore the genes the ability to make vitaman C? Can they produce a chimp that can make its own vitamen C? If not, if they can't even undo a change for one tiny gene, what are the odds all those gene changes reqired for the man-chimp transition could occur in the time allowed?
That brings up another point. If evolution is so powerful that it can add info, why haven't we gotten that gene back? We are no longer mostly fruit eaters, as we were in the Garden. The plains Indians did not have access to vitamen C. A lot of peoples didn't. Wouldn't the ability to make vitamen C be a huge advantage for such peoples? Why did they never aquire the 'reverse mutation' that would restore functionality to this gene? Why don't we have Eskimoes that can make C while most of the rest of us still can't?
If evolution is only the loss or degradation of functionalty, then that argues for ID. An intial large amount of info slowly degrades over time as mutations are aquired.
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