Okies, we all know that amoeba, bacteria, etc., asexually reproduce. Said reproduction is through mitosis, in which the chromosomes (or naked DNA, in some less developed sorts) duplicate themselves, and the cell splits in half, with the new chromosomes ending up in a new cell. The original chromosomes stay together in a greatly depleted cell.
My questions: 1) Is the amoeba with the original chromosomes considered the original amoeba?
2) If yes, do amoeba ever die, other than by disease, environmental depredation, or Lysol?
3) What’s the word for something that, absent something killing them, doesn’t die? Immortal implies never dying, period.
DNA replication is semiconservative. That means each strand of the amoeba’s DNA would combine with a newly synthesized strand of DNA. Both amoeba would have half of the original ameoba’s DNA strands. So you couldn’t identify the “original” by looking at the DNA, AFAIK.
I’ve read that eukaryotes (cells with a well-defined nucleus, in which category you would place the amoeba) have a maximum number of divisions they will undertake before dying, something called the Sonneborn (?) limit, whereas prokaryotes (e.g. bacteria) have no such limit. But I’m not a biologist by trade so my information may well be incorrect or out-of-date.
Is that correct? I thought that was what happened in meiosis. IIRC, in mitosis a new set of DNA is created and wanders off with the new cell. At least that’s what it looks like in those amoeba porn flicks I watch.
But, accepting what you say is correct, does DNA define which is the original amoeba? I know I posited that in my OP, but hey, I’m allowed to change my mind, at least until I get stomped on.
Arnold, I find your reference to the Sonneborn limit fascinating, and I’d like to subscribe to your newsletter. However, how does that fit in with Gene’s semiconservative DNA hypothesis? Does the Sonneborn limit apply if the DNA of the “original” and “new” amoebae intermingle?
Sua
Wee haw! The replies are fast and furious here! Let’s just start a chatroom #!!!hotamoebasex
Yup. It’s known as the Meselson and Stahl experiment.
I think you’re confusing the DNA double helix with the individual strands that make up the helix. Semiconservative replication means that the original double helix splits into two single helices. New helices are synthesized on top of the old helices so you wind up with two double helices. Each double helix has one helix from the original DNA and one newly synthesized helix.
If I could draw a picture it would be easier. If you have your college Bio text book around, I’ll betcha dollars to dinoflagellates it’s in there.
Anyway that means that if you could somehow label the original DNA helices, you’d find that half of it winds up in each amoeba.
I think the Sonneborn limit indicates that eukaryotes have evolved some mechanism or some factor that makes these buggers (and us buggers) mortal.
There seems to be some confusion here. I’m going off old memories for this, so I hope I don’t add to it.
An amoeba has only one chromosome. Therefore, it cannot go through meiosis (in which two homologous chromosome separate into different daughter cells).
DNA replication is, indeed, semiconservative - even in amoebas (amoebae? Nah, too many vowels). Old strand A binds with new strand B in one daugther cell, while old strand B binds with new strand A in the other. This would imply the simplistic view that somewhere out there is an amoeba with the “original” strand A and one with the “original” strand B. Of course, defining the original point is problematic, but let’s ignore that. The reason that this is simplistic is that events like recombinations, mutations, etc, will gradually change and mix up any old strands. Eventually, the “original” strand will look no more like it originally did than any other strand does.
Oh, yes. And prokaryotes do indeed have the ability to divide indefinitely. Good thing, too, or else they’d go extinct pretty quick.
So why don’t ameobae go extinct just as quickly? And I think that in any event, the original question is still just as valid if one substitutes “bacterium” for “ameoba”.
The mechanism that limits the number of divisions in eukaryotes is telomeres. Telomeres are long repetitive stretches of nucleotides at the ends of chromosomes. They don’t code for any proteins, but each time a cell divides, the telomeres in the two daughter cells is shortened. When the telomere gets too short, the cells can’t divide anymore.
Bacteria avoid this by not having telomeres. In fact they don’t have ends of their chromosomes at all. Their chromosomes are a big loop.
I’m not sure how one cell eukaryotes avoid it. If they don’t have a loop chromosome it’s likely that they have an enzyme called telomerase. Telomerase rebuilds the telomeres after division so that they are always the full length.
Humans have a gene to produce telomerase, but it’s turned off in almost all cells. The reason for this is that it is one of the mechanisms that helps suppress cancer. Most cancer cells have the telomerase gene turned on. (Before this was discovered, biologists would mix a few cancer cells with ordinary mammal cells in culture in order to immortalize them. Now they just add telomerase.)
There are a few non-cancer cells, called stem cells, that have the telomerase gene turned on. The best known stem cells produce gametes and blood cells. I believe that stem cells have been discovered in other tissues, although I’m not sure which ones (muscles, I think, but I could be wrong).
Nice responses … OK, I accept that amoebae are not immortal, although if anyone has more info on telomerase, I’d appreciate it. I also appreciate you setting me straight on semiconservative DNA. I should have remembered that.
However, Chronos brings up a good point. Change “ameoba” to “bacterium”. Are they immortal, since they don’t have telomeres? And again, how does one define the “original” bacterium after reproduction, or are neither the original?
In my old biology textbook, the two post-meitosis cells are called daughter cells. Basically, neither cell is the ‘original,’ but each has half of the original DNA and half new DNA.
I don’t have any references, but I think this is correct: bacteria are ageless. The very first living cell, mother to us all, may well still be out there, although:
The extreme, anaerobic environment in which it appeared is much rarer on Earth today, so it will have retreated to one of those (submarine thermal vents?) if it’s still alive.
The individual molecules it’s made of will have been swapped out many times over (we’re all made of its daughter cells!), so it may be more correct to say it occupies the space of, and retains the configuration of, the first living cell.
I’m not sure if the telomerase is nearly as important as was once figured. Once the idea that telomerase extended the potential life span of an organisms arose, it became a hot topic. IIRC, transgenic mice lacking telomerase really didn’t have a significant change in life span.
I’m sure telomerase plays an important role but I think that recent experiments have indicated that telomerase isn’t as necessary for life span as was once thought. Which sucked, because a lot of biotech companies started up with their success hinging on the fact that telomerase was basically a mortality gene. Or so I hear.
Again, this is all IIRC. I’ll be back in my old lab later today and I’ll have access to all my bio journals.
Anyway, I agree with everyone here: there is no “original” amoeba after it divides. That’s more of a sexual reproduction thing.
Shoot. Now I’m trying to remember the morphology of the amoebic chromosome. Look what you people do to me. I did in deed cause some more confusion with my last post, because I stupidly forgot that amoebas are eukaryotic. Duh.
Anyway, SuaSponte, once a single cell divides, you cannot in any way define which is the original. They’re both new. You can look at it two ways - a cell is born at a division and dies at the next division, or a cell lives on forever through its progeny. So the answer to the OP is, as usual, dependant on how you want to look at it.
This also kind of addresses Five’s comments. I don’t agree that “The very first living cell, mother to us all, may well still be out there.” You cannot point to one cell anywhere on the earth and say “You are the original.” The concept just doesn’t make sense. What once was the original (if there was indeed one, which is debatable) is now spread out amongst all cells in one way or another.
As for telomerase, I’m thinking that amoebas have circular chromosomes, explaining away that problem, but I’ll wait for Alphagene to get to his journals and check. In humans, I believe current thought is that it’s an important part of ageing, but it’s only one of many parts. That’s usually what happens in science when you think you’ve found “the” answer. It turns out to be a part of “the” answer.
Bacteria lack the membrane-bound nuclei of eukaryotes; their DNA forms a tangle known as a nucleoid, but there is no membrane around the nucleoid, and the DNA is not bound to proteins as it is in eukaryotes. Whereas eukaryote DNA is organized into linear pieces, the chromosomes, bacterial DNA forms loops. Bacteria contain plasmids, or small loops of DNA, that can be transmitted from one cell to another, either in the course of sex (yes, bacteria have sex) or by viruses. This ability to trade genes with all comers makes bacteria amazingly adaptible; beneficial genes, like those for antibiotic resistance, may be spread very rapidly through bacterial populations. It also makes bacteria favorites of molecular biologists and genetic engineers; new genes can be inserted into bacteria with ease.
Bacteria do not contain membrane-bound organelles such as mitochondria or chloroplasts, as eukaryotes do. However, photosynthetic bacteria, such as cyanobacteria, may be filled with tightly packed folds of their outer membrane. The effect of these membranes is to increase the potential surface area on which photosynthesis can take place.
The cell membrane is surrounded by a cell wall in all bacteria except one group, the Mollicutes, which includes pathogens such as the mycoplasmas. The composition of the cell wall varies among species and is an important character for identifying and classifying bacteria. In this diagram, the bacterium has a fairly thick cell wall made of peptidoglycan (carbohydrate polymers cross-linked by proteins); such bacteria retain a purple color when stained with a dye known as crystal violet, and are known as Gram-positive (after the Danish bacteriologist who developed this staining procedure). Other bacteria have double cell walls, with a thin inner wall of peptidoglycan and an outer wall of carbohydrates, proteins, and lipids. Such bacteria do not stain purple with crystal violet and are known as Gram-negative.
I have looked at as many sites as I can find. Bacteria obtain nourishment, grow and have sex. If they were ageless, this step would not be necessary. Bacteria are not ageless. No organism which is ‘alive’ (by moving food in, converting it to energy for work and expelling waste is ageless. I am willing to be proved wrong.