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Member (Idle past 1433 days) Posts: 20714 From: the other end of the sidewalk Joined: |
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Author | Topic: What IS reproduction? | |||||||||||||||||||
RAZD Member (Idle past 1433 days) Posts: 20714 From: the other end of the sidewalk Joined: |
I'd like to bring together parts of three current threads and look at one aspect that pertains to each of them.
First we have Are there evolutionary reasons for reproduction?:
quote: Then from Definition of Lifequote: and
quote: And finally from Self-sustained Replication of an RNA Enzymequote: Noting that dcarraher apparently draws a distinction (whether it is valid or not) between DNA replication and RNA replication, it seems appropriate to delve into this in a little greater detail. He also says:
quote: Now of course the point of the thread is that the RNA is self-replicating rather than just being replicated within a cell as part of the process of reproduction Is this self-replication life? Without a clear definition of life it is difficult to say. Is a virus alive? This depends on your definition of life. Is this self-replication a feature of life as we know it? Yes, it is common to all organisms during reproduction that all the elements of a cell are replicated. Reproduction is commonly listed as an element of life, as seen in the wiki article above, and a critical part of reproduction is the replication of the material within a cell so that another cell is formed. At its most basic level, cell reproduction occurs by:
This holds for prokaryotes and eukaryotes, for single cell life and multi-cellular life, for asexual reproduction and the reproduction of cells within sexual organisms. As such, I would propose that self-replication of the core molecules is a better defining feature of life than reproduction. Enjoy. Edited by RAZD, : added (c) we are limited in our ability to understand
by our ability to understand Rebel American Zen Deist ... to learn ... to think ... to live ... to laugh ... to share. • • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •
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RAZD Member (Idle past 1433 days) Posts: 20714 From: the other end of the sidewalk Joined: |
Hi Phage0070, thanks.
Isn't that just another way of distinguishing number 6 "Reproduction" from number 3 "Growth"? In fact I take it back; it doesn't really distinguish reproduction from growth at all. Growth for many organisms involves the reproduction of the core molecules, so that definition would be a subset of both growth and reproduction. Perhaps you are confusing the growth of a multicellular organism (by adding cells) with the growth of individual cells.
quote: The cells increase in size. Growth can occur without the replication of the core molecules. A virus replicates without itself exhibiting growth - it hijacks the materials from another system\biology.
quote: Part (a) can be viewed as growth, while part (b) would a part of reproduction that is different from growth. I should add (c): division of material into two or more discrete units. This division is also a critical element of life that is not exhibited by other things that grow (crystals for instance).
Depending on how you viewed things, reproduction wouldn't necessarily require the duplication of the core molecules. If you view reproduction as when organisms split into individuals you could get really technical and mark the point when the DNA itself is split. The reproduction of the other half would then simply be growth; after all it is simply building on the core molecules that already existed in the new individual. A half-split DNA strand then could be half-individual, and growing. And yet a cell that is undergoing the process from replication of the core molecules to division, is distinguishable from other cells that are only exhibiting growth. Or you end up with two kinds of growth, and you need to distinguish one from the other ...
In any case, I view the definition of life as largely irrelevant. "Life" isn't an inherent quality of any organism, it is an emergent property we choose to define (very poorly). A proper definition is not required to understand anything about the organisms, etc. So at what point does this property emerge during the development of life from chemical precursors? Enjoy. we are limited in our ability to understand
by our ability to understand Rebel American Zen Deist ... to learn ... to think ... to live ... to laugh ... to share. • • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •
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RAZD Member (Idle past 1433 days) Posts: 20714 From: the other end of the sidewalk Joined: |
Hi Phage0070,
It is both. For a single-celled organism it is simply the cell increasing in size/mass, but for a multicellular organism it includes the addition of new cells. In other words yes, you are lumping them together. As I said, however, if you include reproduction in growth then you have to define two kinds of growth, reproductive growth and non-reproductive growth. If we define life at the cellular level, then organisms that are multicellular are still included, and the only issue for multi-cellular life then is whether it is a colony or a single organism.
You missed my point; you could technically view a reproducing cell halfway through copying DNA as half-individual, with an extraordinary level of resource sharing between those individuals. The purpose of doing that would be to distinguish growth from reproduction, by making the definition so particular that it turns normal cell reproduction into a nearly inextricable combination of reproduction and growth. No, because I think your conflation of reproductive growth with non-reproductive growth is muddying the waters and that this is what leads to your problems. In the context of defining life there is no need to have reproductive growth and reproduction in the definition, so a parsimonious definition should just stick to non-reproductive growth as an element of life. Likewise, when we discuss reproduction, we should not include the elements that are part of normal (non-reproductive) growth: the self-replication of core molecules and cell division into new cells.
Whenever we decide it qualifies. Think of it like the colors red and yellow; at what point does red become yellow as you move up the spectrum? There are certain points where almost everyone will agree that it is red, or yellow, but in the middle it gets fuzzy. We might even subdivide it into the uncertain range being called orange, but then where does the transition between yellow and orange, or red and orange occur? Our subjective definition of colors is irrelevant to being able to get from one color to another through increasing or decreasing the frequency of the light. It is such an issue because creationists are making what amounts to an argument from incredulity, saying that life cannot come from unlife. This is tantamount to saying that red cannot come from yellow; the dividing line, or lines between subdivisions between those points, can be placed anywhere by the individual observer. In every case the separating point of that distinction would be overcome by an extremely slight modification of frequency. Coming up with a universal definition of colors so that definition can be made would be useless, as the color-creationists would simply reject it so that they could hang on to their belief that you cannot get from red to yellow. Instead we should point out that wherever you draw the line it is a very slight modification to get to the next step, so they will have to show a reasonably large void in the steps. If there was some large gap of light frequencies between red and yellow that could not exist, then they might have a point. The same holds true for creationist claims, if they could produce such a gap. Instead they cling to an unknown subjective definition of life which will travel like a greased-up deaf guy to avoid being nailed down, where the argument from incredulity can be destroyed. It is just a waste of time and we are better off doing something else. Agreed, however this has little to do with what reproduction is or isn't. a b c d e f g h i j k l m n o p q r s t u v w x y z In the context of reproduction as an element of life, we can say that having self-replicating molecules is somewhere in the orange sector: it is something that rocks, water, crystals and other non-living (inorganic) chemical objects do not have, and so it is no longer in the red zone, but somewhere between (a) and (i). Likewise fission, the division of a cell (which is also poorly defined, and somewhat circular in the above definition of life re organization) into two or more similar functional units. Uranium may fission into two distinct elements, but neither of the product elements is uranium. Crystals can fracture into smaller crystals, however this fracturing is not caused by the crystal itself. So again, the elements of reproduction that are critical to the definition\existence of life is that self-replication of core molecules occurs and cell division occurs. Note that I find the element of organization to be poorly defined:
quote: This is circular if we say life has organization and organization means things are built with bits of life. IMH(ysa)o it would be better to say
1. Organization - Living things are are organized into one or more discrete units composed of different functional elements that work as a unit to accomplish metabolism, growth and reproduction. This also helps to draw the distinction between growth and reproduction. Enjoy. we are limited in our ability to understand
by our ability to understand Rebel American Zen Deist ... to learn ... to think ... to live ... to laugh ... to share. • • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •
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RAZD Member (Idle past 1433 days) Posts: 20714 From: the other end of the sidewalk Joined: |
Hi Modulus, thanks.
I appreciate for theoretical discussion it is useful to differentiate these - but this is usually within the context of a certain kind of life where a universal method for discriminating between 'somatic' reproduction and 'germline' reproduction (for lack of better terms) might be available. I'm not sure this has much utility (or feasability) when considering the entirety of life. Though maybe I'm wrong. Do you think it is possible to describe a working understanding of the processes so that in each case we could differentiate between (mere) somatic reproduction of a single cell from germline reproduction leading to a new 'individual'? I'm not sure we need to do this as yet. I'm thinking (a) in the context of the definition of life, a minimal and basic form of reproduction, at the cellular level, and (b) about the original process of reproduction at the early stages of life, and looking to establish what that consists of: the kind of reproduction that would be useful in determining whether abiogenesis has occurred. Perhaps later on we can get to the application of reproduction to multicellular life (and what distinguishes it from colonies of cells).
This might require being able to strictly identify 'individual' which may prove to be interesting... Indeed. As an example we all think of ourselves as individual organisms, and yet we rely heavily on many different kinds of bacteria to help break down and digest food and to protect our skin, etc. so that we are really a colony of many organisms.
Is there some key difference between a bunch of bacteria engaging in binary fission and human skin cells? I honestly don't know. My understanding is that the (eukaryotic) cell reproduction is the same process in these two cases. There is a slight difference (simpler system) for the prokaryotes (no nucleus, no mitochondria). If we take reproduction in prokaryotes first, as an initial simple system, then we can break it down into three basic elements:
As noted above, element (a) is cell growth, and elements (b) and (c) are the elements specific to reproduction. Enjoy we are limited in our ability to understand
by our ability to understand Rebel American Zen Deist ... to learn ... to think ... to live ... to laugh ... to share. • • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •
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