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Why do living organisms spontaneously replicate itself or "procreate" (my understanding is that it does).

From a uni-cellular and micro-organism point of view. Is there some sort of stimulant in the environment? A chemical reaction that causes it? Is is there a physical or emotional motivation? Is it by choice?

Note: I may have some terms used wrongly as I am not a biologist. In this question, I am interested at living organism nearer the forms of single smallest unit of life form.

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2 Answers 2

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If there are enough nutrients, a unicellular organism will "eat" and grow until it reaches a certain size. Through different mechanisms, it can sense that it is large enough and has enough metabolites to divide. Then, the organism will duplicates its DNA, separate the duplicated DNA and divide. (This is called fission for prokaryotes.)

So why does it happen? One of the characteristics of life is replication. In favorable conditions, the organism replicating the fastest will out-compete all other organisms. So there is no motivation or stimulus of some sort needed (or possible), by their very nature cells are made to divide as soon as feasible.

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"One of the characteristics of life is replication" is that part of the definition of life? i.e. if a thing has all characteristic of living organism except "desire" to replication, then it is NOT a living organism? –  Jake Nov 20 '12 at 14:16
    
@Jake yes, well perhaps not "desire" but a thing must replicate itself/reproduce to be classified as alive. –  Rory M Nov 20 '12 at 15:26
    
Yes, reproduction is part of the definition of life. However, this doesn't mean that a sterile animal doesn't live. –  Michael Kuhn Nov 20 '12 at 15:30
    
@RoryM Thinking out loud: Due to pre-perception of a living thing, defining it with "ability to reproduce" seems to suggest that when the thing reproduce, it does so under its controlled will. However if i am asked WHY a kitchen sponge absorbs water with its ability to do so, I am unlikely to suggest that the sponge has a will. In a way, the sponge MUST absorb water. So to answer the "why", either living thing MUST reproduce, or the more approperiate question becomes, "does living thing desire to reproduce" or "can living thing inherently stop (temporarily) reproduction?" Hope I make sense... –  Jake Nov 20 '12 at 17:00
    
In your words, living things "must" reproduce - they have the ability to do so, and if the environment allows it, reproduction will spontaneously (i.e. without "will", "intention" or "motivation") happen. Compared to the original first molecule that had this ability, even the simplest cell is an incredibly complex organism with highly sophisticated control mechanisms which integrate all the necessary requirements for favourable reproduction and scan the environment (both internal and external) for these conditions. –  Armatus Nov 21 '12 at 13:12

This is a very interesting question but the answer (or as much of he answer as is known) can fill a few books. There are many many signals that control cell division.

As a horrible simplification, the cell can be compared to a car parked on a slope with a driver's foot on the brake. If she lifts her foot, the car will roll downhill. In the cell, there are various proteins (P53 is the most famous) that have their metaphorical foot on the brake. Various external and internal stimuli can cause these proteins to stop suppressing replication and the cell will then continue its cycle and replicate.

So, a cell's "natural state" is to replicate, there is a complex network of interacting factors (primarily proteins) that actively block replication in resting cells. When the conditions are "right" (what that means depends on the cell in question) the block is removed and the cell replicates.

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So in your metaphor, rolling down slope is the process of replication? –  Jake Nov 20 '12 at 14:14
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@Jake, yes, the point I am trying to make is that replication is the natural state of cells and needs to be actively prevented for the cells to not proliferate. –  terdon Nov 20 '12 at 19:51

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