The Cell
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There are very few unifying themes in biology. Evolution is one. Cell theory is another. Cell theory was recognized as a coherent theory sometime in the mid-1800s (although Darwin never acknowledged cell theory when he developed his theory of evolution in The Origin of Species). Cell theory has three basic tenets: 1) the fundamental unit of life is the cell, 2) all living organisms are composed of cells, and 3) all cells come from cells. As it turns out, the validity of the three tenets is still under debate. With respect to the first tenet, the "unit of life is the cell" is questionable, since it really depends on your definition of life (i.e. are viruses alive? is the Earth, or Gaia, alive?). With the second tenet, unicellular organisms aren't composed of cells -- a single cell is the cell. Thus, single cellular organisms might really be considered as "acellular". With the third tenet, the question arises: if all cells come from cells, then where did the first cell come from? The answer is, from chemical evolution, and this will be described somewhat in my "origins of life" section. Although there are some difficulties with "cell theory", it gives us a framework from which to develop new theories. For example, origins of life theories -- the first cell had to arise from a non-cell, the question is how did it happen? There are a number of theories out there and I will describe some of them in my "origins of life" section. A vast part of Earth's biodiversity is microbial (ref. 1, p 11046), thus studying single celled organisms is useful for understanding life in general. Cell division is an important property for life, however, there are differences in the mechanisms by which prokaryotes and eukaryotes divide. Bacteria divide by "binary-fission" (for a detailed description, see ref. 2). Eukaryotic cells, on the other hand, divide by different mechanisms: mitosis and meiosis. Meiosis and mitosis are two types of cell division in eukaryotes, and specifically have to do with division of the nucleus. Meiosis, or sexual reproduction, generates four haploid cells from one diploid cell. Germ line cells or gametes (i.e. sperm and eggs) are formed by meiosis and the daughter cells are haploid -- each contain half of the genome from the parent. Mitosis, or asexual reproduction, generates two diploid cells from one diploid cell. Mitosis is how somatic (body) cells divide. For a detailed description of mitosis and meiosis, see any general genetic textbook, or ref. 3. John Maynard Smith and Szathmary (1995) describe a possible scenario of how eukaryotic cell division may have arose from bacterial cell division, but it is hypothetical and somewhat complex, so I'll not go into it here. Eukaryotic cells also undergo what is known as the "cell cycle". The cell cycle has four distinct stages (but this may depend on the species): G1, S, G2, and M. "M" represents mitosis and "S" represents DNA replication or synthesis. G1 and G2 are intermediate "gaps" where the cells are neither undergoing cell division nor DNA replication (however, this doesn't necessarily mean that the cells are at "rest"). Cells may exit the cell cycle from M or G1 by going into a resting state, G0. Cells can re-enter the cell cycle from G0 by going back into G1. One of life's properties is to replicate, for without some form of heritable variation, evolution would not be possible. Genetic information is commonly cited as the "thing" which is replicated. In reality, "replication" is not perfect, as there are errors in copying genetic information. These replication errors are partly responsible for genetic variation -- the very "stuff" of evolution. There is also cytoplasmic inheritance (i.e. inheritance of components of the cytoplasm, such as organelles), and this may not necessarily be equally distributed to the daughter cells. Thus, there may also be variations in structural inheritance, which also takes part in evolution. Here is a simple example of "organismal" replication. A parent cell replicates by dividing into two daughter cells. These two "new" cells can then live on, grow, and replicate. If you start with one cell, it eventually divides into two, and those two cells can each divide into two, so you have four new cells. Four cells divide into eight cells, eight into sixteen, and so on ad infinitum. Cells can divide exponentially over time (assuming there is enough nutrients available). Typical population growth of a bacterial culture begins with a "lag phase" where the cells "sample" the medium they are in (i.e. the cells are in a process of switching genes on or off until they get the "right" set of genes on to uptake nutrients), and this leads to the "exponential phase". Once nutrients run out in the growth medium, the bacterial population enters a "stationary phase" (or a plateau), and ultimately, when the cells use up their internal food reserves, they die off in a "death phase". Some bacteria can divide every 20 minutes under ideal conditions. Let us take a doubling time of 30 minutes (two doublings every hour). In one hour, you get four cells from one. In ten hours (20 doublings), you get 2^20 (2 to the exponent 20) cells, which is about 1 million cells. In 60 hours (120 doublings) you would end up with about 1.33 x 10^36 cells. Let's assume that a single cell can have a volume of about 10^-11 ml. Therefore, after 60 hours the cells would occupy a volume of about 1.33 x 10^25 mL. By way of comparison, the Earth has a volume of about 9.0 x 10^26 mL. So, in just over 60 hours (124 doublings), a single bacteria would have generated a colony the size of planet Earth! Well, that is just ridiculous. We never see anything happen like that in reality. Why? It is because there are other factors involved in the life of a cell. For example, as mentioned above, the environment often has limited resources (once the resources run out, cells tend to die off). How then, does life still exist after some 3.5 billion years since it arose on Earth? By a simple principle: "one man's trash, is another's treasure". One species' waste products may be consumed by a different species with different metabolic pathways. With limited resources, there is competition within and between species ("intra-specific competition" and "inter-specific competition", respectively). With replication errors and other sources of variation, individuals may arise with slightly better or slightly different molecular machinery to adapt to changing conditions. Those individuals that survive to pass on their traits to their offspring----, contribute to the next generation, and the process of change and adaptation continues. Ultimately, a "division of labor" occurs through speciation (i.e. divergence of a single species into two or more new species), complex ecological systems arise -- and to the logical extreme, a global network of life-forms, or "Gaia", emerges. References:
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