BL1102 Essay The Cells Behind The Cells Matriculation Number: 120019783 19 April 2013 1
The Cells Behind The Cells For the first 3,000 million years on the early planet, bacteria were largely dominant. These prokaryotes evolved and existed long before eukaryotes. A random event, most likely an endosymbiotic one, caused the rise of eukaryotes, where one prokaryote engulfed another and soon became dependent on the internalized cell. Supporting this theory, the first eukaryote was a chimera, half archaea and half bacteria (Lane, 2009). Furthermore, around 550 million years ago, the Cambrian Explosion led to the radiation of eukaryotes and a boom in the diversity of life. This type of event usually occurs when genetic promise comes face to face with environmental opportunity (Lane, 2009), demonstrating that eukaryotes had the genetic ability to diversify and were being supported in this process by ideal environmental surroundings. To this day, eukaryotes continue to be the basis for all major life forms on Earth. The rise of eukaryotes can be explained by examining the structure and function of mitochondria, chloroplasts, and even the nucleus, as they provide evidence for endosymbiosis and for prokaryotes as the common ancestor. The Rise of Eukaryotic Cells One theory behind the rise of eukaryotic cells is the Endosymbiotic Theory, introduced in the 1960 s by biologist Lynn Margulis at Boston University. This theory suggests that the predecessors of eukaryotic cells were symbiotic consortiums, meaning that they lived together in symbiotic, even mutualistic, relationships (McGill University, n.d.). This process of endosymbiosis occurred around two billion years ago, where one prokaryote randomly engulfed another and resulted in the fusion of two cells (UCLTV 2009). This act of phagocytosis was accomplished by the extension of the host cell s cytoplasm; the extensions protruded from the 2
cell, fused, and then left the captured object, or cell in this case, in a membrane-bound vesicle within the host cell (Whatley and Whatley, 1981). Instead of being completely digested, the cell became integrated within the host prokaryote over time. The two endosymbionts then benefitted from each other, and eventually, the endosymbionts lost their ability to function independently (McGill University, n.d.). Mitochondria Mitochondria were originally discovered by Richard Altmann in the 19 th century (UCLLHL 2012). Further, and more recent, research has shown that the mitochondrial genome originated from eubacteria. Mitochondrial functions, such as ATP production and mitochondrial protein translation, can be traced back to a bacterial ancestor (Gray, Burger, et al., 2001); mitochondria most likely arose from the endosymbiosis of an alphaproteobacterium (Wilson and Dawson, 2011). As mitochondria became specialized organelles, they lost most of their genes, and their remaining genes were dedicated to supporting the large host cell genomes (UCLLHL 2012). The mitochondrial genome is needed to control respiration, which requires local control and constant feedback (Lane, 2009). The mitochondrial genome underwent streamlining, otherwise known as reductive evolution, by transferring mitochondrial genetic functions to the nucleus. As a result, mitochondrial genes are small, circular like in bacteria, and lack introns (Illingworth, n.d.). In total, mitochondria lost between 96 and 99.9 percent of their genes (Lane, 2009). This process led to a minute loss of coding ability, since many of the genes that were lost were ones whose functions were coded for by unrelated genes in the nucleus. Additionally, some new mitochondrial genes were recruited from the nucleus to complement those of the bacterial ancestor (Gray, Burger, et al., 2001). An examination of the mitochondrial membranes also provides support for the endosymbiotic theory. The outer mitochondrial membrane is 3
synonymous with host cell vacuole and is coded for by the nucleus. At the same time, the inner mitochondrial membrane is homologous with the bacteria plasma membrane, but it is coded for by the mitochondrial genome (Whatley and Whatley, 1981). Chloroplasts Between the development of prokaryotes and eukaryotes, there was an accumulation of oxygen in the atmosphere (McGill University, n.d.). The Great Oxidation Event occurred around 2.2 billion years ago, saturating the air and surface waters of the oceans with oxygen. While this event did not cause bacteria to evolve, it caused a shift in ecology toward bacteria that liked oxygen (Lane, 2009). The Great Oxidation Event is strong evidence for the presence of chloroplasts, since the development of photosynthesis would have caused this dramatic surge in oxygen production, where oxygen was a result of water being split (BBC 2010). Chloroplasts were once free-living bacteria that were then engulfed by the common ancestor of all plants and algae (Lane, 2009). This common ancestor was most likely a cyanobacterium (Wilson and Dawson, 2011). Chloroplasts arose by two consecutive acts of symbiosis and are from multiple origins; there are two possible evolutionary lines. Precursors of green algae replaced phycobilins with chlorophyll b, whereas brown and other algae developed accessory chlorophyll a (Whatley and Whatley, 1981). Chloroplast DNA in red and green algae resembles that of cyanobacteria, suggesting that chloroplasts, like mitochondria, are the result of the fusion of two prokaryotes. Furthermore, plastids in red and green algae have two membranes, which correspond to the inner and outer membranes of cyanobacteria (Campbell, Reece, et al., 2008). 4
Nucleus The nucleus evolved to exclude genes from protein building factories, or ribosomes, in the cytoplasm, but it did not evolve to protect the genes as widely supposed. Eukaryotes developed a mechanism for splicing early in evolution, a process that requires spatial separation which can only be provided by the nucleus. Since they have no nucleus, prokaryotes have no separation between their genes and their apparatus for building new proteins, meaning that their DNA and ribosomes are mixed; thus, it is unknown how prokaryotes get rid of unneeded introns (Lane, 2009). Evidence for endosymbiosis as a cause for the evolution of the nucleus is lacking. The nature of the endomembrane proteins suggests that the eukaryotic endomembrane evolved simultaneously with the nuclear membrane. The first eukaryotic common ancestor lacked nuclear morphology, but its proteins could have contributed to the incremental evolution of the nucleus of the last eukaryotic common ancestor (Wilson and Dawson, 2011). Differences between prokaryotes and eukaryotes Since prokaryotes do not have mitochondria or chloroplasts, they control their respiration over their membrane, limiting their size to a certain extent. As bacteria increase in size, the less they can breathe as it is difficult for respiration to be controlled over a large surface area (Lane, 2009). There are also genetic differences between prokaryotes and eukaryotes. Whereas prokaryotes have single-stranded circular DNA, eukaryotes have a complex double helix wrapped around histone proteins. Although different in structure, genes found in prokaryotes and eukaryotes both encode core processes such as metabolism and DNA processing. These processes tend to evolve slowly and with more difficulty because everything else in the cell 5
depends on them (Lane, 2009). While there are differences between prokaryotes and eukaryotes, there is also an overlap. Bacteria have an internal cell skeleton that is somewhat similar to the cytoskeleton of eukaryotes. There are also some cases of bacteria internally harboring a smaller bacteria; this is quite rare, especially since no bacteria is known to ingest other cells by means of phagocytosis (Lane, 2009). Following the tremendously rare event of endosymbiosis, where one prokaryote engulfed another, the two cells became assimilated and were eventually completely dependent on each other (BBC 2010). The host prokaryote was now much more complex than any prokaryote alone. As a result, these new eukaryotic cells developed chloroplasts, mitochondria, and a nucleus, to which they localized their internal processes. Eukaryotes could now expand in size without incurring large energy costs (Lane, 2009), making cells more efficient than ever before. The rise of eukaryotic cells from a common ancestor defined a new era in the history of the Earth, which led to the evolution of increasingly complex life forms. Word Count = 1268 6
Reference List Campbell, N.A., Reece, J.B., et al., 2008. Biology. 8 th ed. Pearson: San Francisco. Gray, M.W., Burger, G., et al, 2001. The origin and early evolution of mitochondria.[pdf] Available at: <http://www.biomedcentral.com/content/pdf/gb-2001-2-6- reviews1018.pdf> [Accesssed 8 March 2013]. Illingworth, J., n.d. Origin & evolution of mitochondria. University of Leeds. Available at: <http://www.bmb.leeds.ac.uk/illingworth/oxphos/evolve.htm> [Accessed 7 March 2013]. Lane, N., 2009. Life Ascending. London: Profile Books Ltd. McGill University, n.d. Endosymbiotic Theory. [pdf] Available at: <http://lecerveau.mcgill.ca/flash/capsules/articles_pdf/endosymbiotic_theory.pdf> [Accessed 7 March 2013]. Nick Lane: Meet the Author, 2010. [video] London: BBC Meet the Author Series. (Interview by Nick Higham). UCLLHL, 2012. Is complex life a freak accident? [video online]. Available at: <http://www.youtube.com/watch?v=acpq3ixgfne> [Accessed 9 March 2013]. UCLTV, 2009. Mini-lecture: The origins of complex life (UCL). [video online]. Available at: < http://www.youtube.com/watch?v=zj3x4iq91sw&noredirect=1> [Accessed 9 March 2013]. 7
Whatley, J.M. and Whatley, F.R., 1981. Chloroplast Evolution. New Phytologist, 87(2). Available through JSTOR: <http://www.jstor.org/discover/10.2307/2432212?uid=3738032&uid=2&uid=4&sid=211 02103461937> [Accessed 8 March 2013]. Wilson, K.L. and Dawson, S.C., 2011. Functional evolution of nuclear structure. The Journal of Cell Biology, 195(2), 171-181. Available at: <http://jcb.rupress.org/content/195/2/171.full> [Accessed 7 April 2013]. 8