Even though all known life forms have an origin of replication, there is great diversity across the ancient domains of life in how many origins of replication there are and what their structure is. In bacteria, single-celled life forms that have a single circular chromosome, there is only one origin of replication on that one chromosome. Bacteria also possess small loops of DNA called plasmids, not connected to the chromosome, that each have their own origin of replication. Some early experiments conclusively proved that replication is impossible without an origin of replication by removing this sequence from bacterial plasmids that carried resistance to an antibiotic—when this was done, those plasmids were unable to replicate and as the population of bacteria divided, the plasmid was no longer inherited and the bacteria died.
In other life forms, there are multiple origins of replication. The archaea, like bacteria, are single-celled organisms with circular chromosomes, but that unlike bacteria have multiple origins of replication on their single chromosome. Eukaryotes—plants, animals, and a huge diversity of microbes, have more than one chromosome, and their chromosomes are linear instead of circular. Eukaryotes often have many origins of replication on each of their multiple chromosomes—a single human cell may contain more than 100,000 origins of replication.
In other life forms, there are multiple origins of replication. The archaea, like bacteria, are single-celled organisms with circular chromosomes, but that unlike bacteria have multiple origins of replication on their single chromosome. Eukaryotes—plants, animals, and a huge diversity of microbes, have more than one chromosome, and their chromosomes are linear instead of circular. Eukaryotes often have many origins of replication on each of their multiple chromosomes—a single human cell may contain more than 100,000 origins of replication.
Question!
As you may already have known, eukaryotes have multiple, linear chromosomes while prokaryotes like bacteria have only a single, circular chromosome. We also now know that while bacteria have only one origin of replication, eukaryotes can have tens of thousands. Eukaryotes can be much more complex than bacteria (or archaea)--all multicellular organisms are eukaryotes, and eukaryotes generally have larger genomes than prokaryotes. Does this mean that in order for an organism's complexity to increase, its number of origins of replication must increase first?
Current Consensus
An interesting way to ask whether limiting the origin of replication limits the evolution of complexity is by considering something called endosymbiosis. This is the widely accepted theory that the organelles in eukaryotes resulted from engulfment of prokaryotic cells. If we look at organelles like mitochondria and chloroplasts, they are very similar to free-living bacterial cells: their DNA is circular and independently produces some proteins necessary for function of the organelle. One theory is that mitochondria came from Alphaproteobacteria, and chloroplasts from Cyanobacteria (Gray, 2012).
Over time, parts of the engulfed cell's genome were also transferred into the host's genome. The mechanism of this swapping that likely occurred between two circular genomes may also help explain how eukaryote-like linear chromosomes came to be. Some bacteria have a special type of mRNA called group II introns. These RNA molecules not only code for a reverse transcriptase protein that can make DNA from RNA, but also have unique molecular structure that allows them to self-splice into DNA molecules, where they are permanently encoded as DNA by the protein. Transfer of the engulfed prokaryote’s group II introns into the circular genome of the host may have been responsible for what are called "spliceosomal introns," and ultimately led to the formation of linear chromosomes and the nucleus (Garavis, Gonzalez, & Villasante, 2013). Especially if this process occurred again and again as the host and symbiont evolved together, this process might also help explain why eukaryotes have so many more origins of replication than either bacteria or their own organelles.
Based on this evolutionary research, the answer to our question is probably "no." Complexity and eukaryotic features probably developed from interactions between single-celled organisms that at the time had circular chromosomes with a single origin of replication. In this scenario, limits on the origin of replication certainly did not prevent the development of complex life that we observe today, but understanding the origin does help us understand how we got from there to here.
We can look even farther away than bacteria for hints about how the origin of replication works and how it has crossed groups of life forms. Some researchers have discovered similarity between viral origins of replication and those of chloroplasts. viruses have been used as models to study the initiation of DNA synthesis in eukaryotes. The location of the origin of replication in bacteriophage T7 and in chloroplasts appears similar--and some research on this topic has even led researchers to believe that due to historical interaction with viruses, the chloroplast genomes of some plants are straight, rather than circular (Oldenburg & Bendich, 2004). If true, that would be a remarkable discovery contrary to the current knowledge of plant genetics, thanks to research on the origin of replication! Put another way, the discovery of linear chloroplast chromosomes due to viral interaction would mean that a virus inside a bacteria-like organelle interacted with the organelle's genome in a way similar to how the genome of the organelle interacted with the host cell that it itself is inside! Chloroplast DNA in different stages of growth can also use exotic replication methods--with names like recombination-dependent replication, double D-loop replication, and rolling circle replication--that are often seen in viral life cycles (Nielsen, Cupp, & Brammer, 2010), including the well-known bacteriophage T4 that may have infected the bacterial ancestor of chloroplasts. Even at the protein level some similarities have been found: the initiation of DNA replication in herpes simplex virus (HSV) requires an origin binding protein (OBP) that binds to an origin of replication to recruit a replisome, and the same OBP binding has been observed in chloroplasts (Oldenburg & Bendich, 2015).
Over time, parts of the engulfed cell's genome were also transferred into the host's genome. The mechanism of this swapping that likely occurred between two circular genomes may also help explain how eukaryote-like linear chromosomes came to be. Some bacteria have a special type of mRNA called group II introns. These RNA molecules not only code for a reverse transcriptase protein that can make DNA from RNA, but also have unique molecular structure that allows them to self-splice into DNA molecules, where they are permanently encoded as DNA by the protein. Transfer of the engulfed prokaryote’s group II introns into the circular genome of the host may have been responsible for what are called "spliceosomal introns," and ultimately led to the formation of linear chromosomes and the nucleus (Garavis, Gonzalez, & Villasante, 2013). Especially if this process occurred again and again as the host and symbiont evolved together, this process might also help explain why eukaryotes have so many more origins of replication than either bacteria or their own organelles.
Based on this evolutionary research, the answer to our question is probably "no." Complexity and eukaryotic features probably developed from interactions between single-celled organisms that at the time had circular chromosomes with a single origin of replication. In this scenario, limits on the origin of replication certainly did not prevent the development of complex life that we observe today, but understanding the origin does help us understand how we got from there to here.
We can look even farther away than bacteria for hints about how the origin of replication works and how it has crossed groups of life forms. Some researchers have discovered similarity between viral origins of replication and those of chloroplasts. viruses have been used as models to study the initiation of DNA synthesis in eukaryotes. The location of the origin of replication in bacteriophage T7 and in chloroplasts appears similar--and some research on this topic has even led researchers to believe that due to historical interaction with viruses, the chloroplast genomes of some plants are straight, rather than circular (Oldenburg & Bendich, 2004). If true, that would be a remarkable discovery contrary to the current knowledge of plant genetics, thanks to research on the origin of replication! Put another way, the discovery of linear chloroplast chromosomes due to viral interaction would mean that a virus inside a bacteria-like organelle interacted with the organelle's genome in a way similar to how the genome of the organelle interacted with the host cell that it itself is inside! Chloroplast DNA in different stages of growth can also use exotic replication methods--with names like recombination-dependent replication, double D-loop replication, and rolling circle replication--that are often seen in viral life cycles (Nielsen, Cupp, & Brammer, 2010), including the well-known bacteriophage T4 that may have infected the bacterial ancestor of chloroplasts. Even at the protein level some similarities have been found: the initiation of DNA replication in herpes simplex virus (HSV) requires an origin binding protein (OBP) that binds to an origin of replication to recruit a replisome, and the same OBP binding has been observed in chloroplasts (Oldenburg & Bendich, 2015).
Question!
Here's a hard question that cannot be answered entirely by science, but that science can still do a lot to understand. Not everybody agrees that all life on earth shares a common origin. Some biologists believe that the same general type of life evolved multiple times in our planet's early history, and some people accept entirely different, non-evolutionary explanations for the origin of life on earth. Does the ancient diversity in the origin of replication across bacteria, archaea, and eukaryotes affect the hypothesis that all of these domains share a single common ancestor? Do their differences or similarities change the debate about the history of life between those who see it as the result of chemical and evolutionary process and those who have alternative views? Please share your own thoughts here!