Editing Cell cycle (section)

===Evolution of the genome===
The cell cycle must duplicate all cellular constituents and equally partition them into two daughter cells. Many constituents, such as proteins and [[ribosome]]s, are produced continuously throughout the cell cycle (except during [[Mitosis|M-phase]]). However, the chromosomes and other associated elements like [[Microtubule organizing center|MTOCs]], are duplicated just once during the cell cycle. A central component of the cell cycle is its ability to coordinate the continuous and periodic duplications of different cellular elements, which evolved with the formation of the genome.

The pre-cellular environment contained functional and self-replicating [[RNA]]s.<ref name=":2">{{cite journal | vauthors = Nasmyth K | title = Evolution of the cell cycle | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 349 | issue = 1329 | pages = 271–281 | date = September 1995 | pmid = 8577838 | doi = 10.1098/rstb.1995.0113 }}</ref> All RNA concentrations depended on the concentrations of other RNAs that might be helping or hindering the gathering of resources. In this environment, growth was simply the continuous production of RNAs. These pre-cellular structures would have had to contend with parasitic RNAs, issues of inheritance, and copy-number control of specific RNAs.<ref name=":2" /><ref>{{cite journal | vauthors = Cavalier-Smith T | title = The origin of eukaryotic and archaebacterial cells | journal = Annals of the New York Academy of Sciences | volume = 503 | issue = 1 | pages = 17–54 | date = July 1987 | pmid = 3113314 | doi = 10.1111/j.1749-6632.1987.tb40596.x | s2cid = 38405158 | bibcode = 1987NYASA.503...17C }}</ref> 

Partitioning "genomic" RNA from "functional" RNA helped solve these problems.<ref>{{cite journal | vauthors = Maizels N, Weiner AM | title = Phylogeny from function: evidence from the molecular fossil record that tRNA originated in replication, not translation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 91 | issue = 15 | pages = 6729–6734 | date = July 1994 | pmid = 8041690 | pmc = 44276 | doi = 10.1073/pnas.91.15.6729 | doi-access = free | bibcode = 1994PNAS...91.6729M }}</ref> The fusion of multiple RNAs into a genome gave a template from which functional RNAs were cleaved. Now, parasitic RNAs would have to incorporate themselves into the genome, a much greater barrier, in order to survive. Controlling the copy number of genomic RNA also allowed RNA concentration to be determined through synthesis rates and RNA half-lives, instead of competition.<ref name=":2" /> Separating the duplication of genomic RNAs from the generation of functional RNAs allowed for much greater duplication fidelity of genomic RNAs without compromising the production of functional RNAs. Finally, the replacement of genomic RNA with [[DNA]], which is a more stable molecule, allowed for larger genomes. The transition from self-catalysis enzyme synthesis to genome-directed enzyme synthesis was a critical step in cell evolution, and had lasting implications on the cell cycle, which must regulate functional synthesis and genomic duplication in very different ways.<ref name=":2" />